Rise of the Erg—Paleontology and paleoenvironments of the Triassic-Jurassic transition in Northeastern Utah GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 3 2016 © 2016 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. RISE OF THE ERG—PALEONTOLOGY AND PALEOENVIRONMENTS OF THE TRIASSIC-JURASSIC TRANSITION IN NORTHEASTERN UTAH Brooks B. Britt, Daniel J. Chure, George F. Engelmann, and Jesse Dean Shumway A Field Guide Prepared For SOCIETY OF VERTEBRATE PALEONTOLOGY Annual Meeting, October 26 – 29, 2016 Grand America Hotel Salt Lake City, Utah, USA Pre-Meeting Field Trip October 23–25, 2016 GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Three articulated sphenosuchians from the Saints & Sinners Quarry, which is Stop 5 of this field trip. The voids of various colors are weathered concretion sites. i 2016 President Bill Loughlin bill@loughlinwater.com 435.649.4005 2016 President-Elect Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 2016 Program Chair Andrew Rupke andrewrupke@utah.gov 801.537.3366 2016 Treasurer Robert Ressetar rrgeology@gmail.com 801.949.3312 2016 Secretary Tom Nicolaysen tnicolaysen@utah.gov 801.538.5360 2016 Past-President Jason Blake blake-j@comcast.net 435.658.3423 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 2016-2018 Term Craig Morgan craigmorgan@utah.gov 801.422.3761 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 3 2016 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 James I. Kirkland (Editor-in-Chief) — Utah Geological Survey ReBecca Hunt-Foster — Bureau of Land Management Greg McDonald — Bureau of Land Management Martha Hayden — Utah Geological Survey Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 3 2016 1 ABSTRACT This field trip focuses on the Late Triassic-Early Jurassic transition in northeastern Utah. This tran- sition records one of the most striking terrestrial environmental transformations in the history of North America, wherein the fluvio-lacustrine Chinle Formation is transgressed by the vast erg system of the Nugget (Wingate+Navajo)/Navajo/Aztec Sandstones. Exposures in northeastern Utah are ideal for study- ing this transition as they are closely spaced and accessible. The uppermost Chinle Formation beds are lacustrine/fluvial fine-grained sediments which are overlain by increasingly drier, sandy, transitional beds. The non-eolian basal beds of the Nugget Sandstone preserve a Late Triassic ichnofauna, with some sites including Brachychirotherium tracks. Large-scale dune deposits comprise most of the Nugget Sandstone and contain vertebrate (Brasilichnium) tracks and a diverse invertebrate ichnofauna. Interdunal, carbonate, spring mounds, as much as 3 m tall, fed carbonate freshwater lake deposits containing gastropod body fossils and invertebrate ichnofossils. Another lacustrine deposit, located at the Saints & Sinners Quarry, is on the shoreline of a non-carbon- ate interdunal lake/oasis. Over 11,500 bones have been collected from the site and represent two thero- pod dinosaur taxa, sphenodonts, sphenosuchians, a pterosaur, and drepanosaurs (with many complete, three-dimensional, articulated skeletons). In addition to bones, dinosaur trackways are also preserved in shoreline and other interdunal beds. The fauna shows that this interdunal area of the Nugget Sandstone was the site of intense biological activity. The drepanosaurs are chronologically significant in that they are restricted globally to the Late Triassic, indicating that at least the lower one-fourth to one-third of the formation is Late Triassic in age. Rise of the Erg—Paleontology and Paleoenvironments of the Triassic-Jurassic Transition in Northeastern Utah Brooks B. Britt1, Daniel J. Chure2, George F. Engelmann3, and Jesse Dean Shumway1 1Museum of Paleontology and Department of Geological Sciences, Brigham Young University, Provo, UT, 84602; brooks_britt@byu.edu, jdshumway@byu.edu 2Dinosaur National Monument, Jensen, UT 84035; dan_chure@nps.gov 3Department of Geography and Geology, University of Nebraska at Omaha, Omaha, NE 68182; gengelmann@unomaha.edu Citation for this article. Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D., 2016, Rise of the erg—paleontology and paleoenvironments of the Triassic- Jurassic transition in northeastern Utah: Geology of the Intermountain West, v. 3, p. 1–32. © 2016 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. INTRODUCTION The Chinle-Nugget formational transition records one of the most dramatic continental environmental changes in the Phanerozoic of North America, where a fluvial and lacustrine environment is gradually replaced by an eolian-dominated environment represented by a vast erg that covered much of the western U.S. for mil- 2 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 lions of years (Blakey and Ranney, 2008). These formations along the south flank of the Uinta