Paleontology, taphonomy, and sedimentology of the Mygatt-Moore Quarry, a large dinosaur bonebed in the Morrison Formation, western Colorado- Implications for Upper Jurassic dinosaur preservation modes GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 5 2018 © 2018 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. PALEONTOLOGY, TAPHONOMY, AND SEDIMENTOLOGY OF THE MYGATT-MOORE QUARRY, A LARGE DINOSAUR BONEBED IN THE MORRISON FORMATION, WESTERN COLORADO—IMPLICATIONS FOR UPPER JURASSIC DINOSAUR PRESERVATION MODES John R. Foster, ReBecca K. Hunt-Foster, Mark A. Gorman II, Kelli C. Trujillo, Celina A. Suarez, Julia B. McHugh, Joseph E. Peterson, Jonathan P. Warnock, and Heidi E. Schoenstein Theme Issue An Ecosystem We Thought We Knew— The Emerging Complexities of the Morrison Formation SOCIETY OF VERTEBRATE PALEONTOLOGY Annual Meeting, October 26 – 29, 2016 Grand America Hotel Salt Lake City, Utah, USA GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Thin section micrograph of nearly pristinely preserved osteons from a “spongy” and almost rounded dinosaur bone fragment excavated from the Morrison Formation in the Mygatt-Moore Quarry. Micrograph image by Jane Baer (University of Utah) and John Foster (Museum of Moab). i 2018 President Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 2018 President-Elect Peter Nielsen peternielsen@utah.gov 801.537.3359 2018 Program Chair Emily McDermott ekeller@utah.gov 801.537.3389 2018 Treasurer Zach Anderson zanderson@utah.gov 801.538.4779 2018 Secretary Christopher Kravits ckravitsgeo@gmail.com 2018 Past President Bill Loughlin bill@loughlinwater.com 435.649.4005 UGA Board UGA Committees Education/Scholarship Loren Morton lmorton@utah.gov 801.536.4262 Environmental Affairs Craig Eaton eaton@ihi-env.com 801.633.9396 Geologic Road Sign Terry Massoth twmassoth@hotmail.com 801.541.6258 Historian Paul Anderson paul@pbageo.com 801.364.6613 Membership Rick Ford rford@weber.edu 801.626.6942 Public Education Paul Jewell pwjewell@mines.utah.edu 801.581.6636 Matt Affolter gfl247@yahoo.com Publications Roger Bon rogerbon@xmission.com 801.942.0533 Publicity Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Social/Recreation Roger Bon rogerbon@xmission.com 801.942.0533 AAPG House of Delegates 2017–2020 Term Tom Chidsey tomchidsey@utah.gov 801.537.3364 State Mapping Advisory Committe UGA Representative Jason Blake blake-j@comcast.net 435.658.3423 UGA Newsletter Newsletter Editor Bob Biek bobbiek@utah.gov 801.537.3356 UGA Website www.utahgeology.org Webmasters Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Lance Weaver lanceweaver@utah.gov 801.403.1636 Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would like to volunteer. Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. Affiliated with the American Association of Petroleum Geologists. Volume 5 2018 This is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. Earthquake Safety Committe Chair Grant Willis gwillis@utah.gov 801.537.3355 Douglas A. Sprinkel Utah Geological Survey 801.391.1977 GIW@utahgeology.org Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Society of Vertebrate Paleontology Editors Kelli C. Trujillo — University of Wyoming John Foster — Museum of Moab Cary Woodruff — University of Toronto Octavio Mateus — Universidade Nova de Lisboa Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 5 2018 23 ABSTRACT The Mygatt-Moore Quarry is a deposit of several thousand dinosaur bones in the Brushy Basin Mem- ber of the Morrison Formation in western Colorado. The site has been worked for more than 30 years and nearly 2400 mapped specimens have been collected. This study gathered data about the quarry from many sources to investigate the origin of the deposit. The Mygatt-Moore Quarry appears to be an attritional deposit of a relatively restricted diversity of dinosaurs, with few other non-dinosaurian taxa, that accu- mulated in a vernal pool deposit in an overbank setting. Bone modification was mostly by corrosion and breakage by trampling; scavenging was abundant. The paleofauna is dominated by Allosaurus and Apato- saurus (MNI and NIS), with the polacanthid ankylosaur Mymoorapelta less common. The matrix of the main quarry layer includes abundant carbonized fragments of plant material, and the mud during the time of deposition may have been often at least damp and occasionally acidic and dysoxic. The Cleveland-Lloyd Dinosaur Quarry is a close correlate of the Mygatt-Moore Quarry in terms of lithology and taphonomy, but demonstrates significant differences upon close inspection of matrix details and bone modification. Large quarries of fine-grained facies in the Morrison Formation possess a very different preservation mode as well as different taxon and relative abundance profiles from those in coarser sediments, which suggests that more may be learned in the future from taphofacies study of large quarries in mudstone beds. Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado—Implications for Upper Jurassic Dinosaur Preservation Modes John R. Foster1, ReBecca K. Hunt-Foster2, Mark A. Gorman II3, Kelli C. Trujillo4, Celina A. Suarez5, Julia B. McHugh6, Joseph E. Peterson7, Jonathan P. Warnock8, and Heidi E. Schoenstein9 1Museum of Moab, 118 East Center St., Moab, UT 84532; director@moabmuseum.org 2Bureau of Land Management, Canyon Country District, 82 East Dogwood, Moab, UT 84532; rhuntfoster@blm.gov 3Denver Museum of Nature and Science, 2001 Colorado Blvd., Denver, CO 80205; mark.gormanii@gmail.com 4Laramie County Community College, Laramie, WY 82070; kellitrujillo@icloud.com 5University of Arkansas, Department of Geosciences, 226 Gearhart Hall, Fayetteville, AR 72701; casuarez@uark.edu 6Museums of Western Colorado, 550 Jurassic Ct., Fruita, CO 81521; jmchugh@westcomuseum.org 7Department of Geology, University of Wisconsin-Oshkosh, Oshkosh, WI 54901; petersoj@uwosh.edu 8Department of Geoscience, Indiana University of Pennsylvania, Indiana, PA 15705; jwarnock@iup.edu 9Grand Junction Geological Society, P.O. Box 4045, Grand Junction, CO 81502; hsthurmon@gmail.com Citation for this article. Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E., 2018, Paleontology, taphonomy, and sedimentology of the Mygatt-Moore Quarry, a large dinosaur bonebed in the Morrison Formation, western Colorado—im- plications for Upper Jurassic dinosaur preservation modes: Geology of the Intermountain West, v. 5, p. 23–93. © 2018 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. 24 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 INTRODUCTION The Morrison Formation is an Upper Jurassic allu- vial deposit that originally covered at least ~1,000,000 km² of what is now the western United States (Dodson and others, 1980; Turner and Peterson, 2004). The for- mation is well known for having produced some of the largest and most iconic dinosaurs from the Late Jurassic, and it has yielded one of the most diverse and abundant Mesozoic paleofaunas from anywhere in the world. Al- though forms such as Brachiosaurus, Stegosaurus, and Allosaurus are recognized even by non-paleontologists worldwide, the Morrison Formation’s real significance lies in its preserved biota, including more than 90 verte- brate forms (more than 60 of which are not dinosaurs), plus plants, bivalves, gastropods, crustaceans, and oth- ers (Chure and others, 2006). Dinosaur quarries in the Morrison Formation number in the hundreds and include those producing a range from a few bones of one individual up to more than 10,000 bones of multiple individuals and species (Dodson and others, 1980; Foster, 2003). Most quarries are dominated by dinosaurian remains; rare others pro- duce almost exclusively non-dinosaur material. The di- versity of preservational modes is great, particularly be- tween those preserving mostly dinosaurs versus mostly non-dinosaurians, but several taphofacies are consis- tently encountered in widely scattered areas and levels. Among sites producing primarily dinosaurs, a number are considered “large,” defined here as having produced multiple individuals and 500 or more mappable bones (i.e., excluding fragments unmappable at approximately 10 cm = 1 m). Among these are sites such as the Carnegie Quarry at Dinosaur National Monument, Howe Quar- ry, Dry Mesa Quarry, and Cleveland-Lloyd Dinosaur Quarry (Curtice and Wilhite, 1996; Miller and others, 1996; Carpenter, 2013). The greatest number of bones, individuals, and species among these sites appears to be preserved in relatively coarse sand deposits of channel sandstones. These localities also preserve relatively even numbers of each species, often reflective of the relative abundances of the species in the formation as a whole. In contrast, “large” dinosaur quarries (as defined above) preserved in drab mudstone may produce very large numbers of bones, but generally contain fewer species than channel sandstone sites and are often dominated by one or a couple of species (e.g., Cleveland-Lloyd Di- nosaur Quarry, Gates, 2005 and Peterson and others, 2017; Howe Quarry, Bird, 1985; Mygatt-Moore Quarry, Foster and others, 2016). Among the latter mudstone sites, with relatively low diversity and a predominance of just a few species, is the Mygatt-Moore Quarry in western Colorado. This site is unusual among Morrison localities in preserving a moderate diversity of plant macrofossils and palynomorphs, abundant dinosaur bones but of relatively low taxonomic diversity, and rare microvertebrate fossils, all in the same productive layer. This study examines the general paleontology, ver- tebrate taphonomy, and sedimentology of the Mygatt- Moore Quarry as a large dinosaur bonebed and their implications for the origin of the deposit. LOCALITY AND PREVIOUS WORK The Mygatt-Moore Quarry was discovered on March 14, 1981, by J.D. and Vanetta Moore and Pete and Marilyn Mygatt (Armstrong and Perry, 1985; Arm- strong and others, 1987; Mygatt, 1991; P. Mygatt, verbal communication, 2001). It is approximately 2.5 km from the Utah-Colorado state line in far western Mesa Coun- ty, Colorado (figure 1), about 27 km west of Fruita. The quarry (figure 2A) is in the northwestern part of Rab- bit Valley, a drainage that runs south to the Colorado River. The upper rim of the valley is composed of the Cretaceous Cedar Mountain and Naturita (formerly Dakota) Formations (figure 2B), and the floor of most of the northern part of the valley is Salt Wash Member of the Morrison Formation. Most of the hills of the val- ley all around are composed of partially grass-covered slopes of the Brushy Basin Member of the Morrison. The quarry has been worked every summer since 1985 and has produced more than 2300 mapped bones. The quarry has developed into a significant excavation over the years (figures 2C to 2F) but has shown no signs of playing out. In fact, results presented here suggest the bone deposit is much larger than previously suspected. Compared to its fossil diversity, abundance, and the number of years it has been worked, relatively little has been published about the Mygatt-Moore Quarry. Plant 25 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 macrofossils from the Mygatt-Moore Quarry have been studied by Tidwell and others (1998) and the fossil spores and pollen by Hotton and Baghai-Riding (2010). King and Foster (2006) reported on the few non-dino- saurian vertebrates found in the quarry, whereas the an- kylosaurian dinosaur Mymoorapelta—the first Jurassic ankylosaur in North America—was described by Kirk- land and Carpenter (1994) and Kirkland and others (1998, 2010). Surprisingly, these latter two are the only papers that described any element of the dinosaur fauna exclusively. Chin and Kirkland (1998) reported possible herbivorous dinosaur coprolites from the quarry, and Bray and Hirsch (1998) described rare eggshell mate- rial from the site. The occurrence of dinosaur skin at the site, as carbonization and impressions, was reported by Foster and Hunt-Foster (2011). Only two abstracts have been produced dealing with overall site taphono- my (Kirkland and Armstrong, 1992; Foster and others, 2007), and general site characteristics have previously been covered in papers only preliminarily (Kirkland and others, 2005; Foster and others, 2016). GEOLOGIC SETTING Rabbit Valley (and the Mygatt-Moore Quarry) is on the northwestern edge of the Uncompahgre Plateau uplift and northwest of the local uplift of Ruby Canyon along the Colorado River; on its western edge the Un- compahgre borders the laccolithic uplift of the La Sal Mountains, which are to the southwest of Rabbit Valley. This region is on the northeastern edge of the Colorado Plateau. In Rabbit Valley, the Salt Wash Member of the Morrison Formation is about 96 m thick (Armstrong and McReynolds, 1987), and the Brushy Basin Member is about 100 to 140 m thick. The Morrison Formation in the area is underlain by what is called either the Up- per Jurassic Summerville Formation (Carter and Gual- tieri, 1965; Lohman, 1965; Lucas and others, 2006) or the Wanakah Formation (Scott and others, 2001) and the Entrada Sandstone (both Middle Jurassic), and is overlain by the Lower Cretaceous Cedar Mountain Formation and then the Naturita Formation (Stokes, 1952; Young, 1960; Kirkland and others, 1997; Currie and others, 2008; Sprinkel and others, 2012; Carpen- ter, 2014). The Cedar Mountain Formation is equiva- lent to the Burro Canyon Formation of the south and east side of the Colorado River in Colorado and Utah (Young, 1960); the formation is traditionally considered Cedar Mountain in Rabbit Valley only because of the valley’s position north and west of the Colorado River. The Mygatt-Moore Quarry is in the upper part of the Grand Junction Aspen Denver Colorado Springs Interstate 70 In te rs ta te 2 5 Mygatt- Moore Quarry COLORADO 100 km N Front Range San Juan Mountains Elk Range Sangre de Cristo Range Saw atch Range M osquito Range G ore Range Park Range Figure 1. Location of the Mygatt-Moore Quarry in western Colorado (red star). Major outcrops in Colorado of the Morrison Formation in blue-green. 26 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 2. Development and stratigraphy of the Mygatt-Moore Quarry in western Colorado. (A) View of the quarry from the southeast in the 1990s. (B) Stratigraphy of the uppermost Jurassic-Lower Cretaceous contact above and west of the Quarry. Jmbb = upper Brushy Basin Member of the Morrison Formation; Kcm = Cretaceous Cedar Mountain Formation; Kn = Cretaceous Naturita (formerly Dakota) Formation. (C) View of the Mygatt-Moore Quarry in September 1987, look- ing northeast (photo courtesy of Kay Fredette, Museums of Western Colorado). (D) Same view as C in July 2011 showing subsequent development of the quarry. (E) View of the quarry in September 1987, looking east-southeast (photo courtesy of Kay Fredette, Museums of Western Colorado). (F) Same view as E in July 2011. (G) Panorama of the Mygatt-Moore Quarry, looking southeast, in July 2011. 27 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 lower half of the Brushy Basin Member of the Morri- son Formation (Turner and Peterson, 1999, also had the quarry at this approximate level; although Armstrong and McReynolds, 1987, had the quarry low in the upper half). The Morrison Formation is Late Jurassic in age, ranging from the Kimmeridgian (or possibly the latest Oxfordian) up into the early Tithonian (Turner and Pe- terson, 2004; Trujillo and Kowallis, 2015). At the time of quarry deposition, the Morrison Basin consisted of an alluvial plain with rivers flowing generally east out of the western mountains and across the floodplain (Turn- er and Peterson, 2004). Wetlands and lakes existed to the east of what is now western Colorado, and immedi- ately to the south of the Mygatt-Moore area was “Lake T’oo’dichi’” (Turner and Fishman, 1991), a large, alkaline wetland/lacustrine complex in a hydrologically closed basin that had high evaporation rates and groundwater flow blocked to the east (Turner and Peterson, 2004). Other authors have interpreted the evidence differently and see no indication of such a complex (Anderson and Lucas, 1997; Galli, 2014). METHODS The Mygatt-Moore Quarry has been excavated for approximately three to four months per year since around 1990. Before that time, most seasons were ap- proximately two weeks. Mapping of the bones in the quarry has always been from a main datum established near the original point of discovery, consisting of a survey marker-type cap, and a second marker at 40 m due north; these two markers establish the north-south coordinate axis. The grid is measured along meters ap- proximately east-west and north-south of the datum. (It was recognized several years ago that the grid system used year to year is aligned on a bearing of approximate- ly 350o and not true north, though the northern survey marker up slope from the quarry is correctly aligned.) Most bones have been found in an arc approximately southwest to northeast of the main quarry datum; the main quarry layer (may also be referred to as quarry layer) has already been eroded away to the south and east, and it is under the hillside to the north and west. Each bone removed was also assessed for stratigraphic level within the quarry. The main bone layer is approx- imately 1 m thick in most areas; this interval is divid- ed into lower, middle, and upper thirds (each approx- imately 33 cm thick), and as each bone was collected its stratigraphic interval was noted. In areas where the full bone layer is somewhat thinner, each stratigraph- ic level is correspondingly thinner so that the full layer was still divided visually into thirds. This system was developed a number of years ago after abandonment of a measurement of a Z-coordinate in the mapping. The Z-coordinate had been mapped off the top of the so-called “pebble layer” (see below). It was eventually discovered, however, that the “pebble layer” was in- durated (and thus usable) in some areas of the quarry but not others, that where it was present its upper (and lower) boundaries undulated laterally, and that it is al- most non-existent in other parts of the quarry. Thus, the simple three-part lower, middle, and upper bone layer subdivision was adopted. Stratigraphic sections were measured in ten loca- tions along approximately 1.62 km of strike (south- west-northeast) on the north end of Rabbit Valley. West to east, these are (1) Far West Nose, (2) West Nose, (3) Quarry, (4) Cam Stop, (5) Caudal Draw, (6) Chan- nel Lake Traverse 1, (7) Channel Lake Traverse 2, (8) Gravel Hill, (9) East Trail Through Time, and (10) Far East Ridge (figures 3 and 4). Two sections (Quarry and Far East Ridge; figures 3 and 4) covered the complete Brushy Basin Member from the top of the Salt Wash Member of the Morrison Formation up through the Ce- dar Mountain Formation and into the Naturita Forma- tion. The other stratigraphic sections were connected to at least one laterally adjacent section by traceable beds (generally the “fish layer” or various sandstone beds), so that all ten sections could be correlated. Sections were measured with a Jacob staff and Brunton compass set to the local dip of 5° northwest. Five trenches were excavated with a backhoe (fig- ure 5). Three sites were within the quarry excavation and two sites were excavated adjacent to the quarry on either side. These sections were also measured with a Jacob staff. Two drill holes—MM-1, which was about 87 m to the northwest of the quarry, and MM-2, which was about 95 m to the southwest—were bored with a 28 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 track-mounted rig (figure 5). The 2.5-inch-diameter core was logged at the surface. Only 3 to 5 m of core be- low the “fish layer” (see below) was collected and is pre- served in the Museums of Western Colorado paleontol- ogy collections. Positions of the trenches and drill holes were recorded by marking with 40 cm diameter circular orange markers and digitally photographing the whole area with a fixed-wing drone aircraft; the camera had a resolution of approximately 10 cm. Five hand samples were collected from standard lay- ers in each trench; the “fish layer” was sampled from all five. Two samples from the main quarry layer were col- lected from each trench; other samples varied between trenches. Thin sections were made from these samples. Lithologic descriptions of the trenches were recorded in the field and from the hand samples and thin sec- tions. Thin sections were studied under a petrographic microscope and digitally photographed. In addition to the trench hand samples, numerous calcareous nodules were collected from the base of the quarry, and the five of these were thin sectioned (PB-1 through PB-5). Four bones were also thin sectioned to compare their micro- structures; two rounded, pebble-sized bone fragments (with only trabecular bone left) and two fragments con- taining cortical bone. All thin sections are preserved at the Museums of Western Colorado. In addition to the thin sectioning of the calcium car- bonate nodules, two of them plus three other represen- tatives of these nodules (PB-1, PB-2, and PB-7 through PB-9) were analyzed for rare earth element ratios at the Department of Geosciences, Boise State University. Samples were crushed in a mortar and pestle to a fine powder and 125 mg of that powder was added into Tef- lon microwave digestion vessels. The samples were then soaked in 30% trace grade H2O2 for an hour to extract any trace elements from any remaining organic mat- ter. After an hour, a 4.5:1.5:5.5 ml solution of concen- trated trace grade HNO3:HCl:DDI H2O was added to the microwave digestion vessels. Samples were digest- ed on a MARS microwave digester using the pre-pro- grammed 3052XP-15min Xpress method that uses a 15 minute ramp-up time, a 15 minute hold at 190ºC, and Figure 3. Map showing locations and names of the stratigraphic sections measured for this study near the Mygatt-Moore Quarry in Rabbit Valley. Quarry labeled and marked by arrow. Base from Google Earth. 