Mountains have received less attention than equivalent exposures farther south on the Colorado Plateau. Over the last decade, however, intensive work in the Chinle Formation by Randall Irmis (Utah Museum of Natural History), and at the Nugget Sandstone area of Dinosaur National Monument (DINO) by Dan Chure (National Park Service), George Engelmann (University of Ne- braska), and Brooks Britt (Brigham Young University) has shed new light on the Chinle-Nugget transition (Irmis and others, 2015) and the paleontology and pa- leoenvironments of the Nugget Sandstone (Chure and others, 2014a and 2014b). This field trip focuses on recent paleontological dis- coveries in the Nugget Sandstone that provide insights into the rise of the erg, its biota, and paleoenvironments. We have selected sites for their significance and to min- imize travel time so that more time can be spent on the outcrop and less time driving between stops. Several different names and ranks, as summarized by Irmis and others (2015), have been applied to the stratigraphic unit referred to here as the Nugget Sand- stone. Recently, this issue was addressed by Sprinkel and others (2011) and Irmis and others (2015), and both studies recommend the use of the Nugget Sand- stone. The Nugget Sandstone correlates with the Glen Canyon Group, the Wingate/Moenave, Kayenta, and Navajo strata (see references in Doelger, 1987, and anal- yses in Sprinkel and others, 2011). Sprinkel and others (2011) propose using the term Nugget Sandstone in ar- eas where the Kayenta Formation, a fluvial unit present between the Wingate and Navajo Sandstones farther south, is absent. The Nugget Sandstone spans the Triassic-Jurassic boundary, but the position of this boundary is uncer- tain (see references and discussions in Sprinkel and others, 2011; Irmis and others, 2015). With a dearth of body fossils and the absence of volcanic units, including ash, that could be radiometrically dated, there are no calibrated ages for the Nugget. Consequently, dating is limited to stratigraphic bracketing and correlation us- ing vertebrate trace fossils. The Nugget Sandstone rests conformably on the Late Triassic Chinle Formation (Irmis and others, 2015) whereas the marine Middle Jurassic Carmel Formation rests unconformably on top of the Nugget Sandstone. In the absence of age-specif- ic invertebrate fossils, dating of the Nugget is based on vertebrate tracks, with a Brachychirotherium suite in the lower portions of the formation indicating a Late Trias- sic age (Lockley and others, 1992) and the Early Jurassic Grallator, Otozoum, Eubrontes assemblage near the top (Lockley and others, 1992; Lockley, 2011). These traces, however, do not help resolve the position of the Trias- sic-Jurassic boundary within the formation. The Saints & Sinners Quarry fauna sheds additional light on these issues by corroborating the ichnofossil-derived Early Triassic age for the lower portion of the formation with body fossils because the quarry contains a drepanosaur and a sphenosuchian with a rod-like posterior expan- sion of the coracoid. Drepanosaurs are known only from the Carnian to Norian (Renesto and Binelli, 2006; Renesto and others, 2010) and sphenosuchians with such expansions are known only from the Late Triassic (James Clark, George Washington University, person- al communication, 2016). The Saints & Sinners Quarry horizon is 55 m above the base of the lowest contiguous eolian sandstone of the formation (figure 1A), indicat- ing that at least the lower 25% of the formation local- ly is no younger than Late Triassic in age. It should be noted that Molina-Garza and others (2003) conducted a paleomagnetic study of the underlying Chinle Forma- tion in our study area and concluded that the Gartra Member at the base of the Chinle was approximately 207–205 Ma (Rhaetian)—ages that are younger than in- dicated by vertebrate fossils from the lower one quarter of the Nugget Sandstone. Recent summaries of studies of the Chinle Forma- tion in, and adjacent to DINO can be found in Erick- son (2007) and Irmis and others (2015). In the study area, the Nugget Sandstone has received less attention than the Chinle Formation. Much of the work on the Nugget in this area has focused on nomenclature and correlation (Peterson, 1988, 1994; Sprinkel and others, 2011). Previously, little detailed work had been done on the paleontology and paleoenvironments throughout the Nugget, in part because it consists predominantly of eolian dunes, with a sparse fossil record composed pri- marily of dinosaur track sites (e.g., Lockley and others, 1992; Lockley and Hunt, 1995; Lockley, 2011). In 2006, 3 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 Chure and Engelmann began a systematic inventory of the paleontology and paleoenvironments of the Nugget Sandstone in DINO. This intensive study was designed to