29 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 ST RA TI G RA PH IC S EC TI O NS IN R AB BI T VA LL EY , M ES A CO UN TY , C O LO RA D O Fa r W es i N os e W es t N os e Q ua rr y Se ot lo n w ith p os iti on s of ve rt eb ra te fo ss il oc cu rr en ce s gr H n d ay si on e 1C... - -- � red d ay st on e ....._ ....._ w ith p a! eo sl:l l a t t ap ......_ -.. -. ...._ � C lay sto ne � S ilt sto ne � S an ds to ne � L im es to ne I- go ld , re d, an d ta n sa nd ,t on e 80 m M yg at t- M oo re - Q ua rr y: 60 m A J/o sa ur us Ap ar os au ru s M ym oo ra pe /ta D ip lo do t:::U S Ca m ar as au ru s Ce ra to sa ur us 40 m O th ni t!l os au ru s 20 m Om - - - - Hy �o ti, es ize d co rre lat io n or � in ch ou t - - - Tr ac ea bl e be d co rre lat i0 n - - - Fe r 0 ou nd ar y ,... 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S tr at ig ra ph ic se ct io ns m ea su re d fr om th e to p of th e Sa lt W as h M em be r o f t he M or ris on F or m at io n (o r w ith in B ru sh y Ba sin M em be r) up to th e ba se o f t he N at ur ita F or m at io n in R ab bi t V al le y ne ar th e M yg at t- M oo re Q ua rr y, w es te rn C ol or ad o. S ec tio ns a nc ho re d on th e M or - ris on -C ed ar M ou nt ai n co nt ac t. Sa nd st on es a nd o th er m ar ke r b ed s t en ta tiv el y co rr el at ed . D ak ot a = N at ur ita F or m at io n. Q ua rr y an d fiv e ot he r fo ss il oc cu rr en ce le ve ls la be le d an d m ar ke d w ith a rr ow s. 30 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 a 15 minute cool-down. Samples were then evaporat- ed on a hot plate at 90ºC and re-hydrated to 2% DDI HNO3, filtered, and analyzed using solution inductively coupled plasma mass spectrometer (ICP-MS). Samples were analyzed on a Thermo X Series 2 quadrupole ICP- MS for 43Ca, 44Ca, 66Zn, 86Sr, 88Sr, 89Y, 135Ba, 137Ba, all rare earth elements (REE), 208Pb, 232Th, and 238U. Analysis was monitored via an internal 100 ppb (part per billion) Re standard. All REE patterns were plotted relative the NASC (Gromet and others, 1984). Matrix samples for X-ray flourescence (XRF) anal- ysis were collected from within the quarry’s main bone layer, from the same stratigraphic level (about 2 to 3 m below the “fish layer”) but outside the quarry about 250 m to the east (Caudal Draw), and a bone sample from within the quarry. XRF of sediment and bone samples carried out at Indiana University of Pennsylvania using an Innov X Delta Professional handheld XRF analyzer in soil mode. Concentrations of selected elements are given in ppm (parts per million). Sediment samples were ground prior to analysis in a glass mortar and pes- tle. Bone samples were cleaned of matrix with de-ion- ized water, but analyzed whole. The U-Pb (zircon) radiometric age for the Mygatt- Moore Quarry was obtained from an ash-fall bed iso- lated from the quarry mudstone of the main bone layer, collected from the northern part of the quarry (quarry map coordinates 1021N, 998E) on June 6, 2011. The sample was analyzed by the University of Wyoming Geochronology Lab and reported by Trujillo and oth- ers (2014). A variety of zircon morphologies were sep- arated from the mudstone sample; of more than 125 zircons, at least 30 demonstrated characteristics (elon- gate tips, longitudinal bubble trails, transverse chan- nels) consistent with ash grains. Selected zircons were annealed at 850°C for 48 hours and then dissolved in two steps, modified from the chemical abrasion method of Mattinson (2005). The first step used concentrated HF and HNO3 for 12 hours at 180°C. This removed the most metamict domains and surficial Pb. After rins- ing and discarding the leachate, individual grains were completely dissolved in HF and HNO3 at 240°C for 30 hours; the solutions were converted to chlorides and evaporated with 0.05N H3PO4 in preparation for ther- mal ionization mass spectrometry. Nearly every piece of matrix from the main bone layer at the Mygatt-Moore Quarry contains carbon- ized plant fragments. The only plant specimens saved are those that are complete enough to be identified to some taxonomic level. Some but not all of the wood was collected as well. Plants were identified based on com- parisons with publications such as Miller (1987), Tid- well (1990), Tidwell and Medlyn (1992), Ash (1994), Ash and Tidwell (1998) and Tidwell and others (1998, 2006), as well as by consultation with some paleobota- nists. Mollusks were identified using Yen and Reeside (1952) and Evanoff and others (1998). Identifications of fossil vertebrates were compared to museum collections around the country as well as many references including, but not limited to, the fol- lowing: Hatcher (1901), Osborn and Mook (1921), Gilmore (1925, 1936), Madsen (1976), Galton (1980, 1981, 1983, 2007), McIntosh (1990, 2005), Carpenter and McIntosh (1994), Kirkland and Carpenter (1994), Kirkland (1998), Kirkland and others (1998), Madsen Figure 5. Close-up image of the Mygatt-Moore Quarry area showing locations of Trenches 1–5 (bars) and drill hole MM-1 and drill hole MM-2 (circles). Base is from Google Earth. 31 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 and Welles (2000), Upchurch and others (2004), and Tschopp and others (2015). Taphonomic scoring for bones is based on (1) the 0 to 3 scales of Fiorillo (1988) for abrasion, (2) a 0 (none) to 3 (extensive) scale for corrosion (e.g., Eberth and others, 2007), (3) Behrensmeyer’s (1978) 0 to 3 scale for weathering, and (4) a 0 to 3 scale for breakage (0 = none, 3 = broken on most sides; Ryan and others, 2001). Abra- sion was scored based on rounding of otherwise sharp edges, as well as on any surficial scratching. Corrosion relates to chemical or biochemical erosion of bone sur- faces and was scored by the rotting of bone ends or edg- es that was not clearly due to abrasion. That most end and edge loss of bone surface in Mygatt-Moore material is due to corrosion rather than abrasion is evidenced by the occurrence of outer bone-surface spalling, with sediment preserved between the flaking outer sur- face and the spongy inner surface that is then so often found (the spalling outer surface would be long eroded away had the material been abraded). Weathering re- lates to degradation and cracking of bone surfaces due to surface exposure to the elements. Breakage involves pre-burial breaks in the bone that, in most cases, frag- ment the element. Breakage and weathering are fairly straight forward to assess, but abrasion and corrosion can be difficult to distinguish, as in either case the result is rounding of edges. These taphonomic data were col- lected on a randomly selected subsample of the Mygatt- Moore Quarry collections, representing approximately 23.5% of the censused collection. The randomly select- ed subsample of the Cleveland-Lloyd Dinosaur Quarry collection at the Natural History Museum of Utah con- sisted of about 289 bones including those excavated by Gates (2005). Number of identified elements is based on a cen- sus of the collections of the Museums of Western Col- orado, consisting of material prepared through 2013. Minimum number of individuals (MNI) is based on the census data sorted for element but is not based on any single element. In the case of Apatosaurus, for example, the MNI count is based on the presence of a juvenile, a sub-adult and, based on femora, at least two adults. In the case of Allosaurus, on the other hand, the num- ber of individuals is based on femora and metatarsals of adults, plus one juvenile represented by dentaries. A significant percentage of the bone material in the Mygatt-Moore Quarry is fragmented and mostly un- identifiable to element or taxon. Therefore, most bones used for MNI calculations were taken from a relative- ly small overall sample of more complete elements. A true calculation of the number of identified specimens to minimum number of skeletal elements (ratio NI- SP:MNE, e.g., Lyman, 1994) would be difficult, and, if knowable, the ratio would likely be rather high (al- though we did not attempt calculating either for this study). Shed teeth of theropods were censused and mea- sured for maximum basal crown length with a digital caliper. Mean and standard deviation were calculated in a spreadsheet. Allosaurus and Ceratosaurus teeth were distinguished by the more laterally compressed maxil- lary and dentary teeth, and smaller serrations, in Cera- tosaurus, plus the lingual ridges of anterior dentary and premaxillary teeth in Ceratosaurus, as demonstrated by Madsen (1976), Madsen and Welles (2000), and Bak- ker and Bir (2004). Two rose diagrams for orientations of bones were constructed, one bidirectional for bones with no clearly more massive end, and a unidirectional rose diagram for those with a clearly more massive end. Bidirectional bones were measured toward the orien- tation between 0° and 180° and made bidirectional in representation in the computer program. Unidirection- al bones were measured toward the less massive end. The rose diagrams were constructed using 30° intervals in the program Rose.Net (https://en.freedownloadman- ager.org/Windows-PC/Rose-Net-FREE.html). Com- pass readings for the rose diagram data were taken off the quarry map. Voorhies group analysis was based on the group I– III bone categories in Carpenter (2013, table 6), and the work of Frison and Todd (1986) on transportability of elephant bones. It is recognized that the original sheep material studied (Voorhies, 1969) may not be fully com- parable to bones of large dinosaurs, as noted by Gates (2005) and Carpenter (2013); however, the analysis is carried out here to facilitate comparisons with other similar mudstone sites, such as the Cleveland-Lloyd Di- nosaur Quarry, for which such analyses have also been 32 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 conducted previously. There is currently no other sys- tem available, based on flume or river studies of more appropriate (large) analogs, with which to conduct these analyses. A randomly selected, previously unexcavated single grid square (1010 to 1011N, 1009 to 1010E) was select- ed to be excavated and every bone, regardless of size or preservation, was collected. This was done as a possible representation of what an unbiased square meter of the main quarry layer was like, because in the past, excava- tion of the quarry had been biased towards larger and/ or scientifically significant specimens. The test square meter was excavated in the 2013 season and each bone, tooth, and bone fragment was collected and mapped on X, Y, and Z coordinates. The Z coordinate was taken off a datum at the top of the bone layer (below the soil layer). The bones were measured for maximum length, width, and height and identified if possible. Volume was calculated for each bone incorporating the formula for the volume of an ellipsoid cylinder, using the three di- mensions measured. These volumes (as a rough approx- imation of overall size) were then graphed in a histo- gram and the mean volume calculated in a spreadsheet. Articulation ratios for Mygatt-Moore, and other quarries used for comparison, were calculated by di- viding the number of bones in articulation with at least one other bone by the total number of bones mapped from the site. These ratios were calculated by counting the bones on available quarry maps (published and un- published). These ratios were calculated for the Mygatt- Moore, Cleveland-Lloyd, Carnegie, Howe, Howe-Ste- phens, Poison Creek, Little Houston, and Dry Mesa Quarries. RESULTS Geology Measured Sections The top of the Salt Wash Member of the Morrison Formation in Rabbit Valley is at the top of one or sev- eral light-brown, laterally continuous channel sand- stones that can be traced across the valley. The overlying Brushy Basin Member consists of approximately 100 to 140 m of gray, maroon, and greenish-gray claystone with numerous channel sandstone and thin splay sand- stone beds and only a few thin limestone beds (figure 4). The largest laterally traceable channel sandstone beds appear to be at least 250 to 500 m wide. Most channel sandstone beds are medium- to very coarse grained, contain many tabular and trough cross-beds, and bas- al lag deposits. Mudstone beds of the Brushy Basin ap- pear to be smectitic and contain at least some silt-sized grains (Trujillo, 2006). The top of the Morrison Formation was observed in four of the measured sections. In most sections, the top is recognized as a deep maroon-red paleosol overlain by light green, sometimes silty or pebbly claystone of the Cedar Mountain Formation. Approximately 6 m up into the Cedar Mountain is a 0.5- to 2.0-m-thick nodu- lar limestone to paleosol carbonate layer similar to one often found low in the formation farther west in eastern Utah (figure 2B); this unit is consistently at this level low in the Cedar Mountain and is overlain by a thicker unit of silty light green mudstone. The top of the Cedar Mountain is formed by light green claystone or a light- brown sandstone overlain by a basal conglomerate and/ or a soft white sandstone marking the base of the Natu- rita Formation. Immediately above this white sandstone are several meters of dark gray to black carbonaceous, and in some places coaly, mudstone characteristic of the Naturita as well. Above this first dark-gray carbona- ceous mudstone is the first of two thick, yellow-brown sandstone beds that cap the ridges of Rabbit Valley to the west, north, and east. In one measured section (West Nose), the top of the Morrison Formation is at the top of a thick, pebbly and cross-bedded channel sandstone; this unit is overlain by typical light green claystone of the lower Cedar Mountain, with the caliche/nodular limestone layer approximately 5 to 6 m up section. In- terestingly, this channel sandstone pinches out to the west within about 100 m from the West Nose section, so that the Morrison-Cedar Mountain contact is a red claystone and paleosol directly below the light green claystone of the Lower Cretaceous at the next section (Far West Nose) to the west (figure 2B). The Mygatt-Moore Quarry occurs approximately 64 m above the base of the Brushy Basin Member of the 33 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Morrison Formation, in the Quarry section (figure 4) in which the member is 135 m thick. This puts the quarry near the top of the lower half of the Brushy Basin Mem- ber (47% of the way up in the section). Galli (2014) mea- sured a composite section of part of the Brushy Basin Member but had the quarry in a slightly lower position; Turner and Peterson (1999) had Mygatt-Moore Quarry at a slightly lower relative stratigraphic position on their master composite section. In both, however, the quarry was in the upper part of the lower half of the member. Other vertebrate bone sites in the northern part of Rabbit Valley were tied in to the measured sections based on correlations of the channel sandstones in which they occur and were measured and correlated with the network of sections (figure 4). The stratigraph- ically lowest of these is a thin sandstone that contains a partial skeleton of Camptosaurus (Averett locality), a partial skeleton of Allosaurus (Under the Cedar Tree site), and a goniopholidid crocodylomorph osteoderm. These three sites are on the east end of the Trail Through Time (a BLM trail in the Rabbit Valley Research Nat- ural Area that begins at the Mygatt-Moore Quarry). Next higher site in the Brushy Basin Member section is a sandstone that contains a proximal caudal series and several other elements of a diplodocid sauropod (prob- ably Diplodocus); this is also a stop on the east end of the Trail Through Time. The stratigraphically youngest site contains a partial diplodocid skull (and elements of a skeleton still in the sandstone) collected several years ago from the Far East Ridge section east of and above the Trail Through Time. Between these sites and the Mygatt-Moore Quarry there is a channel sandstone ap- proximately 340 m wide east to west. This channel is stratigraphically a few meters above the Mygatt-Moore Quarry. It contains the partial skeleton of a Camara- saurus (a stop on the Trail Through Time) and a single caudal centrum of a theropod dinosaur at the Caudal Draw section a little to the east. These latter sites are stratigraphically younger than the partial Diplodocus from the eastern Trail Through Time, but whether they are similar in age to the diplodocid skull, or are slightly older, is difficult to determine (see figure 4). The “fish layer” above the quarry is a distinct mark- er bed that can be traced more than 500 m to the east of the quarry. Tracing the relationships of beds strati- graphically immediately adjacent to this marker bed indicates that (1) the Camarasaurus channel sandstone pinches out laterally to the west of the sauropod stop along the trail, before it gets to the quarry outcrop, (2) the Camarasaurus channel sandstone scours out the “fish layer” at the Channel-Lake Traverse 2 section, (3) the Camarasaurus channel sandstone pinches out on its east end, west of the Gravel Hill section, and (4) from at least the Caudal Draw section east, the stratigraphic level of the Mygatt-Moore Quarry (about 2 m below the fish layer) no longer consists of medium light-gray clay- stone with abundant plant debris. The Caudal Draw and sections east that level are an often dark-red mudstone devoid of any fossil content (see figure 4). Detailed Quarry Sections Five trenches excavated by backhoe were dug in the quarry area; three were in the back wall of the quarry it- self, and one each was just outside the excavated quarry to the south and east (figures 5 and 6). These trenches were excavated from the “fish layer” down to below the main quarry layer. The base of the main quarry layer to the base of the “fish layer” is 2 to 3 m thick. The main quarry layer unit (defined as a nodule-lag based, medi- um light gray claystone with abundant carbonized plant debris) was approximately 70 cm (in trench 1 south of the main quarry) to 1.5 m (in trench 3 on the northwest edge of the quarry) thick. In trenches 2 and 4, also in the excavated quarry, the main quarry layer was a more typical 1 to 1.2 m thick. The main quarry layer is di- rectly underlain by a soft, light-green silty claystone that sometimes has some white mottling and is generally de- void of fossil vertebrate material, although some speci- mens have been found at the very top of this layer at its contact with the overlying bone layer. Most notable, the soft, light-green claystone layer contains no carbonized plant material at all, unlike the mudstone immediately above it. The main quarry layer is a greenish-gray (5GY 6/1) to medium light-gray (N6) claystone with silt-sized clasts of quartz and minor plagioclase feldspar (in thin section), abundant carbonized plant fragments, and common greenish-colored or white clay-ball clasts that are rounded and about 2 to 5 mm in diameter (figure 6; 34 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Fi gu re 6 . S tr at ig ra ph ic s ec tio ns o f t re nc he s 1 to 5 in a nd n ea r th e M yg at t- M oo re Q ua rr y. Ita lic iz ed n um be rs to r ig ht o f s ec tio ns in di ca te th in se ct io n nu m be rs a nd th ei r s am pl e le ve ls. 35 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 appendix A). Trenches 2 to 4 have dinosaur bone and were in areas from which dinosaur material had been collected nearby in years past. Some of the trench- es have carbonate nodules near the base of the main quarry layer (trenches 1 to 3). Above the main quarry layer is greenish-gray, light-green, and medium-green claystone with no carbonized plant fragments except in trench 2. A thin bed with minor amounts of plant debris is about 1.75 m above the base of the main quarry layer in trench 2. Other trenches were devoid of plant mate- rial above about 1 m above the base of the main quarry layer. To the east, some reddish-colored claystone was present about 1.5 m from the base of the trench 5, un- like any of the other trenches. This and the fact that the main quarry layers are thinner in trench 5 compared to the other trenches, and that the base of the main lay- er in trench 5 lacked clay balls, bones, or nodules, and contained fewer plant fragments, all suggest that trench 5 may be closer to the eastern edge of the bone deposit than the other trenches. The indurated, laminated silty claystone at the top of each trench (the “fish layer”) was consistent except in trench 4 where the layer was mot- tled and laminations were less obvious. Thin Section Micrographs Comparison of thin section micrographs from the different layers across the five trenches indicates subtle differences in the claystone content that are not obvious in hand sample (figure 7; appendix A). The soft, light- green claystone underlying the main quarry layer ap- pears to contain a higher percentage of larger silt-sized clasts than any of the other layers. The main quarry layer also contains a significant percentage of silt-sized grains but also has a high abundance of carbonized plant de- bris, clay balls (some containing silt clasts themselves), and wood fragments; these elements are missing from almost all samples from other layers (figure 7). The layer above the main quarry layer is similar in clay and silt content but is devoid of fossils and clay balls (except one sample with a single small wood fragment). The thin sections of the “fish layer” demonstrate the overall finer grained nature of the silt-sized clasts. None of the silt clasts in the “fish layer” from trenches 1 to 3 approach the size of those from underlying units. Laminations are apparent in some of the thin section, particularly trench 1. The very different non-laminated texture of the “fish layer” thin section from trench 4 is reflected in the unusual mottled appearance in outcrop. The coars- er size of the silt grains in this thin section matches that of many of the underlying layers. The “fish layer” thin section from trench 5 also has slightly coarser maxi- mum silt grain size. The unusual texture for the “fish layer” thin sections of trench 4 (and to a lesser extent, trench 5) is somewhat unexpected because this lami- nated bed is traceable in outcrop for more than a half kilometer to the east. Despite the consistent appearance of the “fish layer” for hundreds of meters in outcrop, the apparent differences in the thin sections may partly ex- plain why, to date, fish skeletons have only been found in the quarry area. In any case, the mottled, non-lami- nated appearance of the “fish layer” at the top of trench 4 reflected in the thin section micrograph is clearly dif- ferent from the other “fish layer” thin sections. Also im- portant to note from the thin sections (figure 7) is the somewhat different appearance of some of the trench 5 micrographs. This again may suggest that the eastern edge of the quarry deposit was near trench 5. Previous exploration excavations in this area turned up little in the way of bone (B. Britt, Brigham Young University, verbal communication, 2014), which is consistent with this interpretation. “Pebble Bed” Nodules A concentration of calcium carbonate nodules commonly found at the base of the main quarry layer is sometimes referred to informally as the “pebble bed.” The “pebble bed” was present in trenches 1 through 3. The nodules are typically smaller and fewer in the southwestern part of the quarry and larger and more numerous in the northern part. The “pebble bed” com- monly contains fragmented and heavily abraded di- nosaur bone. This layer is well indurated only in some places, especially under larger sauropod bones such as a pubis and scapulae. Based on anecdotal evidence from the earlier years of the excavation, this “pebble bed” was more extensively indurated toward the southern part of the current quarry area, but in the past decade or so of excavation, only isolated spots have proved to be 36 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 strongly cemented. In most areas of the northern part of the quarry, the basal layer of the main quarry deposit contains abundant large dinosaur bones and bone frag- ments in relatively soft, plant fragment-rich claystone with a few to abundant, hard calcium carbonate nodules of irregular shape that range from 1 to 10 cm in great- est dimension. Most nodules are approximately 2 to 5 cm across and are devoid of plant material. The south- western part of the quarry that was opened in 2005 and worked most years since, has no well-indurated areas, and, as mentioned above, has fewer and smaller calcium carbonate nodules; however, the southwest area has a greater number of clay balls than the northern part of the quarry. The exact source of the calcium carbonate nodules is uncertain. The calcium carbonate nodules are mostly composed of claystone with some angular to subround- ed, silty to fine-grained sand grains. In hand sample "Fish Layer" Green-gray and light green claystone Quarry Layer Light green claystone Gray claystone Trench 2 Trench 3 Trench 4 Trench 5 Trench 1 Figure 7. Thin section micrographs of samples from trenches 1–5, showing differentiation of layers and range of silt size within quarry layer. Within each trench column, thin sections are numbered in ascending order. Note plant fragments par- allel to bedding in quarry layer samples; also note organics in laminations in “fish layer” samples. Each micrograph view is approximately 2000 µm across. See appendix A for descriptions. 37 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 and when seen in cross section, some of the nodules appear to be laminated and some also have manganese dendrites growing in from their outer edges. Several of these nodules were thin sectioned and were found to contain silt- and fine-sand-sized clasts in a higher concentration than many of the layers sampled from the quarry (figure 8). The silt and sand clasts are not as large as the largest grains in the underlying light-green claystone. The thin sectioned nodules are perhaps most similar in texture to the main quarry layer level from trench 5 or trench 4 and possibly to the lowermost gray claystone in trench 1. The nodules are least similar in texture to the underlying light-green claystone found in the main quarry layer of trenches 1 through 3, and the “fish layer.” The nodules are similar to those in the mudstone layers below, above, and lateral to the quarry, but differ from the mudstone layers in the main quarry layer. This, along with the clay balls in the main quarry layer, and very rare clasts of finely bedded, fine-grained sandstones (and sometimes a basal lag of abraded bone fragments), suggest that the calcium carbonate nodules were washed into the quarry deposits from elsewhere. Taken together, the evidence suggests that the nodules were likely formed in place as incipient soil structures outside of the quarry area but were reworked and rede- posited in the Mygatt-Moore Quarry topographic low by hydraulic influence. This reworking of soil-related calcium carbonate nodules was also suggested by Kirk- land and Armstrong (1992). Total rare REE concentrations for the nodules in the “pebble bed” layer range from 74.8 to 112.5 ppm. There are two REE patterns when normalized to the North American Shale Composite (NASC) (Gromet and oth- ers, 1984). Samples PB-1, 7, and 9 exhibit a light REE pattern typical of shale, which concentrates light REE over middle and heavy REE. Samples PB-2 and 8 are middle REE depleted. This may be due to scavenging of the middle REE by Mn oxides; however, additional analysis needs to be conducted. All samples have a neg- ative Europium (Eu) anomaly typical of a volcaniclastic source rock, consistent with the volcaniclastic nature of the sediment from the Brushy Basin Member of the Morrison Formation. The preliminary REE analysis of several of the calcium carbonate nodules also indicates that the nodules may have mixed together from two sep- arate sources based on the two geochemical signatures (figure 9). More data on these REE signatures should be forthcoming as more nodules are analyzed in the future. Matrix XRF Analysis The Brushy Basin section in the Mygatt-Moore Quarry represents a mudflat deposit, which was ephem- erally wetted (Foster and Hunt-Foster, 2011). The met- al profile detected in the Mygatt-Moore Quarry matrix using XRF analysis is likely the result of apatite diagen- esis in an ephemerally subaqueous environment. Sam- ples analyzed from within Mygatt-Moore Quarry dif- fer from Morrison Formation samples collected at the same stratigraphic horizon outside the bonebed. Spe- cifically, the matrix at Mygatt-Moore Quarry contains Figure 8. Thin section micrographs of calcium carbonate nodules (“pebbles”) P-1 through P-5 from low in main bone layer, showing variable silty texture. Each micrograph view is approximately 2000 µm across. 38 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 elevated K, Ti, V, Ni, Zn, and Pb at levels at least twice those detected outside the quarry (table 1). From within this set, Ni, Zn, and Pb have been reported in obsidians, the chemical composition of which were used as proxies for volcanic ash deposited during the Jurassic (Hubert and others, 1996). Ash would have been incorporated into the sediment of Mygatt-Moore Quarry during ash- fall events, resulting in these elements being found in elevated levels in the matrix and bone. Some elements detected via XRF are enriched within Mygatt-Moore Quarry bone samples relative to Mygatt-Moore Quarry matrix. Elevated levels of metals such as As, Sr, and Pb within apatite are known from other Jurassic deposits, such as the Lower Jurassic Kayenta Formation, as well as the Upper Cretaceous Prince Creek, Hell Creek, and Dinosaur Park Formations (e.g., Goodwin and others, 2007). Di- and trivalent ions substitute for Ca2+ in apa- tite during early diagenesis within bones in contact with water (Trueman and Tuross, 2002). This is one like- ly source of the metals, which are elevated in Mygatt- Moore Quarry bone. A second possibility is organic content within the deposit. Mygatt-Moore Quarry con- tains abundant plant fragments (REF), and organic-rich soil horizons have been shown to be enriched in metals, e.g., Cu, Pb, Mn, Zn, and Hg (Martí and others, 2003). It is possible that decaying dinosaur tissue contribut- ed to the elemental metals found within Mygatt-Moore Quarry, as has been suggested at the Cleveland-Lloyd Dinosaur Quarry (Peterson and others, 2017). Howev- er, the elemental metal profiles detected here contrast with those reported from the Cleveland-Lloyd Dinosaur Quarry in regard to the relative concentrations in bone versus matrix from Cleveland-Lloyd Dinosaur Quarry (Peterson and others, 2017). In contrast, the elemental metal profiles seen here resemble those of other Juras- sic sites whose elemental metal content was determined to be diagenetic (e.g., Hubert and others, 1996; Good- win and others, 2007). Whereas abundant sulfide min- erals, such as pyrite and chalcopyrite, were detected at Cleveland-Lloyd Dinosaur Quarry, the Mygatt-Moore Quarry sediment and bone samples contain consid- erably lower levels of sulfides, implying a sometimes more oxygen-rich setting at Mygatt-Moore than Cleve- land-Lloyd. Despite evidence from carbonized skin at Mygatt-Moore Quarry, the lower levels of sulfides imply that the levels of decay at Mygatt-Moore Quarry were not high enough to utilize the available oxygen, making it unlikely that enough dinosaur tissue was decaying to provide the observed metals. Furthermore, herbivore tissues, which dominate the Mygatt-Moore Quarry assemblage (by number of identified species and min- imum number of individuals [MNI]), generally contain lower concentrations of elemental metals compared to carnivore tissues due to trophic focusing (Vijver and others, 2004). Therefore, the large amounts of dinosaur bone apatite are the most likely source of elevated el- 0.1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu PB-1 PB-2 PB-7 PB-8 PB-9N or m al iz ed N A SC Element Figure 9. Rare earth element geo- chemistry plot of five calcium car- bonate nodules (e.g., PB-1), showing two apparent signature groups and suggesting multiple sources of these “pebbles.” Log-scale Y-axis. 39 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 emental metals within the Mygatt-Moore Quarry rel- ative to Morrison Formation sediment from the same stratigraphic horizon via diagenetic processes. Drill Hole Cores The cores from the two drill holes (figures 5 and 10; appendix B) demonstrated several things. First, the “fish layer,” which is traceable in outcrop for more than 500 m to the east along the Trail Through Time and which is mostly covered beyond trench 1 to the south- west, was present in both drill holes. The drill holes are ~100 m apart indicating that the lake deposit extends a significant distance to the west in general. Second, the distinctive main quarry layer is also present in both cores (figures 10A and 10B); in MM-1 at 17.4 to 17.8 m and MM-2 at 15.7 to 16.6 m. More importantly, the core from MM-2 preserves an approximately 4-cm- long fragment of dinosaur bone from the 16.5 m level in the medium light gray claystone with abundant plant fragments (figure 10C). These results indicate that the main quarry layer is geographically much larger than previously believed, especially to the southwest. The original intent of the drilling was to bracket the western and southwestern edges of the quarry in the directions the main quarry layer goes in the subsurface, but the discovery that the quarry claystone continues at least about 87 m to the northwest (drill hole MM-1) and about 95 m to the southwest (drill hole MM-2) suggests that the main quarry layer may be up to eight times larger than the area excavated so far. The area formed by a triangle link- ing drill holes MM-1 and MM-2 to the quarry and to each other is estimated to be approximately 4130 m2. In contrast, the area of the quarry excavated so far is approximately 500 m2. Age The age of the Mygatt-Moore Quarry has been es- timated by U-Pb dating of ash-fall zircon crystals ob- tained from smectitic mudstone sampled from near Table 1. X-ray flourescence (XRF) analysis results for elements in matrix samples from the Mygatt-Moore Quarry, from the same level as the quarry but about 250 m east of the quarry at the Caudal Draw stratigraphic section, and for bone from the quarry (ppm). *From main bone layer. Bolded values highlight elevated levels compared to values outside the quarry. **From about 2 m below “fish layer." Element Quarry Matrix* (ppm) Caudal Draw Matrix** (ppm) Ratio Quarry Matrix: Caudal Draw Matrix Quarry Bone (ppm) Ratio Quarry Matrix: Quarry Bone P 0 0 - 253,739 0.00 S 9953 0 - 5112 1.95 K 17,129 7517 2.28 8636 1.98 Ca 17,128 95,736 0.18 343,740 0.05 Ti 2267 1088 2.08 2539 0.89 V 175 24 7.29 618 0.28 Cr 39 23 1.69 841 0.05 Mn 1111 1101 1.01 1204 0.92 Fe 10,354 8931 1.16 3443 3.01 Ni 45 6 7.50 167 0.27 Cu 17 12 1.42 48 0.35 Zn 107 26 4.11 86 1.24 As 58 0 - 38 1.53 Sr 150 293 0.51 4880 0.03 Mo 6 0 - 16 0.38 Pb 61 14 4.34 127 0.48 40 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 several dinosaur bones in the main quarry layer. The calculated radiometric age for the quarry was 152.18 ± 0.29 Ma (Trujillo and others, 2014; figure 11), a date that puts the quarry at the Kimmeridgian-Tithonian boundary. This date is very close to that obtained for the famous Reed’s Quarry 9 at Como Bluff, Wyoming (Trujillo and others, 2015), a site that yielded many hundreds of microvertebrates from the Morrison For- mation (Carrano and Velez-Juarbe, 2006). SYSTEMATIC PALEONTOLOGY Preserved Taxa—Main Quarry Layer The macrofossil record of plants from the Mygatt- Moore Quarry is unusual for the Morrison Formation in that it is found among abundant bones of sauropods and other dinosaurs. Unidentifiable plant fragments are abundant in every piece of matrix out of the quar- ry; in fact, as noted above, this plant-debris layer is one way to identify the quarry level even before bones have been located. Among all this preserved plant material, a small percentage is more than fragmentary and can be identified in some cases. The preserved plants from the Mygatt-Moore Quarry include at least 18 species (Tid- well and others,1998; this report), including horsetails (Equisetum figures 12B, 12E, and 12F) cycadophytes (Otozamites, Cycadolepsis, Jensensispermum ), ginkgoes (Ginkgo; figure 12D), Czekanowskiales (Czekanowskia; figures 12N and 12O), and numerous conifers, based 0.09 0.13 0.17 0.20 Pb/ U207 235 0.0228 0.0232 0.0236 0.0240 0.0244 0.0248 148 150 156 152 154 Mygatt-Moore Quarry CA-TIMS Data Mean 206/238 date: 152.18+/-0.29 95% conf. MSWD = 0.56, probability = 0.64 Figure 11. Concordia plot single grain, chemical abrasion, isotope dilution, thermal ionization mass spectrometric U-Pb analyses of zircon from the Mygatt-Moore Quarry, showing approximate age of ~152.2 Ma. Figure 10. Drill cores from the Mygatt-Moore Quarry area. (A) View of medium light gray claystone with abundant carbon- ized plant fragments, characteristics of the main quarry layer, from the 17.7 m depth of drill hole MM-1. (B) View of medium light gray mudstone with abundant carbonized plant fragments from the 16.3 m depth of drill hole MM-2. (C) Side view of core section showing dark spongy-textured dinosaur bone fragment drilled through at the 16.5 m depth of drill hole MM-2, approximately 95 m southwest of the current edge of the quarry excavation. All scales bars = 5 cm. 41 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 12. Plant fossils from the Mygatt-Moore Quarry. (A) Isoetaceae?, MWC uncataloged. (B) Equisetum sp., MWC uncat- aloged. (C) Indet. fern?, MWC uncataloged. (D) Ginkgo?, MWC uncataloged. (E) Equisetum?, MWC uncataloged. (F) Equi- setum sp., isolated tuber, MWC uncataloged. (G) Coniopteris sp., MWC 6000. (H) Indeterminate plant, MWC uncataloged. (I) Indet. conifer, MWC 2190. (J) Brachyphyllum sp., MWC field number MM1205. (K) Coniopteris sp., MWC 2155. (L) Sphe- nopteris?, MWC field number MM1308. (M) Pagiophyllum sp., MWC field number MM1212. (N–O) Part and counterpart Czekanowskia sp., MWC uncataloged. (P) Matrix block showing aligned elongate plant fragments, MWC uncataloged. (Q) Matrix block showing aligned elongate plant fragments from southwest excavation area, MWC 5683. All scale bars = 1 cm, except P which is in cm and Q which is 5 cm. 42 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 mostly on wood, cones, and short shoots (Steinerocau- lis, Behuninia, Conites, Protocupressinoxylon, Mesem- brioxylon, Xenoxylon), but some based on small branch sections (Pagiophyllum, figure 12M; Brachyphyllum, figure 12J). Macrofossil plants reported here for the first time from the quarry include the ferns Coniopteris (fig- ures 12G and 12K) and Sphenopteris? (figure 12L), plus a possible indeterminate Isoetaceae (figure 12A) and the conifer Pagiophyllum (figure 12M). These identi- fications are tentative, but the specimens are illustrat- ed here to show the specimens found since the Tidwell and others (1998) publication; only Figure 12I (indet. conifer) was illustrated previously (Tidwell and others, 1998). A palynological sample from the Mygatt-Moore Quarry, collected and studied by Hotton and Baghai-Rid- ing (2010) along with seven other samples from the Morrison Formation from New Mexico to Wyoming, is unusual in having an abundance of lycopodiophytes (relatives of modern quillworts and clubmosses), ferns, and conifers. The conifers are mostly podocarpaceans, with a number of indeterminate bisaccate forms. It is interesting that at the nearby Fruita Paleontological Area, the conifers identified by pollen are mostly Cu- pressaceae. The abundance of fossilized wood in the Mygatt-Moore Quarry (figure 13) was studied by Tid- well and others (1998) and includes podocarpaceous wood species, which parallels the abundance of podo- carpaceous pollen found in the palynological sample (Hotton and Baghai-Riding, 2010), suggesting that the wood and pollen are of local origin and that podocarpa- ceous conifers grew rather close to the quarry deposit. Several samples of abundant plant fragments in the Mygatt-Moore Quarry (figures 12P and 12Q) show a preferred alignment of long axes of elongate fragments. These have been noted particularly in the southwest part of the excavated quarry. This preferred alignment would seem to suggest orientation by gentle water ac- tion, perhaps on the edge of a pond; the plant material is small and light enough that its being aligned in such a way by unidirectional hydraulic transport seems un- likely. More likely, the water action may have been os- cillatory. Such alignment seems unlikely to result from trampling by large vertebrates, which would seem more likely to produce random orientations. Trampling un- doubtedly occurred at the site, however, and this plant fragment alignment simply indicates that a possibly hydrological influence was overprinting fragmentation Figure 13. Some of the fossilized wood from the Mygatt-Moore Quarry. (A) Smaller wood fragments, MWC uncat- aloged. (B) Larger wood piece, MWC uncataloged. Both scale bars = 10 cm. 43 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 and possible in situ churning of the plant material. Taxa marked with an asterisk (*) are those reported from the Mygatt-Moore Quarry for the first time here. Brief descriptions of each of the specimens are provid- ed using the terms and architectural characteristics of Ellis and others (2009). Specimens that were previously described in alternate papers have been appropriately referenced but not formally described here. PLANTAE Division LYCOPODIOPHYTA Class LYCOPODIOPSIDA Order ISOETALES Family ISOETACEAE INDET.?* Single, partial, lanceolate leaf lamina specimen. Greater than 4.5 cm in length, 0.5 cm in width at the base, and less than 0.2 cm approaching the apex. The base of the specimen is lobate with a wide, obtuse angle. One fragmentary plant specimen is believed to rep- resent a quillwort (figure 12A), a group also identified from the site by spores. Although rare at the site as mac- rofossils, if correctly identified, isoetaceaeans appear to be relatively abundant in the palyonologial record (Hot- ton and Baghai-Riding, 2010). Division SPHENOPHYTA Order EQUISETALES Family EQUISETACEAE EQUISETUM SP. CF. LATERALE* Complete specimens are unknown. Internodes are long and cylindrical. The stem bulges at each node by approximately a millimeter. Leaf organization is free and whorled or occurring in a sheath. Leaves overlap nodes; course parallel; apex acute and convex. Horsetails are represented by several macrofossil fragments (figures 12B, 12E, and 12F); as with most macrofossil plants at Mygatt-Moore, they are not par- ticularly complete. The less typical specimens are sim- ilar to some assigned to Equisetum from Montezuma Creek and Dinosaur National Monument and illustrat- ed by Ash and Tidwell (1998) and Foster (2007). Division PTERIDOPHYTA Order FILICALES Family DICKSONIACEAE CONIOPTERIS SP.