provide basic information that could be used to es- tablish baselines for resource protection, management, and research activities in the Monument (Chure and others, 2014a). This study yielded an unexpectedly rich fossil and paleoenvironmental record, including the discovery of the Saints & Sinners lagerstatten, providing an unprecedented view into the erg’s biota. Study Areas There are two major study areas for the Chinle and Nugget Formations reported in the present work (fig- ure 2). The first is in the western end of DINO around the nose of the Split Mountain anticline and adjacent lands managed by the Bureau of Land Management. The second is around Steinaker Reservoir north of Ver- nal, Utah, about 28 km northwest of the Quarry Visitor Center in DINO. Localities To protect the resources visited in the course of this field trip, we will not provide exact locality data for the sites in this publication, with the exception of field trip Stop 1 (the Cub Creek Petroglyph site) which is a designated stop (no. 14) on DINO’s “Guide to the Tilted Rocks.” This visi- tor stop is for viewing Fremont culture petroglyphs which feature a large lizard and Kokopelli (figure 3A), along with less defined rock art. Carmel Lo w er Ju ra ss ic Chinle U pp er T ria ss ic Formation ? M . J . < 20 0 m 5 5 m Dominant Environment N ug ge t S an ds to ne 2 1 3 6 5 m marine eolian �uvial-eolian �uvial in te rd un e interdune �at st ru ct ur el es s fo rm er d un es w ith c ro ss -b ed re lic s dunes 1 m A B 25 m SW of Quarry bonebeds la cu st rin e eolian interdune Figure 1. Nugget Sandstone stratigraphy and depositional environments in the vicinity of Saints & Sinners Quarry. (A) Sec- tion from the upper Chinle through the Carmel Formations showing the stratigraphic position of the interdune complex within the Nugget Sandstone and depositional environments. (B) Strata within the interdunal portion of the Nugget Sand- stone. The lacustrine units, which contain the three bonebed layers of the quarry are located near the bottom of the interdune unit (see figure 18A). 4 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 Safety Issues The stops on this trip are fairly safe to visit. Those acclimated to lower elevations may find hiking difficult and breathing labored due to the relatively high eleva- tion (~1.5 km/5000 ft). Temperatures in late October are comfortable, and heat should not be a problem. Characteristics of the stops are described briefly below. Stop 1. Cub Creek Petroglyphs: Elevation gain of ~50 m over 0.2 km (one way) hike, following, from road to exposure, the established trail at stop 14 of the Tour of the Tilted Rocks route in DINO. The steepest parts of the trail have rock steps. Stop 2. Cub Creek Chirothere Site: Negligible ele- vation change. 0.7 km hike (one way) following a drain- age floor. Stop 3. Josie’s Cabin: lunch stop. Stop 4. Big Mounds: Negligible elevation change. Hike approximately 0.7 km one-way across several drainages. Some mounds are near the sloping rim of a deep canyon and caution should be exercised. Stop 5. Saints & Sinners Quarry: Short easy hike with approximately 30 m of elevation loss from parking stop to quarry. The quarry is situated at the edge of a canyon and some drops are quite long. Observe caution and do not go to the edge of quarry as the sandstone is often slippery due to loose sand grains and especially lichens that are extremely slick when wet. Stop 6. The Museum of Paleontology at Brigham Young University (BYU), Provo, Utah. STOP 1. CUB CREEK PETROGLYPH SITE: THE CHINLE FORMATION – NUGGET SANDSTONE TRANSITION Stop 1 (figure 3) offers a close look at the base of the Nugget Sandstone, the contact with the underly- ing Chinle Formation, and transitional environments that here form the lower few meters of the Nugget. The Chinle is characterized by mudstone, siltstone, and sandstone deposited in fluvial-lacustrine environments (e.g., Irmis and others, 2015). Elsewhere in the western U.S., the Chinle is often fossiliferous, but body fossils are rare in northeastern Utah and most are fragmen- tary (Irmis and others, 2015). The overlying Nugget is dominated by approximately 200 m of eolian sandstone but includes an interval of a few meters of non-eolian sandstone and mudstone, and occasional, small eolian dunes at its base (figure 1). The thickness and lithologic characteristics of this interval vary from one location to another, even within the study area, so no single site can be considered as typical (Irmis and others, 2015). At the Cub Creek petroglyph site these non-eolian beds are visible and easily accessible. The petroglyphs (figure 3A) were pecked into desert varnish on sandstone cliff faces low in the eolian portion of the Nugget Sandstone. Following the trail northward along the base of the cliff leads to the fluvial unit of the basal