* Complete fronds are unknown and specimens at this site are fragmentary and represented by individual pinnae. The pinna is highly dissected into pinnules and appears to be odd-pinnately compound. The pinnules occur opposite along each pinna rachilla with a decur- rent attachment and are obovate in shape. This fern is represented by four fragments (figures 12G and 12K). This is the first identification of fern macrofossils from the Mygatt-Moore Quarry, although Hotton and Baghai-Riding (2010) identified fern spores in their sample from the site. Why ferns are rare as mac- rofossils may reflect their rarity in the original flora, but this is more likely a taphonomic artifact. Order INCERTAE SEDIS SPHENOPTERIS(?) cf. METZGERIOIDES* Complete fronds are unknown. It is unknown whether the frond is imparipinnate or paripinnate. The overall appearance is feathery and delicate with a ro- bust and linear imparipinnate costa. Shape and course are elongate, feathery and highly dissected. The costa attachment is decurrent and alternate. The pinnules are asymmetrical and deeply dissected. The apex of the pinnules are rounded. The base of the pinnule is indis- tinguishable from the costa and has a decurrent attach- ment. This may be a new fern taxon for the Brushy Ba- sin Member, identified based on this small piece from the Mygatt-Moore Quarry (figure 12L) and a recently discovered, more complete specimen from southeast- ern Utah (S. Ash, University of New Mexico, verbal communication, 2017). Division CYCADOPHYTA Order CYCADALES OTOZAMITES SP. This cycad is known from a single pinna illustrated and discussed by Tidwell and others (1998). 44 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 CYCADOLEPIS(?) SP. Cycadolepis is represented by a single scale (Tidwell and others, 1998), but since then several more possible scale specimens of this cycad have been found. JENSENSISPERMUM REDMONDII This cycad was represented by a single seed in Tidwell and others (1998), and since then several more have been found at the site. Division GINKGOPHYTA Order GINKGOALES GINKGO(?) SP. A single leaf with similarities to a specimen from the Montezuma Creek locality, illustrated by Ash and Tidwell (1998), may represent Ginkgo (figure 12D). It is possible that this specimen instead represents the coni- fer Steinerocaulis radiatus (see below) or a single leaflet from the Bennettitalean, Anomozamites sp. Order CZEKANOWSKIALES CZEKANOWSKIA TURNERI(?) The wet-adapted ginkgophyte Czekanowskia is rep- resented by a single fragmentary fossil (figures 12N and 12O) from the quarry. Division CONIFEROPHYTA BRACHYPHYLLUM RECHTENII This species was identified at Mygatt-Moore by Tid- well and others (1998) based on individual leaves; the specimen illustrated here (figure 12J) is a partial branch that appears to belong to this species of conifer. UNIDENTIFIED CONIFER The counterpart of the specimen illustrated here (figure 12I) was also illustrated and discussed by Tidwell and others (1998), who also noted the broad base of the leaves. This specimen remains unidentified although it was compared to Elatides and Geinitzia by Tidwell and others (1998). BEHUNINIA PROVOENSIS This species is represented by a short-shoot from Mygatt-Moore (Tidwell and others, 1998), and is known from several other sites in Utah and Wyoming. STEINEROCAULIS RADIATUS Already described from Mygatt-Moore by Tidwell and others (1998), this conifer is represented by a num- ber of similar specimens now. CONITES SP. Two seed cones are known from the quarry, and they represent the largest known cones from the Morri- son Formation (Tidwell and others, 1998). PAGIOPHYLLUM SP.* Only fragmentary specimens are known. The rachis is very thick and robust. The pinna can be arranged op- posite to spirally around the rachis. The pinna is dia- mond shaped and comes to an acute, needle-like point at the apex. The base attachment is sessile and the pinna show a rounded attachment to the rachis. Both the base and the apex are rounded. This new conifer identification for the Mygatt- Moore Quarry (figure 12M) was previously known only from the Belt area sites in Montana (Tidwell and others, 1998) and the Temple Canyon Locality (Gorman and others, 2008). PROTOCUPRESSINOXYLON MEDLYNII MESEMBRIOXYLON CARTERII XENOXYLON MOOREI These three types of fossil wood from the Morrison Formation were described as new species by Tidwell and others (1998). All three types of wood exhibited ev- idence of wood-rotting fungi that, in modern woods, most commonly function in moderately warm and wet conditions (either humidity or soil); in some cas- es Mygatt-Moore wood exhibited evidence of collapse, which happens in some modern woods during drying of previously soaked heartwood – this is most common in modern wetland species (Tidwell and others, 1998). 45 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 METAZOA Phylum MOLLUSCA Class GASTROPODA Family VIVIDPARIDAE VIVIPARUS REESIDEI Gastropods have been found in the “fish layer” and just below it, and well above the main bone layer. How- ever, in 2013, several specimens of Viviparus reesidei were identified at the bottom of the main quarry bone layer at its contact with the underlying light green clay- stone. The sample included approximately eight inter- nal shell casts and fragments of shell casts (figure 14), and all were in nodular green claystone devoid of plant material. Phylum CHORDATA Subphylum VERTEBRATA Class AMPHIBIA Clade CAUDATA CAUDATA INDET. Amphibians are uncommon fossils in Jurassic sedi- ments and include only two groups: (1) the modern lis- samphibians (anurans, caudatans and caecilians), and (2) the larger, brachyopoid temnospondyls. Of these groups, only anurans and caudatans are found in the Morrison Formation. Prior to the 2017 season, amphib- ians were unknown from the Mygatt-Moore Quarry, but a new specimen of a caudatan has since been re- covered at the site. The fragile limb bone (MWC 8649) was found isolated, high in the bone-bearing horizon, just a few centimeters below the level where a rhyncho- cephalian was found the same season (see below). It is complete, densely mineralized (no primary bone tissue remains visible in the specimen) and is preserved as a part (figure 15A) and counterpart. There is no ossifica- tion of the epiphyses, as is typical in living caudatans, and the diaphysis is asymmetrical with a deeply concave inflection directed toward the midline of the limb. The element is 8.75 mm in length and is much longer than the 2.2 mm radius of Iridotriton from Dinosaur Nation- al Monument, which also has a straighter shaft than this specimen. MWC 8649 thus likely represents a different, potentially undescribed caudatan taxon. This element is strikingly similar to the morphology of the ulna in extant Necturus and likely represents either a forelimb (radius or ulna) or hindlimb (tibia or fibula) element in this extinct taxon. Unfortunately, as distal limb ele- ments are not diagnostic at lower taxonomic levels, it is impossible to identify the genus to which this limb belongs or to erect a new genus based solely on this el- ement. MWC 8649 is not referable to Anura, based on its gross morphology. In anurans, the fore- and hindlimb elements are much straighter and there is notable fusion between the radius and ulna, and likewise between the tibia and fibula. No fusion is evident in this specimen. Additionally, the elongate tarsals (tibiale and fibulare) also tend to be fairly straight elements, and are inconsis- tent with the morphology seen in MWC 8649. Class REPTILIA Order RHYNCHOCEPHALIA RHYNCHOCEPHALIA INDET.* In 2017, the first rhynchocephalian fossil was discov- ered at the Mygatt-Moore Quarry after more than three Figure 14. Gastropod Viviparus reesidei from the base of the Mygatt-Moore Quarry, MWC field number MM1724. Width of specimen approximately 13 mm. 46 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 15 caption on following page. 47 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 decades of excavations (figure 15B). The microfossil was found high in the bone-bearing horizon where the mudstone has a higher sand content than lower in the section. At this level, large complete vertebrate remains are not as abundant as they are lower. The element is a partial right dentary (MWC 8671) that is preserved in lingual view and contains five spade-shaped acrodont teeth, each bearing small denticles along the margins. The dentary measures 6.9 mm in length and is broken on both ends, suggesting that taphonomic factors such as trampling or predation/scavenging were at play prior to burial. Although the dentary is too fragmentary to be diagnostic at lower taxonomic levels, MWC 8671 re- sembles a juvenile Eilenodon with unworn teeth or pos- sibly could be referred to the genus Opisthias (M. Jones, University of Adelaide, written communication, 2017). REPTILIA INDET. A small reptilian vertebral centrum from the Mygatt-Moore Quarry (figured 15E to 15G) may belong to either a small ornithopod dinosaur or perhaps a go- niopholidid neosuchian crocodylomorph, the latter of which do not have procoelous vertebrae (Steel, 1973), making identification of small reptilian vertebral cen- tra from the Jurassic particularly difficult. This verte- bra also has unfused neurocentral sutures so the neural arch and spine are missing. A fragment of a very hollow limb bone (figure 15J) was found in the quarry but is too incomplete to iden- tify. Based on the degree of hollowness, however, King and others (2006) suggested that this specimen may represent a pterosaur limb element. A third, very small, reptilian is represented by a sin- gle, tiny conical tooth, approximately 2 mm tall (figure 15H). This specimen was found by screen washing the matrix in a jacket containing a juvenile sauropod femur. The tooth has slightly wrinkled enamel on the lingual surface and is slightly laterally compressed just at the tip, though it is mostly conical. The tooth is larger than those of most lizard specimens from the Morrison For- mation, and it is not of certain crocodylomorph affinity. It is generally similar to some teeth assigned to embry- onic sauropods from the Lower Cretaceous of France (Barrett and others, 2016), although even this assign- ment for the specimen would be tentative. Small reptilian elements were found in 2005 in the soft green mudstone immediately below the main bone layer in the southwestern part of the quarry. These asso- ciated elements include three vertebrae, a fragmentary limb bone, a girdle bone, and several small fragments (figures 15I and 15K to 15L). The vertebrae measure 7 to 9 mm long and 4 to 7 mm in diameter; the centra have wide neural canals and laterally widely spaced pedicals with detached neural arches and exposed sutures. The girdle bone, possibly a scapula, is 30 mm long and has a somewhat blade-like shaft and a preserved “glenoid.” The limb bone is preserved in several elements, approx- imately 42 mm long total, and appears hollow to some degree. The material suggests either a relatively large rhynchocephalian or possibly a turtle; in the latter case, the girdle bone would be an ilium, not a scapula. Clade CROCODYLIFORMES Family GONIOPHOLIDIDAE GONIOPHOLIDIDAE INDET. Goniopholidids are represented by a single vertebra at the Mygatt-Moore Quarry (MWC 1902). This verte- bra is small but is very slightly procoelous and has the ventrally projecting hypapophysis typical of neosuchian Figure 15 (figure on previous page). Fossils of amphibian(?) and small reptiles from the Mygatt-Moore Quarry. (A) Small limb element of Caudata(?) indet., possibly a fibula, MWC 8649. (B) Lingual view of partial right dentary(?) of rhyncho- cephalian, possibly Opisthias?, MWC 8671. (C) Small verte- bra of a goniopholidid neosuchian crocodyliform in anterior view, MWC 1902. (D) Same in right lateral view. (E–G) Small vertebra (possible neornithischian or goniopholidid) in dor- sal, ventral and posterior(?) views, respectively, MWC 5634. (H) Tiny reptilian tooth ~2 mm tall in mesial or distal view, MWC 5444 (image courtesy of Andy Heckert, Appalachian State University). (I) Partial limb elements of reptile?, part of MWC 5999 (associated with K and L). (J) Small hollow limb element (pterosaur?), MWC 5728. (K–L) Associated small bones of indeterminate reptile, MWC 5999. (K) Three verte- brae in ventral view. (L) Pectoral or pelvic girdle element (?). All scale bars = 1 cm, except A, B, and H which are 1 mm. 48 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 posterior cervical to anterior dorsal vertebrae (figures 15C and 15D). At least one small tooth of a goniopho- lidid crocodylomorph was found at the quarry as well, but no other such material has been found in nearly 30 years of excavations. Clade DINOSAURIA Order SAURISCHIA Suborder THEROPODA Family CERATOSAURIDAE CERATOSAURUS NASICORNIS Ceratosaurus is represented in the fauna by nine isolated shed teeth characterized by either ridged lin- gual enamel surfaces (in the case of mesial teeth of the dentary and premaxilla) or by large strongly laterally compressed, finely serrated distal teeth of the dentary and maxilla (figure 16A; Gilmore, 1920; Madsen, 1976; Madsen and Welles, 2000). The holotype of Ceratosau- rus (USNM 4735) is from the Marsh-Felch Quarry in Garden Park, near Cañon City, Colorado (Gilmore, 1920). Ceratosaurus magnicornis was named from a partial skull and skeleton from the Fruita Paleontolog- ical Area, and C. dentisulcatus was named from anoth- er partial skull and skeleton from the Cleveland-Lloyd Dinosaur Quarry (Madsen and Welles, 2000; Sanders and Smith, 2005). Rauhut (2003) suggested that the two more recently named species of Ceratosaurus from the Morrison Formation were not sufficiently differentiat- ed from the type species, and Carrano and Sampson (2008) formally synonymized C. magnicornis and C. dentisulcatus with C. nasicornis. Although this taxon is only identified from teeth out of the Mygatt-Moore Quarry they are tentatively referred to Ceratosaurus na- sicornis, because there are no differences between the Mygatt-Moore material and the other specimens. Ceratosaurus is relatively common in the area Figure 16. Theropod teeth from the Mygatt-Moore Quarry, just a few of many. (A) Anterior tooth of Ceratosaurus, showing lingual ridges. (B) Anterior tooth of Allosaurus, note lack of lingual ridges. (C) Laterally compressed and finely serrated max- illa or dentary tooth of Ceratosaurus. (D) Large but less laterally compressed and more coarsely serrated maxilla or dentary tooth of Allosaurus. All scale bars = 1 cm. 49 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 17. Skeletal material of Allosaurus from the Mygatt-Moore Quarry. (A) Left dentary of juvenile in lingual view, MWC 5440. (B) Right dentary fragment in labial view, MWC 5441, juvenile and associated with A, probably the same individual. (C) Left lacrimal in lateral view, MWC 5309. (D–E) Cervical vertebra, MWC 4010, in posterior and left lateral view. (F) Dor- sal vertebra in anterior view, MWC 2861. (G) Right scapula in lateral view. (H) Right ilium in lateral view. (I) Left femur in anterior view. (J) Metatarsals II–IV. (K) Manus claw, MWC 3730. All scale bars = 10 cm. 50 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 around Mygatt-Moore, as it has been found at three other sites nearby: (1) the Twin Juniper Quarry, just 800 m south of Mygatt-Moore in Rabbit Valley, produced a partial skeleton of an articulated Apatosaurus and a sin- gle, large, shed tooth of Ceratosaurus, and (2) two sites in the nearby Fruita Paleontological Area, just 25 km to the east. These Fruita Paleontological Area specimens include the partial skeleton at the Eriksen Ceratosaur Quarry (Madsen and Welles, 2000; Sanders and Smith, 2005) and isolated teeth found at the Tom’s Place Quar- ry (J.R. Foster, personal observation). Clade TETANURAE Family ALLOSAURIDAE ALLOSAURUS FRAGILIS Bones of Allosaurus are the most abundant verte- brate fossils at the Mygatt-Moore Quarry (Foster and others, 2007). This theropod is represented by more than 240 elements and at least 360 teeth in the quarry (figures 16B and 16D). Represented elements include skull, girdle, and limb elements, with dorsal and caudal vertebrae particularly abundant (figure 17). The materi- al is identified as Allosaurus based on the presence of (1) a single lateral pneumatic fossa on the cervical centra (versus two in Ceratosaurus), (2) absence of prespinal chonos on the dorsal vertebrae (versus presence in Cer- atosaurus), (3) tapering of neural spines of anterior- to mid-caudal vertebrae so that the distal end is narrower than the base in lateral view (“beveled cranial margin of the spine” in Holtz and others, 2004; distal end and base same width in lateral view in Ceratosaurus), (4) scap- ula with a less expanded proximal end than in Cerato- saurus, and (5) tibia with the astragalus contact ridge on the antero-distal surface that rises well up the shaft (versus very low in less derived theropods). None of the theropod material out of the Mygatt-Moore Quarry is referable to the Megalosauridae or Torvosaurus. Teeth are identified as Allosaurus based on the lack of lingual ridges on the anterior teeth and on the smaller size of the tooth crown, less lateral compression, and moder- ate-size serrations relative to Ceratosaurus and Torvo- saurus (Bakker and Bir, 2004). There are several named species of Allosaurus, and although few authors agree on which are valid (e.g., A. atrox and A. fragilis in Paul, 1988; A. fragilis and A. maximus in Smith, 1996, 1998), it is possible that only three species of allosaurids were present during Morri- son times: the stratigraphically older-ranging Allosau- rus n. sp. (Chure, 2000), the more common (and more robust) A. fragilis (Madsen,1976), and the rare and stratigraphically high Saurophaganax maximus (Chure, 1995; see Rauhut, 2003, and Holtz and others, 2004). Dalman (2014) named A. lucasi based on material from southwestern Colorado, although this taxon has not yet been addressed by others. Carpenter (2010) document- ed significant variation in the Allosaurus fragilis pop- ulation from the Cleveland-Lloyd Dinosaur Quarry, which suggests that some of the variation used to define species of Allosaurus in the Morrison Formation may be suspect. The material of the Mygatt-Moore Quarry is not large and robust enough to be S. maximus but is more robust than Allosaurus n. sp. The complete dorsal vertebrae from Mygatt-Moore lack the accessory hor- izontal processes along the bases of the neural spines seen in S. maximus (Chure, 1995). Although autapo- morphies of Allosaurus nov. sp. are missing from the Mygatt-Moore material due simply to lack of elemen- tal overlap, the overall robustness of the vertebrae and girdle elements differentiates this material from that species. Additionally, a left lacrimal of Allosaurus from the Mygatt-Moore Quarry preserves the pneumatic re- cesses at the base of the cornual process; the presence of these is one of the autapomorphies of A. fragilis (Chure, 2000). The Mygatt-Moore Quarry material is therefore referred to this species. Suborder SAUROPODA Family DIPLODOCIDAE Subfamily APATOSAURINAE Genus APATOSAURUS A large number of the sauropod elements out of the Mygatt-Moore Quarry may be referred to the diplodo- cid genus Apatosaurus, with all regions of the skeleton present and cervical and caudal vertebrae particularly abundant (figure 18). The generic identification is based on the following characters: (1) short and robust cervi- cal ribs, (2) cervical ribs set well below and lateral to the centrum, (3) anterior caudal vertebrae have “wing-like” 51 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 18. Skeletal material of the Apatosaurus from the Mygatt-Moore Quarry. (A–B) Cervical vertebra MWC 1916 in left lateral and ventral views. (C–D) Cervical vertebra MWC 3829 in left lateral and anterior views. (E–F) Cervical vertebra in left lateral and dorsal views. (G) Posterior cervical vertebra in anterior view with associated dorsal rib and chevron, as preserved in quarry. (H) Close-up of cervical rib of cervical vertebra. (I–J) Caudal vertebra in anterior and right lateral views. (K) Large left femur measuring 1876 mm overall length; robusticity index for this element puts it in a range of overlap between slender Apatosaurus and robust Diplodocus, so identification is tentative. (L) Right tibia in posterior view. All scale bars = 10 cm except for K which is 25 cm. 52 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 caudal ribs (similar to Diplodocus and Barosaurus) but generally lack well-developed lateral pneumatic fossae (pleurocoels) in the centra, (4) coracoids with roughly quadrangular lateral profile, and (5) proximal condyles of the ulna sub-equal in length (Upchurch and others, 2004). The material from the Mygatt-Moore Quarry may be referred to Apatosaurinae based on the following synapomorphies of Tschopp and others (2015; numbers in parentheses refer to their synapomorphy identifiers): (1) cervical ribs project well beneath centrum so that the diapophysis/tuberculum is approximately the same length as the posterior process, (2) no paired pneumatic fossae on the ventral surface of anterior cervical verte- brae, and (4) anterior process of posterior cervical ribs is absent or reduced to a short bump-like process. Be- cause we treat the clade Apatosaurinae of Tschopp and others (2015; Apatosaurus + Brontosaurus) as consisting of several species but all within Apatosaurus (Foster and Peterson, 2016), we identify the Mygatt-Moore material as belonging to Apatosaurus. APATOSAURUS CF. LOUISAE By Upchurch and others (2004) criteria, the Ap- atosaurus material from the Mygatt-Moore Quarry may best be referred to the species A. cf. louisae, first identified from Dinosaur National Monument (Gilm- ore, 1936). Among the recognized species of Morrison apatosaurs, A. ajax and A. parvus may be ruled out as candidate species for the Mygatt-Moore material due to the cervical vertebrae not having a width:height ra- tio of 1.5 or greater and to the scapular acromial ridge being at less than a 90-degree angle to the long axis of the scapular blade, respectively. Foster (2015), however, noted that the interspecific variation in these characters among apatosaurines was slight and that, regardless, the small sample size left the characters potentially statis- tically insignificant. The Mygatt-Moore material is in some respects similar to A. excelsus by Upchurch and others’ (2004) criteria but bears greater similarity to material of A. louisae. As in A. excelsus, the cervical ribs terminate anterior to the posterior end of the centrum; as in A. louisae, the proximal ends of the sacral ribs are on the anterior part of their respective centra, the ante- rior caudal centra are ventrally keeled (a “ventral mid- line ridge”), and the beveled glenoids of the scapulae face anteriorly, ventrally, and medially (Upchurch and others, 2004). The Mygatt-Moore material is thus ten- tatively assigned to Apatosaurus cf. louisae. Identifying the Mygatt-Moore material to species based on synapomorphies identified by Tschopp and others (2015) is more ambiguous. The quarry’s apato- saurine material includes few skull elements, but of four autapomorphies of A. ajax, three are in the skull; the fourth, “elliptical depression between the lateral spinal lamina of caudal neural spines and the postspinal lam- ina,” is true of most anterior caudal vertebrae from the site. (A possible juvenile apatosaur basioccipital-basal tubera, figures 21A and 21B, does not seem to match autapomorphy 3 for A. ajax.) However, there are 28 au- tapomorphies for A. louisae (Tschopp and others, 2015) and of these the Mygatt-Moore material is clearly lack- ing characters 11 and 12 (the rest of them are unclear in the preserved material or cannot be confirmed because the material was found disarticulated). Of seven auta- pomorphies of Brontosaurus excelsus listed by Tschopp and others (2015), most are ambiguous regarding the disarticulated Mygatt-Moore material, but character 1 does not match the type and referred material (some Mygatt-Moore cervical vertebrae do have median tu- bercles) and character 3 is highly variable within the Mygatt-Moore material. The Mygatt-Moore apatosau- rine material does not match characters 5, 8, or 9 for B. parvus (Tschopp and others, 2015; character 1 is true only of a few anterior cervical vertebrae, but not most cervicals among Mygatt-Moore material), and it matches character 5 but not character 2 for B. yahnah- pin (Tschopp and others, 2015). Based on these results, it is difficult to assign the Mygatt-Moore material with confidence to any one apatosaurine species defined in Tschopp and others (2015). Most elements out of the quarry either lack the right data for most characters or their disarticulated condition prevents certainty when characters depend on skeletal position. Most importantly, for many char- acters for which we can identify their states, the mate- rial is contradictory by matching some autapomorphic characters, but not matching others. This would not be 53 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 unexpected if there are multiple species in the sample, but we assume the Mygatt-Moore apatosaurine materi- al represents a single species because most other large dinosaur quarries in the Morrison Formation contain a single species of each genus of sauropod (Foster, 2003). We therefore are relying, until contradictory evidence is found, on the autapomorphies of Upchurch and oth- ers (2004) and identify the Mygatt-Moore apatosaurine material as A. cf. louisae. Discussion – Some of the material assigned to Ap- atosaurus consists of juvenile elements, including at least one cervical rib, a femur and a scapula (see SAU- ROPODA MISC. section for figures). Juvenile elements have been recognized from the Mygatt-Moore Quarry for some time, and juvenile material in general is not especially rare in the Morrison Formation (Foster, 2005a). A very large, complete femur from the quarry (1876 mm in length) may belong to Apatosaurus (figure 18K) and represents the second longest complete femur ever found in the formation (after the type specimen of Brachiosaurus FMNH 25107). In 2016, Museums of Western Colorado crews collected the nearly complete skull of an apatosaurine (with cervicals 1–3 nearly in articulation) from Mygatt-Moore (McHugh and others, 2017); this specimen is still being prepared. Subfamily DIPLODOCINAE DIPLODOCINAE INDET. Eight elements, all caudal vertebrae, may be referred to non-apatosaurine diplodocids based on the “wing- like” caudal ribs, deep pleurocoels, and ventral exca- vations of the anterior caudal vertebrae. The mid-cau- dal vertebrae are elongate with small pleurocoels and groove-like ventral excavations (figure 19). These cau- dals are distinct from those of Apatosaurus and are similar to Diplodocus and Barosaurus (Hatcher 1901; McIntosh, 1990; Ostrom and McIntosh, 1999). Distin- guishing caudals of Diplodocus and Barosaurus is rather difficult, as most of the differences between these two Morrison diplodocines are in the cervical vertebrae and the number of dorsals (Curtice, 1996; McIntosh, 2005). Barosaurus may have mid-caudal centra that are rela- tively shorter and less deeply excavated ventrally than in Diplodocus (McIntosh, 1990; Foster, 1996), but quanti- fying these differences is difficult. The material from the Mygatt-Moore Quarry is therefore identified only as Di- plodocinae indet. Numerous isolated teeth of diplodoc- ids are found in the quarry, and some of them may have belonged to Diplodocus or Barosaurus individuals, but Figure 19. Elements of Diplodocinae indet. from the Mygatt- Moore Quarry. (A) Right lateral view of an anterior cau- dal vertebra collected in 1989, MWC uncataloged. (B–C) Mid-caudal vertebra in lateral and ventral view, MWC 3707. All scale bars = 10 cm. 54 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 the teeth cannot be distinguished from Apatosaurus. Clade MACRONARIA Family CAMARASAURIDAE CAMARASAURUS SP. CF. LENTUS Camarasaurus sp., the most abundant dinosaur in the Morrison Formation overall (Foster, 2003), is rare at the Mygatt-Moore Quarry. It is represented by 23 bones, mostly caudal vertebrae, a braincase, two dorsal vertebrae, a scapula (distally expanded), three pelvic el- ements, several limb elements, a dentary fragment with teeth of a juvenile, and 35 shed teeth, mostly of adults, but with some juveniles represented. These elements are characteristic of Camarasaurus within the Morri- son Formation in having (1) anterior caudals with sim- ple caudal ribs, no pleurocoels, rounded unexcavated ventral outlines, and short transversely expanded dor- sal ends of the neural spines, (2) dorsal vertebrae with transversely expanded (“fan-shaped”) neural spines (figure 20), (3) particularly elongated metacarpals, (4) robust, spoon-shaped teeth, and (5) antero-posteriorly narrow, transversely elongate supratemporal fenestra, compared with diplodocids (Osborn and Mook, 1921; Gilmore, 1925; Berman and McIntosh, 1978; Madsen and others, 1995). The two dorsals do not preserve the neurocentral sutures well enough to determine if they are the high-pedicel-bearing type of Camarasaurus grandis or the low morphology of C. lentus (McIntosh, 1990), but all specimens are smaller than those assigned to C. supremus. The caudal neural spines appear to be gradually transversely expanded near the distal ends, in the manner of C. lentus and not like C. grandis or C. supremus (Ikejiri, 2005). SAUROPODA MISC. Many specimens cannot be assigned to lower tax- onomic levels but are clearly sauropod elements. Most of these belong to adult sauropods, but a number of juveniles are represented, some of them very young. Juvenile sauropod elements represented in the quarry include a diplodocid basioccipital (figures 21A to 21C), a Camarasaurus dentary fragment (figure 21D), a possi- ble Apatosaurus cervical (figure 21E), a possible Cama- rasaurus dorsal (figure 21G), and several Apatosaurus elements (figures 21H to 21J). The sample represents at least three juvenile sauropods (MNI). Order ORNITHISCHIA Clade ANKYLOSAURIA Family POLACANTHIDAE MYMOORAPELTA MAYSI The polacanthid ankylosaurian Mymoorapelta is represented by more than 160 elements from all parts of the skeleton (figure 22; Kirkland and Hunt-Foster, in preparation), making this genus the third most abun- dant vertebrate in the Mygatt-Moore Quarry. Partic- ularly abundant are lateral spines, osteoderms, dorsal and caudal vertebrae. Based on comparative size of ele- ments, there are at least two individuals present; occur- rence of some ankylosaurs osteoderms stratigraphically high within the quarry suggests that more than two in- dividuals may in fact be represented. Mymoorapelta was the first Jurassic ankylosaur identified in North Amer- ica (Kirkland and Carpenter, 1994). Polacanthid anky- Figure 20. Posterior dorsal vertebra of Camarasaurus from the Mygatt-Moore Quarry. (A) Anterior view. (B) Posterior view, MWC 2864. Scale bars = 10 cm. 55 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 21. Juvenile sauropod specimens from the Mygatt-Moore Quarry. (A–C) Basioccipital of a juvenile diplodocid in dorsal, ventral, and posterior view, MWC 7224. (D) Fragment of right dentary of sub-adult Camarasaurus with several teeth in lingual view, MWC 5009. (E–F) Two cervical vertebrae of very young indeterminate sauropods in dorsal view, MWC 1917 and MWC 3828. (G) Dorsal vertebra of indeterminate juvenile sauropod (possibly Camarasaurus) in left lateral view, MWC 3630. (H) Left cervical rib of young Apatosaurus, MWC 1925. (I) Left femur of juvenile Apatosaurus in anterior view, MWC 5439. (J) Left scapula of juvenile Apatosaurus in lateral view, MWC 1848. All scale bars = 5 cm. 56 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 22. Mymoorapelta elements from the Mygatt-Moore Quarry. (A–B) Braincase fragment in left lateral (internal) and right lateral views, MWC 5435. (C) Cervical vertebra in anterior view, MWC 6737. (D) Dorsal vertebra in right lateral view, MWC 1801. (E) Anterior caudal vertebra in anterior view, MWC 1805. (F) Two distal caudal vertebrae. (G) Ulna, MWC 5643. (H) Lateral spine, MWC 2678. (I) Osteoderm. (J) Lateral spine. All scale bars = 5 cm. 57 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 losaurs have since been found at a number of other sites in other areas of the Morrison Formation, including western Colorado, Utah, and Wyoming (e.g., Kirkland and others, 1998; Tremaine and others, 2015). Clade NEORNITHISCHIA OTHNIELOSAURUS CONSORS Othnielosaurus was found in the quarry in 2008, the first new dinosaur taxon for the quarry in 18 years at the time. It is represented by a small maxilla or dentary fragment with three teeth in matrix (figure 23; MWC 5966). The teeth are tiny (0.9–1.4 mm in mesiodistal length), diamond-shaped, and contain two to four den- ticles each, characteristic of some Othnielosaurus from other localities (Galton, 1983, 2007) and perhaps dis- tinct from other small neornithischians of the Morrison Formation such as Drinker (Bakker and others, 1990), and very distinct from Dryosaurus (Galton, 1981). The teeth also lack the cingula of Stegosaurus and ankylo- saurs. Carpenter and Galton (in press) suggest that Oth- nielosaurus is a junior synonym of Drinker nisti. DINOSAURIA? INDET. Eggshell Angustiprismatic Morphotype A 43 x 34 mm fragment of what was estimated to have been an originally 10 cm diameter egg was found in the Mygatt-Moore Quarry during the 1990s (figure 24). This calcitic eggshell is of an unnamed type that is considered a dinosauroid-prismatic basic type, and is also unique to the Mygatt-Moore Quarry within the Morrison Formation (Bray and Hirsh, 1998). Preserved Taxa—“Fish Layer” Phylum MOLLUSCA Class GASTROPODA Several indeterminate gastropods have been identi- fied in the “fish layer” beds. These are not well preserved and are difficult to identify. Phylum ARTHROPODA Clade CRUSTACEA Class BRANCHIOPODA Order DIPLOSTRACA A number of “conchostracans” have been found in the claystone just below the “fish layer” (Kirkland and Carpenter, 1994) (figure 25). All are in light gray mud- stone devoid of plant material. The term conchostracan has been abandoned after determination that the group was paraphyletic. The taxa formerly within “Conchostraca” are now placed within several fami- lies within the Diplostraca (Martin and Davis, 2001). Figure 23. Neornithischian jaw fragment with three teeth, MWC 5966, probably Othnielosaurus. Scale bar = 1 cm. Figure 24. Dinosaur egg in the Mygatt-Moore Quarry, photo taken in the field. Counterpart in upper left; egg still in situ in lower right below U.S. nickel, MWC 1900 and 1901. Photo courtesy of Jim Kirkland, Utah Geological Survey. 58 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 These bivalve crustaceans are capable of dormancy in dry habitats and are thus able to survive in ephemeral wetlands. Class MALACOSTRACA Order DECAPODA Unnamed crayfish A nearly complete crayfish fossil was found in the “fish layer” during the 1990s, and this specimen has not yet been described or named. A second fragmentary crayfish specimen was discovered at the site in 2005 (figure 26I). Phylum CHORDATA Superclass OSTEICHTHYES Clade PALAEONISCOIDEA MORROLEPIS SCHAEFFERI Several specimens of the coccolepid palaeoniscoid Morrolepis have been found since the description of the type specimen by Kirkland (1998) (figures 26B to 26D). Some of these specimens show details of caudal and dorsal fin morphology better than the type, due to bet- ter preservation. Another specimen preserves a skull, possibly of Morrolepis (figure 26G). Class ACTINOPTERYGII Subclass NEOPTERYGII Order INDET. “HULETTIA” HAWESI The type specimen of “Hulettia” hawesi is from the Fruita Paleontological Area, but the species was soon af- ter identified from the “fish layer” at the Mygatt-Moore Quarry. There are now at least five specimens known from the “fish layer,” including a scale patch, two scale patches with partial caudal fins, an anterior third of a fish with the skull, and a nearly complete fish except for the skull (figure 26A). The latter two specimens have been found since 2005. This species probably belongs in a separate genus from Hulettia, from the marine Sun- dance Formation (J. Kirkland, Utah Geological Survey, verbal communication, 2003), which is why the generic name is in quotations here. Infraclass TELEOSTEI Order LEPTOLEPIFORMES Family LEPTOLEPIDAE CF. LEPTOLEPIS A partially complete teleost fish was described by Figure 25. Diplostracan arthropods (“con- chostracans”) from the Morrison Formation, includ- ing the upper part of the Mygatt-Moore Quarry just below the “fish layer” in gray claystone matrix without plant material. (A) Lioestheria sp., MWC 1854, from the Wolny Site near Grand Junction, Colorado (Lucas and Kirkland, 1998). (B) Unidentified diplostracan from upper levels of Mygatt-Moore, MWC 1862. (C) Unidentified diplostracan from upper levels of Mygatt-Moore, MWC 1865. All scale bars = 5 mm. 59 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 26. Species from the “fish layer” above the Mygatt-Moore Quarry. (A) The possible semionotid fish “Hulettia” hawesi, MWC 5564. (B) The coccolepid fish Morrolepis schaefferi, holotype, MWC 440. (C–D) Referred specimens of Morrolepis, MWC 5306 and MWC 5305. (E) Teleost cf. Leptolepis, MWC 3722. (F) Actinopterygii indet., MWC 5941. (G) Skull of possi- ble Morrolepis, MWC 5307. (H) Undescribed crayfish. (I) Partial crayfish, MWC 5566. All scale bars = 1 cm. 60 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Kirkland (1998) out of the “fish layer” (figure 26E). This specimen is missing most of the skull and is the only confirmed teleost from the site. ACTINOPTERYGII INDET. Several other specimens are known from the “fish layer,” most consisting of partial or isolated fins, and these generally cannot be identified to a lower taxo- nomic level. One partial fish skeleton, preserved in the left lateral view in its posterior portion and in oblique right-dorsal view in its anterior portion (figure 26F), is somewhat more complete but still awaits identifica- tion. TAPHONOMY Sample Size Over 30 years of work at the Mygatt-Moore Quar- ry, several thousand bones have been collected and mapped. The quarry map contains nearly 2400 mapped elements, whereas the census of the data in the Muse- ums of Western Colorado collections consists of more than 900 elements. Relative Abundances of Fossil Vertebrates The sample of 897 identifiable bones indicates that the Mygatt-Moore Quarry is dominated by sauropod dinosaurs and the theropod Allosaurus fragilis (figure 27). By number of elements, the most abundant verte- brate taxa by far are indeterminate sauropods and Ap- atosaurus cf. louisae, followed by Allosaurus and then Mymoorapelta (figure 27A). All other taxa are rare, including the otherwise common Morrison sauropod Camarasaurus. By minimum number of individuals, Allosaurus is most abundant followed by Apatosaurus and then indeterminate sauropods (figure 27B). Allosaurus is also particularly abundant based on the tooth sample out of the quarry (figure 28). The tooth sample indicates a normal distribution of basal crown length of Allosaurus teeth and a larger sample, contrary to what was found previously with a smaller example from the quarry by Foster (2005b). Preserved Elements The total sample inventoried in the Museums of Western Colorado, including taxonomically unidentifi- able material, consisted of nearly 1900 elements (table 2). Although fragmentary bones were most abundant, 0 50 100 150 200 250 300 N = 897 N u m b er o f I d en ti fie d E le m en ts Apato sa uru s s p. Allo sa uru s f ra gilis M ym oora pelta m aysi Cam ara sa uru s s p. Dip lo docin ae in det. Cera to sa uru s n asic orn is Oth nielosa uru s c onso rs Gonio pholid id ae in det. Reptil ia in det. A Sauro poda in det. Reptil ia in det. B A 0 1 2 3 4 5 6 7 8 Adult Sub-adult Juvenile N = 24 M in im u m N u m b er o f I n d iv id u al s Apato sa uru s s p. Allo sa uru s f ra gilis M ym oora pelta m aysi Cam ara sa uru s s p. Dip lo docin ae in det. Cera to sa uru s n asic orn is Oth nielosa uru s c onso rs Gonio pholid id ae in det. Reptil ia in det. A Sauro poda in det. Reptil ia in det. B B Figure 27. Relative abundances of vertebrate taxa from the main quarry layer at the Mygatt-Moore Quarry. (A) By num- ber of identified elements. (B) By minimum number of indi- viduals and with relative age designated. 61 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 teeth, vertebrae, and ribs were well represented (figure 29). Girdle, limb, and dermal bones were about equal- ly represented, and skull elements were comparatively rare. An analysis of the preserved elements by modified Voorhies bone distribution groups (Voorhies,1969; Carpenter, 2013; see Methods) for the sauropods and Allosaurus subsamples of the inventoried collection (figure 30) indicates that the percentage representation of elements in highly transportable (Group 1) to trans- port resistant (Group 3) categories closely matches the representation of the elements in the respective (sau- ropod and Allosaurus) skeletons. This suggests that the better-preserved, identifiable material from the quarry was not likely transported into the quarry area as an as- semblage nor winnowed to its current condition with Element Number of Identified Specimens Cranial Braincases 4 Dentaries 4 Other skull elements 7 Teeth 438 Axial Cervical vertebrae 56 Cervical ribs (isolated) 5 Dorsal vertebrae 75 Dorsal ribs 86 Rib fragments 192 Sacra 6 Caudal vertebrae 190 Chevrons 54 Appendicular Scapulae 10 Coracoids 6 Humeri 10 Radii 4 Ulnae 3 Carpals 3 Metacarpals 15 Ilia 5 Pubes 7 Ischia 9 Femora 17 Tibiae 14 Fibulae 11 Astragali 4 Metatarsals 38 Phalanges 56 Unguals 22 Other Dermal spines 20 Osteoderms 70 Miscellaneous elements 25 Fragments 869 Total: 1897 bones 438 teeth Table 2. Mygatt-Moore Quarry bone sample by element. All taxa combined. 0 10 20 30 40 50 MYGATT-MOORE QUARRY THEROPOD TEETH ALLOSAURUS N = 375 Range: 3.0-26.2 St. Dev.: 3.96 Basal Crown Length (mm) N u m b er o f T ee th 5 10 15 20 25 Mean: 13.6 0 1 2 3 4 5 CERATOSAURUS N = 9 Range: 3.0-26.2 St. Dev.: 3.96 THEROPODA INDET. N = 35 Range: 4.9-22.2 St. Dev.: 5.25 Basal Crown Length (mm) N u m b er o f T ee th 5 10 15 20 25 Mean: 14.1 Mean: 21.8 A B Figure 28. Size distributions of theropod dinosaur teeth from the Mygatt-Moore Quarry, measured along the basal crown length. (A) Allosaurus. (B) Theropoda indet. plus Ceratosau- rus. 62 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 highly transportable elements washed elsewhere. Rath- er, the Voorhies element analysis results seem to indi- cate that at least the identifiable material was essentially autochthonous to the local quarry area (fragmentary and rounded; i.e., transported allochthonous material, however, is also abundant in the quarry). Age Distribution The main bone layer sample includes, by minimum number of individuals of dinosaurs and other verte- brates, four juveniles, one sub-adult, and 17 adult-sized animals (figure 31). Such a distribution with many adults, fewest number of sub-adults, and moderate representation of juveniles is suggestive of an attrition- al morality assemblage with delayed burial (Eberth and others, 2007, their figure 5.7). This argues against a non-selective catastrophic mortality for the animals preserved and indicates that the skeletal material was Teeth 18.7% Fragments 38.3% Vertebrae 13.7% Ribs and Rib frags. 12.1%Dermal Spines/ Osteoderms 3.8% Phalanges/ Unguals 3.3% Hindlimb 3.6% Pelvic 1.2% Forelimb 1.5% Scapulae/Coracoids 0.7% Chevrons 2.3% Skull Elements 0.6% TOTAL MYGATT-MOORE QUARRY SAMPLE BY ELEMENT N = 2335 Figure 29. Frequency of bones from the Mygatt-Moore Quarry sample by element. 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Group 1 Group 2 Group 3 Group 1 Group 2 Group 3 Pe rc en t o f S am p le Pe rc en t o f S am p le VOORHIES GROUPS SAUROPODS VOORHIES GROUPS ALLOSAURUS Observed N = 322 Observed N = 173 A B O b se rv ed Ex p ec te d Ex p ec te d Ex p ec te d Ex p ec te d Ex p ec te d Ex p ec te d O b se rv ed O b se rv ed O b se rv ed O b se rv ed O b se rv ed Figure 30. Voorhies group analysis for sauropod and Allosau- rus bones from the Mygatt-Moore Quarry. Group 1 is highly transportable, Group 2 less so, and Group 3 is least trans- portable. Observed is the percentage of elements within each group actually preserved in the sample; Expected is the per- centage of elements in the respective skeletons. Samples un- usually high in Group 1 elements would be expected to have been perhaps washed into the area, whereas a Group 3-dom- inated sample suggests a lag deposit in which only the most transport-resistant elements are left in a winnowed assem- blage. The fact that in both cases here, the observed distribu- tion of elements is similar to what would be expected out of a skeleton suggests the bones are essentially autochthonous and are neither significantly transported in nor winnowed. 