Nugget, which consists of alternating tan sand- stone and brown-red mudstone beds (figure 3B and C). The slope at this locality is the purple interval at the top of the Chinle Formation (figure 3C, bottom right), which here is largely covered by colluvium. The upper- most Chinle is marked by an informally named “purple interval” that includes a distinctive, striped sandstone and grayish-red siltstone/mudstone layers that are ex- UTAH COU T 5 km Vernal Jensen DINO US 40 Gree n R ive r Figure 2. Nugget Sandstone exposures in the vicinity of Dinosaur National Monument. Mapped area indicated by the shaded area in the inset map of the state of Utah. Orange = Nugget Sandstone exposures. Green = Utah portion of Dinosaur National Monument. Modified from Irmis and others (2015). 5 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 posed at its top at Stop 1; Irmis and others (2015) place the boundary between the Chinle and Nugget at the top of these beds (figure 3B and C), just above the colluvi- um. The formation boundary is at the base of a channel sandstone that overlies the grayish-red mudstone, just above Irmis and others’ (2015) “red stripe sandstone” (figures 3C and 4). Branching, horizontally oriented, hemicylindrical features (downward-protruding ridg- es approximately 10 cm wide) stand out on the sole of the sandstone. These may be burrows or large, round- ed, desiccation cracks. Close inspection of the sole of the sandstone, especially of the branching features, re- veals many cylindrical burrows 5 to 8 mm in diameter that groove, or penetrate the surface (figure 5). These small traces can also be seen penetrating the sandstone in cross section. This lowest sandstone in the Nugget Sandstone varies in thickness over the extent of its out- crop at Stop 1, from as much as 0.5 m to as little as 10 cm. A thin to almost absent mudstone separates the basal sandstone from a thicker sandstone above. The mudstone has well-developed mudcracks, and the over- lying sandstone contains rip-up clasts of this mudstone. Above the mudstone in the Nugget is an interval of fluvial sandstones and red mudstones/siltstones ap- proximately 2 m thick. At the southern end of the out- crop, this interval consists of massive to cross-bedded sandstone beds 0.5 m or less in thickness, alternating with red, laminated mudstones and siltstones. Tracing Figure 3. Nugget Sandstone ex- posures at Stop 1, Cub Creek petroglyph site. (A) Petroglyphs chipped into desert varnish on the Nugget Sandstone cliff face. (B) Exposures as seen from the road at the petroglyph site. The lower slope is the upper Chinle Formation. The cliff is the lower part of the Nugget Sandstone. (C) Cliff base. This is the location of a measured section described in detail in Irmis and others (2015). The white dashed line marks the boundary between the Chinle and Nugget Formations as in- terpreted by Irmis and others (2015). 6 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 this interval along the outcrop to the north, the sand- stones and mudstones of the upper part thin and pinch out, whereas the basal sandstone of the interval thick- ens. At the northernmost end of the outcrop, this inter- val consists almost entirely of sandstone with only thin partings of mudstone. Above the fluvial interval just described, the cliff continues upward as uninterrupted sandstone. The low- er 1 to 2 m thick interval of the sandstone is fluvial, as indicated by small, low-angle cross-beds. This interval is overlain by the large-scale, high angle cross-beds of eolian sandstone that are typical of most of the overly- ing portions of the Nugget Sandstone. The contact be- tween these two facies is not pronounced, although the lowest 1 to 2 m of the eolian sandstone is marked by soft-sediment-deformation features. Based on sorting and grain size, it is likely that the fluvial sandstone beds were reworked from eolian dunes. Figure 4. Full section at Stop 1, Cub Creek petroglyph site. (A) Measured section including the top of the Chinle Formation and the base of the Nugget Sandstone (modified from Irmis and others, 2015). (B) Exposure where the section in (A) was measured. The white dashed line marks the formation boundary. The scale is 18 cm long. 7 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 Nowhere in the Chinle-Nugget section exposed at Stop 1, or elsewhere in the study area, is there evidence for an unconformity (Irmis and others, 2015). The transition from the fluvial-lacustrine environments of the Chinle to the dominantly eolian environments of the Nugget is gradual. In accord with Walther’s Law, stacked, conform- able facies were also coeval within the depositional basin. Thus, the purple interval of the top of the Chinle Forma- tion, the interbedded sandstones and mudstones of the basal Nugget Sandstone, and the eolianites of the balance of the