63 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 exposed for some time before burial, which is consis- tent with other taphonomic factors noted about the site (see below). Bone Distribution The nearly 2400 mapped bones from the Mygatt- Moore Quarry show a pattern of disarticulation and only slight association in rare instances (figure 32). Of the entire mapped sample only eight bones are in artic- ulation with at least one other one (three pairs of dorsal vertebrae and one pair of cervical vertebrae, all of Apa- tosaurus; two of the three dorsal pairs appear to be from the same sub-adult individual). This ratio of articulated bones to the total number (0.00337) is lower than for any other major quarries of the Morrison Formation studied here (table 3). The almost totally disarticulated nature of the sample out of the Mygatt-Moore Quar- ry has been one of its notable features since it was first worked. From the quarry map (figure 32) it is also ap- parent that the orientation and distribution of material in the quarry is random (see below); bones are scattered in all parts of the quarry, lightly in some areas but clus- tered in some others. The main bone layer is approximately 1 m thick and the occurrences of bones within this interval are con- centrated near the basal 33 cm or so. More than half of the bones collected from the 2000–2013 seasons were found in that lower third of the deposit (figure 33). El- ements from the upper third accounted for only a little more than 15% of the sample. The vertical distribution of fossils in the quarry area was also illustrated qualita- tively in sections by Kirkland and Carpenter (1994) and Kirkland (2006). Based on the number of mapped bones to mapped area, there appear to be approximately 4.9 mapped bones per square meter of the quarry. This density, however, is based on the bones large enough to map and record. Because very small and fragmentary bones have been noted as common in the quarry, but have not historical- ly been documented, an unexcavated square meter was selected from the eastern part of the quarry to excavate and collect everything found to record the density of material within this test area. This nearly cubic meter (m3) of matrix contained about 100 randomly distribut- ed bones (figure 34); mostly these are small fragments, but the sample also included four theropod teeth. The results showed a relatively random orientation within the meter but concentrated in the lower portion. The fragments were mostly unidentifiable and small, with a mean volume of 6.78 cm3 and a maximum volume of about 47 cm3 (figure 35). The excavated m3 is thought to be typical of the deposit overall because it is from an area close to where large dinosaur bones have been ex- cavated. It is from neither within the current excavation nor close to what appears to be the edge of the deposit to the east. The material was found within typical me- dium light-gray claystone containing abundant carbon- ized plant fragments. If the many small bone fragments randomly distributed within this 1 m3 indicate what is typical of the quarry, there may have been approximate- ly 50,000 individual bone pieces within the about 500 m2 of the quarry excavated so far. Extrapolated to the potential area of the quarry demonstrated by the drill coring (see above), the entire deposit may contain as many as 463,000 bones, based on the fully sampled sin- gle m3, and as many as 23,150 mappable bones, based on the density demonstrated by the quarry map (figure 0 5 10 15 20 Juveniles Sub-adults Adults M in im u m N u m b er o f I n d iv id u al s N = 22 Age Profile Figure 31. Age profile of the vertebrates out of the Mygatt- Moore Quarry, discounting indeterminate reptiles. Sample is high in adults, lower in juveniles, but is mostly missing mid-sized animals. 64 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 32. Quarry map of Mygatt-Moore Quarry main quarry layer, through the 2013 season. Scale bars marked in meters. 65 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 32). The abundance of small fragmentary bones, along with larger better preserved bones and more complete dinosaur elements in the same deposit, may suggest two sources for the material. As mentioned above, the quarry map (figure 32) appears to show a random orientation of the bones at Mygatt-Moore. Azimuth orientations of bones from the quarry, measured off the map and categorized as either bidirectional or unidirectional, suggest that indeed the orientation is random. Neither bidirectional bones nor those that are unidirectional demonstrate any strongly preferred azimuth orientation (figure 36) such as would be expected if there were current influence on the ma- terial (Toots, 1965). Random orientations of bones as demonstrated by figure 36 suggest scattering by scav- enging and other process other than hydraulic current influence. Bone Preservation and Modification A large number of bones in the quarry are fairly well preserved and complete. Most of the identifiable ele- ments that have been collected over the years are nearly complete, with only moderate degradation by abrasion, corrosion, weathering, and breakage. Among the bone modifications are green fractures that appear to have occurred when the bones were in or on the muddy ma- trix near the surface, well before lithification. Examples of these breaks include splintered, broken sauropod ribs (figures 37A and 37B), a broken and displaced Allosau- rus metatarsal (figures 37E and 37F), and an Allosaurus distal tibia broken off before burial. A random survey of a subsample of the total collections inventory (446 spec- imens, or about 23.5% of the bone collection) indicates that approximately 61.6% of bones have post-mortem, pre-burial breaks that in many cases probably resulted from trampling. Only about 0.46% of the bones have pathologies (table 4). One of these includes a sauropod ulna with a slightly forked, distally-pointing bone spur along the shaft near the distal end of the element (MWC 5044). Another common modification on bones is tooth marks left by carnivorous theropod dinosaurs. Approx- imately 4.62% of the bones out of the quarry have some Quarry State Lithology Number of Mapped Bones Bones in Articulation Articulation Ratio Map Reference Mygatt-Moore Quarry CO Mudstone 2372 8 0.00337 This paper Cleveland-Lloyd Quarry UT Mudstone 6197 123 0.01984 Miller and others (1996) Carnegie Quarry, DNM UT Sandstone 2993 1848 0.61744 G. York; Carpenter (2013) Howe Quarry WY Siltstone 1668 644 0.38609 Bird (1985) Howe-Stephens Quarry WY Mudstone 649 217 0.33436 Ayer (1999) Poison Creek Quarry WY Mudstone 469 127 0.27079 Erickson, unpublished (1984) Dry Mesa Quarry CO Sandstone 416 33 0.07933 Miller and others (1991) Little Houston Quarry WY Siltstone 255 65 0.25490 Pagnac, unpublished (2014) Table 3. Articulation ratios for several larger quarries in the Morrison Formation. 0 100 200 300 400 500 600 High Medium Low 15.6% 26.1% 58.3% Stratigraphic Level Within 1 m-thick Bone Layer Number of Bones Collected N = 971 Figure 33. Stratigraphic distribution of vertebrate material within the main quarry layer showing concentration of most elements in the lower and middle thirds of the deposit. Each level is approximately 33 cm thick. (There is no correlation between bone size or volume and level at which they occur; large bones have been found high in the deposit, and many small ones are found low.) 66 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 indication of tooth marks (table 4), which is perhaps not surprising given the more than 400 teeth of theropods that have been found in the deposit (98.3% of these teeth appear to be shed and were probably lost during scavenging, as only a few percent of the 419 teeth count- ed for this study had intact roots). These teeth are so abundant that they are often found in close association with bones of herbivores. One tooth was found in close proximity to the pleurocoel of an Apatosaurus cervical vertebra (figure 37G), and in another case nine teeth were excavated in 2005 from around a single sauropod femur. Among the distinctly tooth-marked bones from the quarry are an Apatosaurus ischium (figure 37D), an Apatosaurus pubis, and the type ilium of Mymoorapelta (Kirkland and Carpenter, 1994). Abrasion of bones was noted in about 24.5% of the bones surveyed. It was characterized commonly by sin- gle, small, shallow, and randomly oriented scratches on the bone surface in some cases, but most often by rounding of originally sharp edges of bones or broken elements of them, a common characteristic of trans- port modification. In some cases, abrasion was difficult to distinguish from corrosion. If a bone was not sig- nificantly broken otherwise, it was assumed that most wearing of edges was more likely due to corrosion than transport abrasion. The corrosion of bone from Mygatt-Moore was found to be concentrated on the edges of vertebral cen- tra and on the ends of long bones such as limb elements and ribs (figure 37C), where cortical bone is thin and trabecular bone close to the surface. Most limb bone- shafts, for example, where cortical bone is thick, are unaffected by corrosion. The nature of this bone loss is confirmed as corrosion rather than abrasion by the documentation of thin and delicate spalled outer bone surfaces separated from trabecular bone in cross sec- tions of several Mygatt-Moore bones collected and thin sectioned in the 1990s (J. Kirkland, Utah Geological Survey, verbal communication, 2013). Weathering was noted in 25.2% of bones from the Figure 34. X-, Y-, and Z-coordinate plot of 98 bone positions within the square-me- ter test pit in the Mygatt-Moore Quarry, showing the density of distribution and the material’s concentration lower in the main quarry layer. This may be typical of most of the quarry. 67 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 sample and was generally characterized by cracking and decay of the outer surface of the bone. The degree of weathering (i.e., average weathering stage) of bones from the quarry, however, was generally low. The majority of bones surveyed were either un- abraded (0) or only slightly so (1, figure 38A). A num- ber of bones demonstrated an abrasion condition of 1 to 2 and none was at 3. Similarly, most surveyed bones were unweathered (0) and none reached class 3. Corro- sion and breakage were much more common (figures 38B and 38D), and for both of these a number of bones were in classes 2 and 3. In contrast, a surveyed subsa- mple of the collection from the Cleveland-Lloyd Dino- saur Quarry (at the Natural History Museum of Utah) indicates much less modification in all four categories (figure 39); none of those elements reached class 3 (in case of weathering, class 1 was the highest recorded for any bone). Although the degree of corrosion and break- age within the Cleveland-Lloyd subsample was higher than it was for abrasion and weathering, all were low- er than for the Mygatt-Moore Quarry subsample. The Cleveland-Lloyd Dinosaur Quarry sample was, howev- er, museum-based, meaning that there was a collecting 0 10 20 30 40 50 60 MYGATT-MOORE QUARRY TEST METER Approximate Bone Volume (cubic centimeters) 10 20 30 40 50 N u m b er o f E le m en ts Mean: 6.78 cc Figure 35. Histogram showing the size distribution of bone pieces collected from the test pit square-meter, measured by calculated bone volume. Most pieces are small and fragmen- tary. Figure 36. Rose diagrams of bone azimuth orientations in the Mygatt-Moore Quarry. (A) Bidirectional plot for long bones with no heavier end. (B) Unidirectional plot for long bones with a heavier end. Both distributions show no strong preferred orientation. Weak lower frequency in west-north- west–east-southeast orientation in both. 68 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 bias in favor of better preserved material from the start. The sample studied from the single test meter in the Mygatt-Moore Quarry, interestingly, showed a different pattern from the collection inventory subsample. The test meter consisted of a full recording of every bone piece within a square meter, regardless of size or iden- tification potential. Because of that fact, we perhaps should not be surprised that the results for the test me- ter (figure 40) are quite different from the quarry collec- tions subsample, and from the Cleveland-Lloyd Dino- saur Quarry subsample, for that matter. In fact, recent detailed work at the Cleveland-Lloyd Quarry indicates a similar pattern in that most bones in the matrix (gen- erally uncollected) are small and fragmentary (Gates, 2005; Peterson and others, 2017), and thus the collec- tions sample surveyed here is skewed in favor of better preservation by collection bias. Many bones at Mygatt- Moore are abraded, more are corroded, a number are lightly weathered, and a large majority are broken quite badly, many rounded into a “bone pebble” shape. The test meter results may indicate that numerous alloch- thonous, fragmentary bones were transported into the site, although these were not necessarily transported long distances, nor out of the home ranges of the ani- mals they belonged to (see Behrensmeyer and Rogers, 2017). Histological slides of two very rounded bone “peb- bles” (containing no outer bone surface), and of two bone fragments containing sections of the outer sur- face of the elements as well, indicate very different states of preservation at a microscopic scale. Whereas at fine scale, some bone specimens appear nearly pris- tine (figure 41C), and others are very well preserved (figure 41B), others have some or a great number of micro-fractures in the bone cell structure (figures 41A and 41D). The range of preservation of these elements suggests that these apparently allochthonous bone frag- ments may come from several different source areas. Especially interesting is that most rounded and abrad- ed bone “pebble” specimens (figures 41C and 41D) are, respectively, the best and worst preserved pieces at a microscopic scale among the four that were thin sec- tioned; there was no way to predict this prior to sec- tioning. The nearly perfect preservation of the osteons in one specimen (figure 41C) and the heavily fractured nature of the same in another (figure 41D) suggest very different pre-burial (or possibly digenetic) histories of these elements and perhaps a complex set of origins for the “background” majority of small bone fragments that comprises the bone sample out of the quarry. Dinosaur Skin At least two patterns of dinosaur skin have been found in the Mygatt-Moore Quarry (Foster and Hunt-Foster, 2011), one identified as that of a sauropod consists of ~30 mm diameter hexagonal scales (figure 42A) and another consisting of much smaller polygonal scales (figures 42B and 42D) that belongs to an indeter- minate dinosaur. These skin sections are preserved as either carbonized patches or similarly partially carbon- ized impressions of scales in matrix. In some cases, the Mygatt-Moore Quarry Collections (N=446; ~23.5%) Cleveland-Lloyd Dinosaur Quarry Collections (N=282; ~4.6%) Tooth marks 4.62% 0.73% Pathologies 0.46% 1.82% Juveniles 1.61% 6.72% Sub-adults 1.16% 16.42% Table 4. Comparison of percentage occurrence of tooth marks, pathologies, juveniles, and subadults among museum collec- tions from Mygatt-Moore and Cleveland-Lloyd Dinosaur Quarries. Percentages after N values indicate approximately how much of full museum sample was surveyed for this study. 69 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Figure 37. Examples of taphonomic modifications of bones from the Mygatt-Moore Quarry. (A) Sauropod rib with splin- tered fracture, MWC field number MM1260. (B) Close up of splintered fracture in A. (C) Corrosion of the end of a long bone, MWC 2882. (D) Distal end of theropod tibia with fresh break of lower half of shaft; break surface was surrounded by matrix when found. (E) Deeply gouged tooth marks on distal end of a sauropod ischium (arrow), MWC 4011. (F–G) Frac- tured Allosaurus metatarsal as found, MWC field number MM1398. (H) Theropod tooth preserved in matrix almost in a pleurocoel of an Apatosaurus cervical vertebra (upside down), showing the often close association of sauropod material and shed theropod teeth, MWC 5046. All scale bars = 10 cm. 70 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 skin pieces are folded over within the matrix and are separated by up to 1 cm of matrix. DISCUSSION Paleoenvironmental Setting The stratigraphic sections around Rabbit Valley suggest that the quarry area itself was surrounded by an alluvial system of poorly-drained to well-drained flood- plains and sandy to very gravelly river channels. These channels may have been meandering to anastomosing (e.g., Cooley and Schmitt, 1998), as they are only trace- able laterally for at most several hundred meters and are noticeably less laterally extensive than the channel sandstones of the underlying Salt Wash Member. The sections also indicate a great degree of lateral variabili- ty within the Brushy Basin Member strata as mudstone 0 50 100 150 200 250 300 350 0 1 2 3 Classes N u m b er o f E le m en ts N = 441 ABRASION MYGATT-MOORE QUARRY COLLECTION 0 50 100 150 200 250 0 1 2 3 Classes N u m b er o f E le m en ts N = 441 CORROSION 0 50 100 150 200 250 300 350 0 1 2 3 Classes N u m b er o f E le m en ts N = 441 WEATHERING 0 50 100 150 200 0 1 2 3 Classes N u m b er o f E le m en ts N = 443 BREAKAGE A B C D Figure 38. Bone modification of material from the Mygatt-Moore Quarry collections inventory, by classes and type of mod- ification. Generally, the severity of modification goes up from classes 1 to 3; 0 = no modification. Modification less than average for whole quarry due to this being a collected museum sample of better material. 71 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 bed, channel sandstone, and the “fish layer” beds were all documented to change or pinch out laterally within tens to hundreds of meters. The lateral variability is matched by almost univer- sal changes in the vertical stacking patterns (figure 4). These vertical components document changing paleo- environments over time, and within the quarry area itself we can document several changes. A transition from a permanent or ephemeral overbank topographic low into an ephemeral pond and then to a permanent but small (and probably shallow) lake is indicated by the lithologic variability of the quarry interval. Later, a return to floodplain deposition with a nearby river rep- resented by the channel sandstone at the Camarasaur Stop. The main quarry layer demonstrates features of a pond or pool. The lack of bones east of trench 5 and the reddish-colored mudstone at the quarry level at the 0 50 100 150 200 250 300 350 0 1 2 3 Classes N u m b er o f E le m en ts N = 281 ABRASION CLEVELAND-LLOYD QUARRY COLLECTION 0 50 100 150 200 250 0 1 2 3 Classes N u m b er o f E le m en ts N = 281 CORROSION 0 50 100 150 200 250 300 350 0 1 2 3 Classes N u m b er o f E le m en ts N = 282 WEATHERING 0 50 100 150 200 0 1 2 3 Classes N u m b er o f E le m en ts N = 282 BREAKAGE A B C D Figure 39. Bone modification of material from the Cleveland-Lloyd Dinosaur Quarry collections survey, by classes and type of modification. Generally, the severity of modification goes up from classes 1 to 3; 0 = no modification. Modification less than average for whole quarry due to this being a collected museum sample of better material. 72 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Caudal Draw stratigraphic section, the different char- acter of trench 5 in both trench stratigraphy and thin sections, and the fact that drill holes MM-1 and MM-2 showed that the main quarry layer is thinner and slight- ly higher stratigraphically, all suggest that the quarry layer is a convex-bottomed, laterally restricted lens. The minimum area of the deposit is approximately 18,200 m2 and may be the deepest near the present northern part of the quarry (dimensions about 130 x 140 m, mea- sured drill hole MM-1 to trench 5 and drill hole MM-2 to trench 4). Although this is an approximately rectan- gular area, the actual deposit is probably roughly oval. Paleoenvironment of the Main Quarry Layer The base of the main quarry layer (with abundant plant debris and bone) appears to begin with deposition in a quiet, perennial pond, in that the very base of the 0 10 20 30 40 50 0 1 2 3 Classes N u m b er o f E le m en ts N = 94 ABRASION MYGATT-MOORE QUARRY TEST METER A B 0 10 20 30 40 50 0 1 2 3 Classes N u m b er o f E le m en ts N = 91 CORROSION 0 5 10 15 20 25 30 35 40 0 1 2 3 Classes N u m b er o f E le m en ts N = 71 WEATHERING 0 10 20 30 40 50 60 70 80 0 1 2 3 Classes N u m b er o f E le m en ts N = 98 BREAKAGEC D Figure 40. Bone modification of material from the Mygatt-Moore Quarry test pit square-meter. Generally, the severity of modification goes up from classes 1 to 3; 0 = no modification. 73 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 quarry (at the contact with the underlying light-green silty claystone) is a layer that contains several viviparid gastropods. The sediments that contain these gastro- pods, however, do not have any plant fragments. So, these gastropods occur in a basal layer or unit rather different than the actual main bone layer just a few cen- timeters higher in the section. The main quarry layer has some seemingly contra- dictory taphonomic characteristics. Some character- istics suggest very wet conditions, possibly perennial standing water; others suggest wet or damp conditions but only ephemeral water. Some evidence suggests the bones in the quarry are locally autochthonous, whereas other rounded and abraded elements seemingly indi- cate transport a moderate distance; it is unlikely even heavily modified bones were transported great distanc- es or mixed into areas outside the ranges of the animals they derive from (Behrensmeyer and Rogers, 2017). Al- though there is no clear evidence of miring of dinosaurs at the Mygatt-Moore Quarry as there is at some oth- er sites (e.g., Howe Quarry, Farlow in Bird, 1985), the abundance of theropod material might at first suggest a predator trap. Even though the sample is dominated by just two of the present dinosaur taxa, the age profile Figure 41. Thin section micrographs of samples from bone fragments out of the Mygatt-Moore Quarry. (A) Relatively well preserved bone fragment, BG-1, nevertheless showing some micro-fracturing. (B) Well preserved bone fragment, BG-2, showing fairly good preservation even at microscopic scales. (C) A “spongy” and almost rounded bone fragment, BP-1, that preserves nearly pristine osteons. (D) Another poorly preserved, rounded bone fragment, BP-2, with highly micro-fractured osteons. All thin section micrograph views are approximately 2000 μm across. 74 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 suggests an attritional assemblage (see below). The Mygatt-Moore Quarry “bone layer” may repre- sent an ephemeral pond surrounded by conifers, simi- lar to a wooded vernal pool, although there is a paucity of direct evidence to confirm this. Amphibians, often characteristic of modern vernal pools may be very rare at the site due to taphonomic bias. The unit geometry (see above) and the mudstone (with some silt) lithology indicate that it is not a channel and is likely some type of overbank deposit. The presence of many bone fragments (almost a ‘background’ of small, nearly rounded frag- ments), calcium carbonate “pebbles,” and small clayballs indicates that a significant portion of the material was washed in, probably during flooding of a nearby chan- nel. The range of micro-preservation characteristics in the histological bone samples, the appearance of two Figure 42. Dinosaur skin specimens from the Mygatt-Moore Quarry. (A) Sauropod skin patch with large hexagonal scales, MWC 6718. (B) Patch of smaller scales belonging to an indeterminate dinosaur, MWC 1903. (C) Pattern of very small tubercles, possibly superimposed on larger hexagonal pattern, MWC 5537. (D) Another patch, partly carbonized but also preserved as impression; this specimen is also folded over on itself, discovered during the 2013 season, MWC field number MM1687. 75 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 distinct geochemistry signals in the calcium carbonate nodules, and the different colors and range of the silt and clay content in the clayballs, all in the lower main quarry layer, suggest that there were multiple sources for the allochthonous bones, “pebbles,” and clay clasts in the quarry. In addition, these sources mixed during a single phase of emplacement, though probably not a single episode. Variation in clay clast, nodule, and bone characteristic does not appear to stratigraphically cor- relate. Despite the “background” of small fragmentary bones (allochthonous), a significant number of nearly complete, well-preserved, but often rotted (corroded), bones are present in the main quarry layer, which prob- ably represent an autochthonous component. Small, elongate plant debris that is often aligned might indicate light (possibly oscillatory) water influ- ence during deposition of the bone layer. The fact that the rose diagrams both show no preferred orientation suggests that whatever current there was, it was not a unidirectional current and not strong enough to align dinosaur skeletal material. The total lack of fish from the main quarry layer, the fact that not a single turtle element has ever been confirmed from the layer, and the extreme paucity of neosuchian material all suggest that permanent water was probably not present at the site at the time of depo- sition of the main bone layer. The restriction of “proso- branch” gastropods to layers above and below the main bone layer indicates that the quarry area was a locality of perennial water, but there is no direct evidence to in- dicate this with certainty from the bone layer itself. That “conchostracans” are present just below the “fish layer” suggests that the area may have been an ephemeral wa- ter hole at times, but again there is no direct evidence of these ephemeral pond inhabitants in the bone layer. Actinopterygian fish are restricted to the overlying “fish layer” and indicate perennial water with certainty, but only well after dinosaur quarry (bone layer) deposition was over. In addition, the fact that other sites such as the Small Quarry (Garden Park), Quarry 9 (Como Bluff), and the Little Houston Quarry (Black Hills) seem to represent ponds, and also preserve abundant turtles, neosuchians, and fish (Foster, 2001, 2003), suggests that if these aquatic and semi-aquatic taxa were present at the Mygatt-Moore Quarry at the time, they would have been preserved. Their absence may be due to a mechan- ical taphonomic bias that filtered out smaller animals (Behrensmeyer and others, 1979), but this bias seems unlikely for taxa as robust as turtles. If these animals were present in large numbers at the Mygatt-Moore Quarry, it would seem at least a few would be preserved. The abundance of plant material and carbonized plant fragments throughout the main quarry layer at the Mygatt-Moore Quarry suggest that the soil was acidic. The high degree of corrosion (or “rot” along with result- ing spalling) in the quarry is also probably indicative of acidic conditions in the quarry mud at the time, as bone preservation can deteriorate in lower-pH condi- tions (Baxter, 2004). The effects of low pH on bones can sometimes be quite variable even between samples in the same soil, however (Nicholson, 1996). Preservation of dinosaur skin, not just as impressions in matrix, but more often as carbonization, suggests occasional dys- oxia in the mud of the deposit, if not necessarily in any standing water that may have been present. Dysoxic or anoxic conditions may have enhanced the frequency of plant preservation also (Briggs, 1999). The presence of contemporary wood-rotting fungi in a number of the wood specimens from the quarry (Tidwell and others, 1998) would also seem to suggest that the environmen- tal setting was wet and possibly humid. Like the Cleveland-Lloyd Dinosaur Quarry, the Mygatt-Moore Quarry bone layer matrix is elevated in several elements such as Pb, As, Mn, K, and S (Peterson and others, 2017). It is unknown if this phenomenon is due to original conditions in the deposit, diagenetic fac- tors, or perhaps is a result of decomposition of carcass- es at the site (Peterson and others, 2017). As discussed above (matrix XRF analysis), it seems more likely that the elevation of several elements is the result of diage- netic processes at the Mygatt-Moore Quarry. Perhaps perennial water was present at the Mygatt- Moore Quarry during deposition of the bone layer, but were there environmental conditions that kept small- er aquatic and semi-aquatic animals out? The acidic soils and dysoxic conditions apparent at the site during deposition of the main quarry layer might have been 76 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 such factors to make the area unappealing to these taxa. However, the fact that turtles, crocodylians, and fish are abundantly preserved in the dark, carbonaceous (pre- sumably somewhat acidic) mudstone and the (some- times dysoxic; mummifying) sandstone beds of Late Cretaceous deposits suggest that such conditions were neither always avoided by the animals nor detrimental to their preservation. The paucity of neosuchians and the lack of fish and turtles from the main quarry layer may well be a paleobiological pattern then, and the pos- sibility remains that the mud that now comprises the main quarry layer was only seasonally under water. The presence of trampled bones suggests that living animals frequented the deposit area, stepping on bone already in the mud during times of low (or no) water. This trampling is evidenced by many bones broken clearly after the element was in the mud but still exposed at or near the surface; unhealed, splintered fractures are relatively common and were documented in the field as having occurred prior to collection by matrix adjacent to broken surfaces. Documented field evidence, such as vertical changes in mudstone bedding within the quar- ry and post-depositionally broken bones, also suggests that many bones were churned in the soft mud, again presumably by trampling. Among observed elements that seem to have been preserved in soft wet mud were multiple bones preserved at an angle to horizontal, thin bones with fresh unabraded edges preserved in the ma- trix on edge, and at least one shed theropod tooth found on end (vertically) with the tip down in the matrix (no associated skull material). The frequent presence of carnivores and abun- dant scavenging in the quarry area are indicated by the abundance of Allosaurus bones, the abundance of shed theropod teeth, and the moderate amount of tooth marks on bones. The extremely high rate of disarticula- tion and significant disassociation compared with other large quarries in the Morrison Formation may be due to trampling by dinosaurs (as with scatter of modern elephant bones by elephants; Haynes and Klimowicz, 2015) and scattering by scavenging theropods. Some modern bones, however, can also be found scattered by lake waters, surface sheet flow, and possibly wind along lacustrine margins in autochthonous deposits (Co- bo-Sánchez and others, 2014). Therefore, a number of factors may have been acting in scattering the material at the Mygatt-Moore Quarry. Several of the plant types preserved in the quarry are associated with wet environments, including horsetails, Czekanowskia (Tidwell and others, 1998), and lycopo- diophytes. Lycopodiophytes today are common ele- ments of some vernal pools (Keeley, 1990) and, in some settings, live in slightly acidic water (Wang and others, 2002). Ferns and conifers are also particularly abun- dant, and plenty of ginkgoes and cycadophytes are also present. The fact that podocarpaceous wood and pollen are both found in the quarry (Tidwell and others, 1998; Hotton and Baghai-Riding, 2010) indicates that some of the abundant fossil wood at Mygatt-Moore is almost certainly local and therefore that the surrounding area was probably wooded. Among the plants common at the Mygatt-Moore Quarry, Equisetum and ginkgoes are two of the better candidates for plant fodder for herbiv- orous dinosaurs, whereas Podocarpaceae were probably less appealing (Gee, 2010). Vertebrate age profiles in hypothetical standing populations would have many juveniles and fewer adults, and non-selective catastrophic mortality would result in a similar profile (Eberth and others, 2007). At- tritional mortality could result in an age profile with many juveniles and adults and fewer mid-age classes. Rapid burial of such a profile would result in a simi- lar distribution, whereas delayed burial would favor preservation of adult-size classes (although Myers and Storrs, 2007, noted that with sauropod remains, rate of burial may have less influence on preservation effects than with smaller taxa). Still, delayed burial of an attri- tional assemblage may result in an age profile with few mid-sized animals, more juveniles, and an abundance of older animals (Eberth and others, 2007) and this is what is observed at the Mygatt-Moore Quarry. The ex- tremely high degree of disarticulation of the material seen at the quarry (lowest measured articulation radio among large Morrison sites) is also consistent with de- layed burial (Myers and Fiorillo, 2009). Despite the un- usual abundance of two taxa (Allosaurus and Apatosau- rus) in the quarry, the age profile seems to indicate an attritional assemblage with non-rapid burial. Therefore, 77 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 the deposit does not appear to be the result of a cata- strophic mortality of dinosaurs. The slow burial would seem support the existence of an ephemeral overbank pond, but the high abundance of just two taxa is harder to explain. Among several scenarios, it is possible that the deposit built up over multiple environmental stresses (e.g., occasional droughts), with the high abundance of Apatosaurus resulting from a population frequenting the water source repeatedly during times when the rest of the floodplain was drying out. That apparently-shed sauropod teeth (mostly Diplodocidae but with a number of camarasaurids) are relatively abundant at the site suggests that in fact the animals were feeding nearby, and it is estimated that population densities on the Morrison landscape would have allowed for a handful of individuals of each sauropod species per km2 (Farlow and others, 2010). Possibly, a regional apatosaur population tended to frequent the area seasonally, coincidentally when stresses on the herd were highest, while other sauropod taxa migrated through during other, less stressful times (J. Kirkland, Utah Geological Survey, verbal communication, 2013). There seems to be some evidence of regional migration among sauropods from the Morrison (Fricke and others, 2011). At times when the population was in the area it may have had higher densities of animals because the group was absent at other times, and because presumably at times when the apatosaurs were there, the other sauropods were away. An alternative hypothesis is that a local apatosaur population frequented the area year- round, preferring an environmental or dietary aspect of the surrounding area, to the general competitive exclusion of other sauropods. In this scenario, the attritional assemblage would also have been built up over time as the area was populated long-term by a high percentage of Apatosaurus. Although we were able to discern an attritional versus catastrophic origin for the bone assemblage, distinguishing between the two above scenarios to explain the abundance of Apatosaurus relative to Camarasaurus and Diplodocinae indet., may be difficult. The facts that the macro-plant and pollen- spore assemblages are thus far almost unique among plant sites in the Morrison (Hotton and Baghai-Riding, 2010), and that the Mygatt-Moore Quarry is one of the only apatosaur-dominated sites in the formation, suggest that perhaps something about the flora of the local habitat was attractive to these particular sauropods. Still, determining whether this attraction was seasonal or perennial may be impossible. Comparison to Other Quarries Cleveland-Lloyd Dinosaur Quarry The major quarry to which the Mygatt-Moore Quarry compares most closely is the Cleveland-Lloyd Dinosaur Quarry in the Morrison Formation of the San Rafael Swell of east-central Utah (Stokes, 1985; Miller and others, 1996; Bilbey, 1999; Gates, 2005; Peterson and others, 2017). Similarities to the Mygatt-Moore Quarry include mudstone matrix, many disarticulated bones, highly mixed arrangements of the bones, many Allosaurus elements (MNI = 49), and a relatively thin bone layer (Richmond and Morris, 1996; Gates, 2005; Hunt and others, 2006; Foster and others, 2016). Differ- ences of Cleveland-Lloyd Dinosaur Quarry compared to Mygatt-Moore Quarry include (1) lack of carbon- ized plant material in the mudstone, (2) higher degree of articulation, and (3) greater dominance by Allosau- rus primarily. Additionally, as found in this study, the Mygatt-Moore Quarry has a much higher percentage of shed carnivore teeth (versus rooted), has slightly more tooth marked bones, has more fragmented, weathered, abraded, and corroded bone, and has more allochtho- nous clasts and bone fragments. Mygatt-Moore Quar- ry also demonstrates less evidence of hydraulic sorting and more random azimuth orientations of the bones as compared to the Cleveland-Lloyd Dinosaur Quarry (see Gates, 2005). The Cleveland-Lloyd Dinosaur Quar- ry has only a single Apatosaurus element (Foster and Peterson, 2016), whereas the Mygatt-Moore Quarry has no elements clearly belonging to Stegosaurus; the rea- sons for these taxa being rare or missing are unclear, as they are otherwise very common overall in the Morri- son Formation. The Mygatt-Moore Quarry also has a lower percentage of juveniles and sub-adults than does the Cleveland-Lloyd Dinosaur Quarry. 78 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Howe Quarries The Howe Quarry (Bird, 1985) and the Howe-Ste- phens Quarry (Ayer, 1999) are large, multi-taxic sites in the Morrison Formation of northern Wyoming. Both differ from the Mygatt-Moore Quarry in that the matrix is a silty to slightly sandy mudstone with more direct evidence of fluvial influence (more so at Howe-Ste- phens). The Howe Quarry contains an abundance of diplodocine sauropod material relative to other taxa, and in this sense, it is similar to the Mygatt-Moore and Cleveland-Lloyd Dinosaur Quarries. The Howe Quar- ry also preserve a relative abundance of dinosaur skin (Czerkas, 1992). Little Houston Quarry The Little Houston Quarry is in the thin Morrison Formation of the Black Hills in northeastern Wyoming (Foster and Martin, 1994; Foster, 2001). This quarry is in a laterally restricted, ribbon-like abandoned channel deposit with interbedded green claystone and laminated gray-green siltstone. Articulation of dinosaur bones is moderate, density of the bone material is high, and mi- crovertebrate material is very abundant, even in among the sauropod bones in some levels. This site preserves a diversity of large dinosaurs and especially small reptiles, mammals, and fish, along with an abundance of tur- tles and neosuchian crocodyliforms. This preservation mode is common in the northern and eastern parts of the Morrison Formation (Foster and McMullen, 2017) and is characteristic of the Small Quarry and Quarry 9 as well (Foster, 2001, 2003). This mode is taphonomi- cally very different from the Mygatt-Moore and Cleve- land-Lloyd mode of preservation. Carnegie Quarry, Dinosaur National Monument The Carnegie Quarry at Dinosaur National Mon- ument in Utah is a good representative of the tapho- nomic mode of preservation so common among large dinosaur quarries in the Morrison Formation in that it occurs in a thick, pebbly sandstone (Lawton, 1977; Car- penter, 2013). The site preserves a diversity of dinosaurs, most of the reasonably common ones in the formation, in fact, and in high numbers. There are thousands of bones in this quarry and probably more than 120 in- dividuals represented. The degree of articulation is the highest measured for this study. Sedimentologically and taphonomically channel sandstone deposits like the Carnegie Quarry are very different from large sites in mudstone like Mygatt-Moore and Cleveland-Lloyd Di- nosaur Quarries. Preserved Diversity by Site One interesting aspect of the preservation of dino- saur taxa at some of the large quarries in the Morrison Formation is the variation in the relative abundance of the most common dinosaur taxa. For example, within the Morrison overall, the most abundant dinosaurs are, in descending order, Camarasaurus, Allosaurus, Stego- saurus, Apatosaurus, and Diplodocus. Each of these is slightly less abundant than the next highest in rank abundance. If we look at large quarries in mudstone and siltstones (fine-grained sites), we notice that (1) this pattern does not hold up and that the relative abundanc- es among these five taxa are highly uneven, with some quarries missing one genus entirely, (2) one or two taxa often dominate, with the other taxa rare, and (3) the dominant taxon or taxa are variable (figure 43; Foster and others, 2016). Although the Mygatt-Moore Quarry has a number of Allosaurus individuals, it is dominat- ed by Apatosaurus by number of identified elements (as opposed to MNI). Cleveland-Lloyd Dinosaur Quarry is even more dramatically dominated by Allosaurus, and the Howe Quarry is dominated by diplodocines. The pattern demonstrated by large quarries in channel sandstones, however, more closely reflects the pattern in the Morrison Formation overall (figure 43). All of the “big five” dinosaur genera are present at all sites, relative abundances are much more even, and no site is domi- nated by one taxon to the degree that the fine-grained sites are. These different relative abundances probably reflect the relative size of the sampled geographic area and the amount of time averaging within each deposit. The geographic area from which large Morrison river channels were drawing skeletons could well have been rather extensive, particularly considering the possible areas of the individual drainage basins on a relatively 79 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 05010 0 15 0 20 0 Minimum Number of Individuals Cam ara sa uru s Allo sa uru s Stegosa uru s Apato sa uru s Diplodocus M o rr is o n F o rm at io n O ve ra ll 012345678 Cam ara sa uru s Allo sa uru s Stegosa uru s Apato sa uru s Dip lo docin ae M yg at t- M o o re Q u ar ry 01020304050 Cam ara sa uru s Allo sa uru s Stegosa uru s Apato sa uru s Dip lo docin ae C le ve la n d -L lo yd Q u ar ry 024681012 Cam ara sa uru s Allo sa uru s Stegosa uru s Apato sa uru s Dip lo docin ae H o w e Q u ar ry Minimum Number of Individuals 051015202530 Minimum Number of Individuals Cam ara sa uru s Allo sa uru s Stegosa uru s Apato sa uru s Diplodocus C ar n eg ie Q u ar ry , D in o N at ’l M o n 0. 0 0. 5 1. 0 1. 5 2. 0 2. 5 3. 0 3. 5 4. 