formation represent different depositional facies that were laterally adjacent and graded into one another on a regional scale. The relatively wetter fluvial-lacustrine facies of the Chinle Formation graded laterally into the semi-arid fluvial system with intermittent, small dunes (not preserved at this stop) of the basal Nugget Sandstone which in turn graded downslope into the dune fields of the Nugget (May, 2014). STOP 2. CUB CREEK CHIROTHERE SITE: NON-EOLIAN ENVIRONMENTS IN THE BASAL NUGGET SANDSTONE Although generally thought of as an eolian unit, the basal part of the Nugget Sandstone was deposited primarily in fluvial environments (Sprinkel and oth- ers, 2011; May, 2014; Irmis and others, 2015). We have not measured a section at Stop 2, but here the fluvial base of the Nugget is approximately 10 m thick (figure 6), consisting of fine- to medium-grained, moderately well-sorted sandstone with small-scale cross-beds (fig- ure 7). Elsewhere, this same interval includes small eo- Figure 5. Partially bioturbated, large-scale desiccation crack, or possible burrow, at the base of the Nugget Sandstone, ex- tending into the underlying mudstone. The feature contains numerous small burrow traces in various orientations. Small divisions on left side of the scale equal 1 cm. 8 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 lian dunes (May, 2014). At Cub Creek, vertebrate tracks occur on two bed- ding planes separated vertically by about 10 cm (Ander- son and others, 2011; Anderson, 2013). The tracks occur over a total of 123 m2 of exposed bedding surface, with track density as high as 4.4 tracks/m2. Tracks occur as manus-pes pairs (hand-foot), but because of the density of the tracks and their poor preservation, few trackways can be identified. Most or all tracks are undertracks, of- ten poorly preserved with little morphological detail. Pes impressions are longer (as much as 30 cm) than wide and manus impressions wider than long (figure 8). Asymmetrical push-up rims are rare. Only one type of track is present at Stop 2, and poor preservation of the tracks makes assignment to an ich- notaxon problematic. The fact that all tracks are ma- nus-pes pairs indicates that the track-maker was a likely obligate quadruped. Lockley and Hunt (1995) identified well-preserved Brachychirotherium isp. from the base of the Nugget at the nearby Bourdette Draw tracksite (figure 9). This occurrence is significant because Brachy- chirotherium is regarded as diagnostic of Triassic age (Lucas and others, 2006). The pedal claw impressions in the tracks at Stop 2 show five digits of unequal length (figure 10). This is unlike the tridactyl pedal impressions of ornithopods and theropods. These cannot be prosau- ropod pedal impressions, which consist of four, elon- gate digits as opposed to five (Lockley and other, 1992). We feel confident that the Stop 2 tracks are chirothere tracks, but a more specific ichnotaxon assignment is not warranted. In addition to Stop 2, there are two oth- er chirothere sites within the monument with similarly Figure 6. Horizontally bedded, non-eolian basal beds of the Nugget Sandstone at Stop 2. The overlying cliff faces are com- posed of thick, large-scale cross-bedded eolianites. 9 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 preserved tracks. Both sites are within the fluvial beds in the lower, non-eolian portions of the Nugget Sandstone. STOP 3: LUNCH AT HISTORIC JOSIE MORRIS SETTLER CABIN, DINOSAUR NATIONAL MONUMENT Josie Morris was born in the latter part of the 19th century and raised in remote Browns Park, Utah, north of present-day DINO. This rugged upbringing gave her a strong sense of independence and resourcefulness. In 1913, Josie decided to homestead in the Cub Creek area where she built her own cabin and lived until she died of complications from a broken hip in 1963. She mar- ried five husbands and divorced four. She was a color- ful character, a friend of outlaws such as Butch Cassidy, and was tried and acquitted for cattle rustling when she was in her 60s (McClure, 1985). We will lunch at Josie’s homestead, which includes her cabin, chicken coop, cattle pond, fences, orchards, and shade trees. Her cab- in is open to the public, so wander in and step back for a few moments into the life of a pioneer and colorful character. STOP 4. LARGE CARBONATE MOUNDS: INTERDUNAL LAKES OF THE ERG SYSTEM Stop 4 shows some of the features of the interdunal carbonates that occur within the Nugget Sandstone in the study area. Nugget Carbonate Beds Carbonate beds make up part of interdunal facies at Figure 7. Small-scale cross-beds in the basal non-eolian section of the Nugget Sandstone. Scale bar in cm. 10 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 three different areas (figure 11) in the overall study area. These carbonate localities include exposures at Stop 4, as well as other nearby outcrops (figure 11, Area 1), a larger occurrence consisting of a layer along the rim of a mesa west of Steinaker Reservoir north of Vernal (fig- ure 11, Area 2), and exposures in a Nugget Sandstone hogback on the south flank of the Blue Mountain anti- cline cut by Cocklebur Draw (figure 11, Area 3). All the carbonate beds in the study area are of lim- ited lateral extent (<0.5 km along strike). Some of the beds include mound structures. All are dolomitic and incorporate sand-sized quartz grains. There are calcite infills within post-lithification fractures and cavities. Carbonate layers are exposed south of Cub Creek (figure 11, Area 1), along the rims and walls of canyons cut by a small drainage that flows into the Green Riv- er near where the Cub Creek Road (State Route 149) crosses the drainage. The carbonate layers are exposed along the rim on the north and south sides of the main, west-draining canyon, in the east and west walls of the westernmost tributary canyon that drains northward into the main canyon, and high on the divide between the tributary canyon just mentioned and the next can- yon to the east. The carbonate beds are not vertically stacked, that is, we have not observed carbonates repeat- ed through a significant vertical thickness of section. Figure 8. (A) Overview of chirothere trackway site at Stop 2. (B) Chirothere manus-pes pair showing proportions of impres- sions. Scale bar in cm. 11 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 The single exception consists of two carbonate hori- zons separated by < 2 m of sandstone. In the absence of horizontal marker beds and the laterally discontinuous nature of interdune horizons in the Nugget Sandstone, we cannot tell whether carbonates occur within a pref- erential horizon within the Nugget or are distributed through a greater portion of the section. This is true between and even within the carbonate exposure areas noted above. Carbonate beds exposed in the canyons south of Cub Creek occur as horizontal layers and as several mound structures. The carbonate layers vary in thick- ness from 10 to almost 100 cm. Beds within the car- bonate horizons range in thickness from 1 to 10 cm and are usually massive, but some contain fine, wavy lam- inae. At Stop 4, the carbonate mounds occur on both the north and south rims of the main canyon directly opposite each other. Large Mounds Carbonate beds are exposed along the north rim of the main canyon. The most prominent features of these carbonate units are mound structures, which are preserved and dissected to varying degrees (figure 12). There are at least five mounds. These mounds are relatively large, the largest being at least 10 m across Figure 9. Brachychirotherium pes print from basal beds of Nugget Sandstone in Bourdette Draw. Scale bar in cm. Figure 10. Deep chirothere pes im- pression at Stop 2 showing differing lengths of toes. Scale bar in cm. 12 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 at its greatest preserved extent. They are approximate- ly aligned, with a nearly west-northwest trend. These mounds and intervening horizontal layers extend more than 110 m along the canyon rim. The mounds are hemispherical in shape, and their peripheries merge contiguously with a horizontal car- bonate layer that continues to the next mound (figure 13). Where erosion has exposed the core of a mound, laminae are inclined away from the center. The core can also be seen as an area where the carbonate laminae are brecciated and separated by intruded sand and the un- derlying sand units rise up in the center of the mound (figure13B and C). Also visible in the area of the large mounds is the sandstone underlying the mound structures. The east- ernmost and largest of the mounds is perched on the canyon rim in such a way that it and the underlying sandstone are exposed in cross section through the mound core (figure 13B). This reveals soft-sediment deformation in the sandstone below the mound, in which reddish sandstone has intruded upward through light-colored, partly-consolidated sandstone into the base of the mound core. Soft-sediment deformation in the sandstone beneath the carbonate layer is also vis- ible to a depth of 10 m or more in the wall of a small, south-draining canyon that cuts across the trend of the mounds. The deformation consists of contorted beds and fractured and offset, partly consolidated sandstone layers. Parrish and Falcon-Lang (2007), Parrish and Dor- ney (2009), Parrish and others (2016) have described similar mound structures in the Navajo Sandstone near Moab, Utah. They interpret them as the result of upwell- ing of groundwater beneath the mounds, and this hy- pothesis fits the characteristics of mounds in Cub Creek Canyon equally well. Associated with the large mounds at Cub Creek are abundant Taenidium isp. invertebrate