0 Cam ara sa uru s Allo sa uru s Stegosa uru s Apato sa uru s Dip lo docin ae M ar sh -F el ch Q u ar ry 024681012 Cam ara sa uru s Allo sa uru s Stegosa uru s Apato sa uru s Dip lo docin ae B o n e C ab in Q u ar ry 012345 Cam ara sa uru s Allo sa uru s Stegosa uru s Apato sa uru s Dip lo docin ae D ry M es a Q u ar ry Minimum Number of Individuals M U D ST O N E/ SI LT ST O N E SI TE S SA N D ST O N E SI TE S O V ER A LL P A T TE R N Fi gu re 4 3. C om pa ris on o f r el at iv e a bu nd an ce (b y M N I) o f t he fi ve m os t c om m on d in os au rs o f t he M or ris on F or m at io n at ea ch o f t hr ee fi ne -g ra in ed (m ud st on e/ sil ts to ne ) f ac ie s q ua rr ie s a nd e ac h of fo ur sa nd st on e fa ci es q ua rr ie s, w ith c om pa ris on to th e M or ris on F or m at io n ov er al l s am pl e. Re p- re se nt at io n an d re la tiv e ab un da nc es o f t ax a in sa nd st on e fa ci es a re m or e re pr es en ta tiv e of th e fo rm at io n ov er al l t ha n th e oft en v er y un ev en re pr e- se nt at io n an d re la tiv e ab un da nc e in la rg e m ud st on e fa ci es si te s. 80 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 flat floodplain at the time (Behrensmeyer, 1982). Al- though sauropod bones and carcasses probably would not have been transported great distances in recogniz- able form (Myers and Storrs, 2007), it would not be sur- prising if the area from which a dinosaur deposit in a channel sandstone was drawn was in fact far broader than that of a local pond or overbank topographic low. There is variation in the amount of time represented by different sites within the fine-grained site subsample, with the Mygatt-Moore Quarry perhaps representing an attritional and possibly more time-averaged sample than Cleveland-Lloyd Dinosaur Quarry, for example. The amount of time-averaging in an attritional mud- stone site was probably still far less than that of a chan- nel sandstone deposit like the Carnegie Quarry, which appears to include several lag beds superimposed in a thickness of several meters. Both Mygatt-Moore and Cleveland-Lloyd Dinosaur Quarries are much thinner beds of about 1 m thickness. This pattern of uneven relative abundances at mud- stone sites and more diverse and evenly represented taxa at sandstone sites (figure 43) suggests that large quarries of dinosaur bones in the Morrison Formation fall into at least two major taphonomic modes. Many sites in the Morrison consist of paucispecific deposits of a few in- dividuals and up to a few hundred bones, sometimes also dominated by one or a few taxa, in poorly drained floodplain mudstones (e.g., Tucker, 2011). Very large deposits of multiple hundreds or thousands of bones are often either (1) diverse sandstone quarries with rel- atively even abundance distributions among the most common dinosaurs, or (2) less diverse fine-grained (mudstone or siltstone) quarries with sometimes wildly uneven abundance distributions among the most com- mon dinosaurs (some missing entirely). There are un- doubtedly a number of the former deposits in the Mor- rison Formation channel sandstone beds with many bones and many taxa (e.g., Carnegie Quarry, Dry Mesa Quarry), and undeveloped examples can be seen in the Morrison Formation at least in Colorado and Utah Figure 44. Reconstruction of the Mygatt-Moore Quarry, 152 million years ago. An Allosaurus feeds on the carcass of an Ap- atosaurus. Shed teeth and tooth marks of carnivores suggest that theropod dinosaurs regularly concentrated their feeding on the large muscles of the hind limbs and pelvis. ©Todd Marshall 81 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 (e.g., Split Rock, Mill Canyon). Perhaps there are yet undiscovered deposits of the latter kind, large potential quarries in mudstone that sampled a Camarasaurus- or ankylosaur-dominated fauna, for example. These sites have the potential to tell us a little more about the vaga- ries of preservation and dinosaur paleobiology than do more time-averaged and geographically blended sites. CONCLUSIONS The Mygatt-Moore Quarry is Late Jurassic in age (152.18 +/-0.29 Ma), very close to the Kimmeridg- ian-Tithonian boundary. The Mygatt-Moore Quarry is in a claystone that has slightly higher (and coarser) silt content than the surrounding claystone beds. Claystone beds that are lateral to the quarry deposit are different in color and silt content and, in some cases, include sandstone beds near quarry level away from the site. The bones were probably deposited on a topographi- cally low part of the floodplain, possibly within 800 to 1000 m of a river channel to the east (figure 4). The quarry was at least about 130 m by 140 m across, possibly an oval shape. The area may be as much as 18,200 m2 as currently known from excavations, trenches, and drill cores. The thickest part of the quar- ry is in the northern area of the current excavation; it thins to the east, southwest, and northwest. The base slopes upward in those latter directions as well, so that the bone layer is somewhat lens-shaped (convex down). The quarry preserves both autochthonous and al- lochthonous bone components. There is a group of mostly fragmented and almost rounded bones that was probably washed in early in the deposit’s history from a significant distance and a second group of more intact bones that were sourced from skeletons of animals that died near or at the point of deposit itself. As a probably perennially damp to seasonally flood- ed low area of the floodplain, the quarry area was very likely occupied and immediately surrounded by a great- er abundance of conifers, horsetails, and, to a lesser de- gree, ferns than the next most outlying areas. Abundant plant material in the quarry bone layer matrix was likely preserved due to acidic soil conditions, which are con- ductive to plant preservation. Relatively low pH during deposition of the bone deposit seems likely, although at the earliest stages when the “pebble layer” was depos- ited, pH may have been higher. Dampness from high moisture content in conifer wood in the quarry area led to rotting of wood by fungi. Preservation of carbonized skin of dinosaurs (and plant material) suggests the mud of the deposit may have been dysoxic to anoxic. The calcium carbonate nodules that are abundant near the base of the bone layer appear to be washed in from at least two different source areas, but how far they were transported is difficult to assess. They formed in place as incipient soil nodules but were then reworked and deposited at the quarry after transport. Their origin could be from a different area than the rock clasts and clayballs in the matrix of the main bone layer. Much of the initial input to the bone layer (basal portion) thus appears to have been from elsewhere. The abundance of Allosaurus bones and teeth is al- most certainly a result of a combination of environmen- tal stresses on the animal (drought?) causing slightly higher mortality of the population and increase in the number of individuals in the area, probably drawn in to scavenge on carcasses of both herbivorous and carniv- orous dinosaurs (figure 44). Although the abundance of shed teeth and tooth-marked bone clearly indicates scavenging, there is no direct evidence of predation oc- curring at the site. The abundance of Apatosaurus appears to be from attritional mortality of a resident population of animals that frequented the area either year-round or season- ally. If these animals were around all year, it may have been due to an affinity for the Mygatt-Moore Quarry area’s unique mix of plant types. Other sauropods like Camarasaurus were probably competitively excluded or simply did not like the fodder in the area and so would be preserved less frequently. Although it is possible that acidity or oxygen-poor conditions excluded small aquatic taxa or caused them not to be preserved, this is not the case at other, very wet sites in the Morrison Formation (e.g., Little Hous- ton, Quarry 9). We believe the near total lack of aquatic and semi-aquatic animals represents their actual rari- ty during deposition of the deposit due to infrequent (possibly only seasonal) presence of standing water at 82 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 the site. The extremely high degree of disarticulation of bones in the quarry appears to be a result of scaveng- ing of carcasses by predators, trampling and churning of the bones in mud by living animals, and relatively long exposure before the bones were locked in the sed- iment and fully buried. Still, the exposure time was not long enough for there to be significant weathering of the bone material. ACKNOWLEDGMENTS We first need to thank the Mygatt and Moore fami- lies for their discovery and reporting of the site in 1981 and their early work at the site. The current research project was made possible by a grant from the National Landscape Conservation System in 2010. Past funding for work at the Mygatt-Moore Quarry, and on its collec- tions, came from the David B. Jones Foundation, Chuck Safris, Dinamation International Society, and the Mu- seums of Western Colorado. Thanks to Ken Carpenter and Bucky Gates for critical reviews of the manuscript. Lance Eriksen started working on the quarry for the Museums of Western Colorado and was followed by curators (in order) Harley Armstrong, Brooks Britt, Rod Scheetz, John R. Foster (lead author), and Julia B. McHugh (co-author). Harley Armstrong in particu- lar has been a continuing encyclopedia of archival in- formation about the quarry. Jim Kirkland worked the site for Dinamation International Society for years and has been an invaluable resource about the geology and paleontology of the site, particularly about specimens collected long ago that we eventually relocated in the collections during the inventory. Volunteers from the Museums of Western Colorado have been the backbone of field and lab work for the site for 30+ years and are probably too numerous to name, but we will try to cover as many as we can: Mike Perry, Dorothy Stewart, Kay Fredette, Bonnie Carter, Walt Williams, Ray Bley, Don Boden, Darrell Bay, Tom Lawrence, Tom Schroer, Dale Jones, Dorothy Stewart, Dick Peirce, Bob Whitehorne, Gary Spever, Ken Stadt- man, Dee Hall, Nancy Colaizzi, Ralph Nash, Larry Marshall, Lyn Farber, Norm Cyphers, Bill and Ginger Mitchell, John Doyle, Marty Doyle, Edie Mangano, Gary Anderson, Marge and Walt Averett, Mel Moyer, Maury Jean and Gordon Peterson, Dave Wolny, Grace Brown, Gaylord Headrick, Fred Savage, Charlie Neill, George Moorehead, Bob Smith, Carter Burnham, and many others. Museums of Western Colorado staff that have worked frequently at the site over the years include Don Chaffin, Mike Perry, Don Kerven, Josh Smith, Vaia Barkas, Lorin King, Alex Morrow, Zach Cooper, Krista Brundridge, Jacob Gottlieb, Nehali Dave, Lucia Herre- ro, Kelsie Abrams, Tom Temme, Rob Gay, Chris Racay, and Mitch Lukens. Dinamation interns from the past years include Matt Bonnan, Jason Head, and Jonathan Weinbaum. Thanks also to the thousands of dig participants of the Museum of Western Colorado and Dinamation paleontology expedition programs of the past 25 years or so; many of the most interesting discoveries in the quarry were made by these folks. The Grand Junction Field Office of the Bureau of Land Management has been critical to the research at the Mygatt-Moore Quar- ry and Museums of Western Colorado research at the site has been conducted under paleontological resourc- es use permits C-60150a and COC75354; Liz McReyn- olds, Nikki Grant-Hoffman, Scott Gerwe, Harley Arm- strong, Katie Stevens, Collin Ewing, Bruce Fowler, Greg Gnesios, and Wade Johnson have all been particularly helpful. Robert Bakker and Jack McIntosh provided ex- pertise and advice early in the operation of the quarry. The City of Fruita Department of Public Works and John McBride have been generous in assisting with re- moval and shifting of overburden and with lifting out large field jackets. Rick Charlesworth helped assemble much improved shade tents in 2012. Special thanks to Associated Builders and Contractors for significant overburden removal in 2006 that has made work at the quarry safer and easier over the past 12 years. Harold Bollan led preparation on many of the Di- namation-collected specimens, with help from staff and volunteers such as Dave and Marilyn Younie, Marlies Aeberlic, Lee Campbell, Bob Marsh, Kim Clark, Bob Gabbert, Robin Prunty, Roxie Mobley, Marilyn Soko- losky, Duane and Pat Hogue, Rick Adleman, Sandy and Bill Hood, Pat and Lee Mitchell, Bob Waters, Judy Bay, 83 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Phyllis Bollan, Chris Mays, Chris Mays, Jr., and Russell Bucher. Rex Cole (Colorado Mesa University) and Jane Baer and Randy Irmis (University of Utah) provided microscope access, micrograph assistance, and image rescue. Study of the Cleveland-Lloyd Dinosaur Quar- ry material at the Natural History Museum of Utah (NHMU) was facilitated by Randy Irmis and Carrie Levitt-Bussian. Assistance with data collection at the NHMU courtesy of Ruby Foster. Jonathan Cooley (Colorado Mesa University) has worked at the Mygatt-Moore Quarry for many years in cooperation with museum crews, and two of his young crew members found the (so far) only Othnielosaurus specimen. Kay Fredette loaned the 1987 field photos in figures 2C and 2E. Mike Perry assisted in measuring the stratigraphic sections. Bruce Erickson (Science Muse- um of Minnesota) and Darrin Pagnac (South Dakota School of Mines and Technology) provided research quarry maps of the Poison Creek and Little Houston sites, respectively. San Juan Drilling did the drill cores. Ben Miller helped map the quarry area trenches and drill holes with the fixed-wing drone. Andy Heckert (Appalachian State University) shot the 2 mm tooth from the Mygatt-Moore Quarry on an SEM. 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Geology of the Intermountain West 2018 Volume 5 APPENDIX A – GEOLOGIC THIN SECTIONS Descriptions of hand samples and thin sections made from them. Specimens numbered within each trench from stratigraphically lowest upward. Trench 1 #5 – Clay (90–95%) with black mineral or organic carbon laminations; 5–10% angular to subrounded quartz silt; less silt and smaller grains than main quarry layer; “fish layer.” #4 – Clay (80%) and silt to fine sand (20%) that is sub-angular to subrounded and mostly quartz; few clayballs and mostly without silt; some carbonized plant fragments. #3 – Clay (80–85%) and silt to fine sand (15–20%) that is sub-angular to subrounded and mostly quartz; angular to sub-angular clayball clasts of various colors, some purely clay, some with minor amounts of silt grains within them; many carbonized plant fragments; “Main bone layer.” #2 – Clay (80–85%) and silt to fine sand (15–20%) angular to subrounded; silt grains mostly quartz but with at least one plagioclase feldspar. #1 – Clay and mostly angular to some rounded silt, mostly quartz with a few non-quartz, non-feldspar silt grains; tiny amounts of detrital micas; 85–90% clay and about 10–15% silt. Trench 2 #5 – Clay (90–95%) with black mineral laminations; 5–10% angular to subrounded quartz silt; less silt and smaller grains than main quarry layer; “fish layer.” #4 – Clay (95%) and silt to fine sand (5%) that is sub-angular to subrounded and mostly quartz; few clayballs and mostly without silt; some carbonized plant fragments and at least one small plant stem or woody plant fragment. #3 – Clay (95%) and silt to fine sand (5%) that is sub-angular to subrounded and mostly quartz; no clayballs; some carbonized plant fragments. #2 – Clay (85–90%) and silt to fine sand (10–15%) sub-angular to subrounded; silt grains mostly quartz but with at least one plagioclase feldspar; rounded to sub-angular clayballs of various colors to 9 mm diameter, some with and some without internal silt grains; many carbonized plant fragments and one small wood frag- ment; “Main bone layer.” #1 – Clay and mostly sub-angular to subrounded silt, mostly quartz; 80–90% clay and about 10–20% silt, no clayballs. Trench 3 #5 – Clay (90–95%) with black mineral laminations; 5–10% angular to subrounded quartz silt; less silt and smaller grains than main quarry layer; “fish layer.” #4 – Clay (95%) and silt to fine sand (5%) that is angular to subrounded and mostly quartz with at least one plagioclase feldspar; some light hematite mottling. #3 – Clay (90–95%) and silt to fine sand (5–10%) that is sub-angular to subrounded and mostly quartz; no clayballs; some carbonized plant fragments. #2 – Clay (90–95%) and silt to fine sand (5–10%) angular to subrounded; silt grains mostly quartz and minor amounts of plagioclase feldspar; carbonized plant fragments; one bone fragment. #1 – Clay (90–95%) and silt to fine sand (5–10%), angular to subrounded; silt grains quartz and minor amounts of plagioclase feldspar; abundant carbonized plant fragments; some clayballs; “Main bone layer.” 91 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Trench 4 #5 – Clay (90–95%) and silt to fine sand (5–10%); angular to subrounded quartz and minor amounts of rock fragments as silt grains; mottled but no laminations; “fish layer.” #4 – Clay (95%) and silt to fine sand (5%) that is angular to subrounded and mostly quartz with minor pla- gioclase feldspar grains; single carbonized plant fragment; mottled; no clayballs. #3 – Clay (95%) and silt to fine sand (5%) that is angular to rounded and mostly quartz; no clayballs; very rare carbonized plant fragments. #2 – Clay (90–95%) and silt to fine sand (5–10%) angular to rounded; silt grains mostly quartz; some carbon- ized plant fragments; one clayball; “Main bone layer.” #1 – Clay (85–90%) and silt to fine sand (10–15%), angular to subrounded; silt grains quartz and minor amounts plagioclase feldspar; no plant material; no clayballs. Trench 5 #5 – Clay (90–95%) and silt to fine sand (5–10%) with dark laminations; angular to subrounded quartz silt grains; “fish layer.” #4 – Clay (95%) and silt (5%) that is angular to subrounded and mostly quartz with minor plagioclase feldspar grains. #3 – Clay (95%) and silt to fine sand (5%) that is sub-angular to rounded and mostly quartz. #2 – Clay (90–95%) and silt to fine sand (5–10%) sub-angular to subrounded; silt grains mostly quartz with some minor amounts of plagioclase feldspar; some carbonized plant fragments. #1 – Clay (95%) and silt to fine and medium-fine sand (5%), sub-angular to subrounded; silt grains quartz and minor amounts plagioclase feldspar and rock fragments; some carbonized plant material; one clayball; “Main bone layer.” Pebbles (P-1 through P-5) Clay (90–95%) and silt (5–10%); silt grains angular to subrounded and mostly quartz; silt grains finer grained but more numerous than some in-place silty claystone; edges of pebbles same as center; dendrites growing in from some edges; P-5 is laminated but otherwise no different from the others. P-1 is somewhat laminated, too. All appear to be calcite cemented nodules, no chert nor rock fragments. 92 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 APPENDIX B – CORE DESCRIPTIONS Mygatt-Moore Quarry Drill Hole MM-1 June 25, 2013 Logged by J. Foster UTM: 12S 0670796E, 4340409N 105m ~WNW of 1000, 1000 quarry datum cap Top 107 cm soil 274 cm light gray claystone 25 cm darker gray claystone 107 cm light gray claystone 180 cm maroon to brick-red, slightly silty mudstone (some minor light gray mottling) 140 cm light gray to light green claystone 134 cm gray mudstone, becomes silty near base 173 cm deep red silty mudstone 36 cm light gray mudstone 33 cm silty gray and purple mudstone 127 cm light gray silty mudstone 13 cm deep red to purple and green-gray mottled and laminated siltstone 13 cm deep red mudstone 33 cm silty gray mudstone, with black and white (dentritic?) staining in bottom half 228 cm light gray silty mudstone 18 cm purple and light gray laminated siltstone (“fish layer”) 8 cm very dark gray, crumbly mudstone 8 cm light gray and purple mottled mudstone 84 cm light gray and light greenish-gray mudstone 38 cm light gray mudstone with abundant carbonized plant fragments (main quarry layer) 8 cm green mudstone 137 cm light gray to greenish-gray slightly silty mudstone 71 cm gray, silty mudstone 30 cm light gray and greenish-gray very fine grained sandstone Bottom Mygatt-Moore Quarry Drill Hole MM-2 June 25–26, 2013 Logged by J. Foster and H. Schoenstein UTM: 12S 0670809E, 4340274N 118m ~SW of 1000, 1000 quarry datum cap Top --- loose gravel 23 cm green and red mottled mudstone 61 cm maroon-red mudstone 93 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado–Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 20 cm green-gray mudstone 23 cm gray siltstone to very fine grained sandstone 25 cm dark gray mudstone 25 cm green-gray crumbly mudstone with yellow staining of the cracks 97 cm light gray claystone 41 cm dark reddish gray mudstone 145 cm light gray to light greenish-gray mudstone and siltstone 51 cm dark gray mudstone, some reddish mudstone near base 221 cm light greenish-gray mudstone, with golden-tan mineralization in cracks near the base 48 cm red and light greenish-gray mottled mudstone 183 cm dark red mudstone, with some light gray mottling 71 cm light greenish-gray mudstone 51 cm red mudstone mottled with light gray 129 cm light gray to white siltstone and slightly silty mudstone 30 cm green-gray mudstone 165 cm light gray to white, finely laminated silty mudstone with some very fine grained sandstone 8 cm light gray siltstone with thicker, slightly purple laminations (“fish layer”) 13 cm dark gray mudstone 8 cm light green siltstone, slightly laminated purple (“fish layer”) 33 cm light green mudstone 48 cm medium green mudstone 94 cm medium greenish-gray to gray mudstone with moderate to abundant carbonized plant fragments; some layers of light green, rounded clayballs to 5 mm diameter; dinosaur bone fragment in core at 16.5 m down (main quarry layer) 33 cm light green, soft mudstone (“clay layer”) 33 cm light green mudstone 38 cm medium green mudstone 15 cm medium green to reddish mudstone 299 cm medium to light green and maroon-red mottled, silty mudstone Bottom