traces in the sand- stone immediately beneath the carbonate layer (Good, 2013, 2014). They are particularly well represented in exposures on the eastern periphery of the largest mound (figure 14). Here they consist of numerous, randomly Area 3 Area 1 Area 2 Vernal Cub Creek Rd US 40 Steinaker Reservoir U S 19 1 G re en R iv er 10 km NorthU T A H Figure 11. Interdune carbonate occurrences in the study area. The small polygons labeled Areas 1, 2, and 3 indicate where carbonates are exposed. Area 1 includes Stop 4 on this field trip. 13 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 oriented, tubular traces typically 1 to 2 mm in diameter. There are several larger meandering burrows, 10 to 12 mm in diameter, with meniscoid backfills. Carbonate beds extend 200 m eastward along the north rim. At the level of the mound structures, erosion has stripped away any in situ carbonate, but there are some large (~0.5 m), loose carbonate blocks and several smaller pieces that indicate it was present; at least as a layer and possibly including additional mound struc- tures. Over part of this same interval, a thin (10 to 20 cm) layer of horizontally bedded carbonate is present perhaps 2 to 3 m stratigraphically lower than the lev- el of the mounds. These horizontal beds often exhibit fine, wavy laminae and there is a single, small mound structure about 30 cm across and about 20 cm thick. Crinkled laminae in this small mound suggest that it is stromatolitic in origin. Small Mounds South and west across the main canyon from the large mounds there are carbonate beds at what is prob- ably the same stratigraphic level, including four mound structures (figure 15). These mounds are similar in morphology to those to the north, but are significantly smaller, the largest being about 2 m across. As with the mounds across the canyon to the north, these mounds are in line, oriented northwest-southeast, and about equally spaced, spanning a distance of about 8 m. The carbonate mounds pinch out or are lost to erosion to the east, but a horizontally bedded carbonate layer connects Figure 12. Carbonate exposures at Stop 4. (A) View is to the south of carbonate mounds exposed along the north rim of a canyon beyond (not visible). Arrows indicate carbonate mounds. (B) View to the east of the same area showing the same mounds. 14 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 to the mound structures along a persistent outcrop to the north and west where it is usually 10 to 25 cm thick. Gastropods occur as external molds within the hori- zontally bedded carbonate layer, along exposures 20 to 40 m north and west of the mound structures (figure 16). These fossils occur at a wide range of angles to bed- ding. Patchy exposures and float show that the carbon- ate layer extended farther north and west to, and a short distance along, the south rim of the main canyon (figure 17). These last exposures along the rim are nearly 1 m thick in places, and show evidence of disturbance and brecciation. Carbonates are also exposed in the walls of the westernmost canyon that empties northward into the main canyon of this small drainage system. They occur as horizontal layers 10 to 20 cm thick within associated interdunal facies that are as much as 2 m thick. STOP 5. THE SAINTS & SINNERS QUARRY—A LAGERSTATTEN IN THE ERG The Saints & Sinners locality (figures 1 and 18) pre- serves a wealth of bones, tridactyl theropod tracks, and an isolated vertebrate burrow, all about 55 m above the base of the eolian portion of the Nugget Sandstone. This site is BYU locality 1442, and coordinates are in the files of the BYU Museum of Paleontology. This is the most Figure 13. Large carbonate mound structures at Stop 4. (A) Gray-weathering carbonates of mound draped over white and red sandstone that is higher at the center of the mound. The inclination and thinning of the carbonates away from the center is also apparent. (B) Mound dissected by erosion, viewed from below. The cross section exposes apparent soft-sediment de- formation in the underlying red and white sandstones. (C) Carbonate in the core of a dissected mound structure separated and surrounded by sandstone. Divisions on left of scale equal 1 cm. 15 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 Figure 14. Trace fossils in sandstone immediately beneath a carbonate layer peripheral to one of the large mound structures at Stop 4. Traces of varying size and orientation include burrows that exhibit meniscoid backfill structure and other possible burrows or root traces. Divisions on the scale equal 1 cm. Figure 15. Small mounds (arrows) in carbonate bed exposed along the rim on the south side of a canyon at Stop 4. Although smaller in scale, these features are similar in all respects to the large mounds on the north side of the canyon. 16 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 prolific body-fossil-producing site of any of the Late Triassic to Early Jurassic sand ergs in terms of number of bones, number of individuals, and taxonomic diver- sity. The site was first excavated in 2009 and continues to be actively worked. To date, over 11,500 specimens (articulated skeletons, bones, and partial bones) have Figure 16. Gastropods preserved as external molds in carbonates on the south rim of the canyon a short distance from the small carbonate mounds at Stop 4. (A) Carbonate slab with numerous gastropod molds of varied size and orientation. (B) Silicone rubber cast of a gastropod (mold of a natural external mold) from carbonates along the south rim of the canyon. Most of the gastropods have this high-spired morphology. Figure 17. Carbonate exposures along the south rim of the canyon as seen from Stop 4. The small carbonate mounds and the location of gastropod fossils are indicated by arrows. 17 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 been prepared with thousands more awaiting prepa- ration. All preparation is done by BYU students at the university’s Museum of Paleontology, which will be the last stop of our trip.  The vertebrate fauna (table 1) consists of two sphe- nosuchian genera, a drepanosaur, two theropod genera (a coelophysoid and a medium-sized taxon of unknown affinity), and a dimorphodontid pterosaur. An informal tally (many specimens are not yet in the database) indi- cates there are at least 76 individuals, 41 of the smaller sphenosuchian and 20 of the coelophysoid. The Saints & Sinners Quarry, BYU locality 1442 (figures 1 and 18A), is located along a flank of the Sec- tion Ridge anticline, a Laramide structure that strikes southwest to northeast in northeastern Utah, along the south flank of the Uinta Mountains uplift. The anticline deforms and exposes Paleozoic to Cretaceous strata. The resistant Nugget Sandstone forms a marked cuesta above the strike valley of the Chinle and Moenkopi For- mations. A series of small canyons, developed in fault zones, breach the cliff-forming Nugget cuesta, which locally dips about 20° to the southwest. The Saints & Sinners Quarry is on about a 4 by 10 m shelf at the top of the cliff wall of one of these canyons (figure 18A). All of the shelf is bone bearing, but only 19 m2 have been excavated (figure 19A). This stratigraphic interval is traceable to the southwest of the quarry into a small al- cove bounded by small cliffs (figure 18A). Although no bones are present in those cliffs, they provide exposures crucial to understanding the depositional environment of the quarry and our detailed stratigraphic section (fig- ure 1) was measured in the alcove (figure 18A). From our vehicles we will walk to the rim of the cuesta to look down at the Chinle Formation and the lower Nugget Sandstone at the base of the cliff that conformably over- lies the Chinle Formation. From the overlook we will walk into the quarry to observe its various units, then down into the alcove to put the bone-bearing unit into context with under- and overlying strata. The Quarry Discovery The site of what is now known as the Saints & Sinners Quarry was first seen by two of us (Dan and George) on July 8, 2007. We were wrapping up a long, hot day of prospecting for fossils and interdunal deposits in the Nugget Sandstone. Looking north across a canyon, we spied a vertical outcrop on the opposing canyon wall containing well-defined interdunal deposits (figure 18A, alcove). As it was the last day of the field season, we decided to visit the outcrop the following year. In Taxon, high Taxon, mid Taxon, low Size, approx. Common name Notes # In di vi du al s Body fossils Plantae Gymnospermophyta Cycadeoidophyta cycadeoid bennettitalian "cycad" isolated frond, rhachi, petioles Animalia Sauroposida "Reptilia" incerta sedis drepanosaurid 40-cm-long bird-like head, digging arms articulated/associated/disarticulated >5 Lepidosauromorpha sphenodontian A, normal-jawed 30-cm-long tuatara-like "lizard" isolated jaw elements 7 Lepidosauromorpha sphenodontian B, slender-jawed 30-cm-long tuatara-like "lizard" isolated jaw elements Crocodylomorpha sphenosuchian A, primitive 20 to 50-cm-long crocodylomorph, terrestrial articulated/associated/disarticulated >41 Crocodylomorpha sphenosuchian B, large 1.5-m-long crocodylomorph, terrestrial braincase, dermal ossicles 1 Pterosauria dimorphodontid 1.5-m-wingspan pterosaur single individual, partial skull + phalanx 1 Dinosauria, Theropoda, coelophysoid 1.5 to 3-m-long predatory dinosaur disarticulated 20 Dinosauria, Theropoda, medium-sized 7-m-long predatory dinosaur teeth, partial vertebrae 1 Trace fossils Invertebrata Skolithos < 8 mm diameter invertebrate burrows in dune facies Planolites < 8 mm diameter invertebrate burrows in dune facies Vertebrata Grallator ~15-cm-long small tridactyle tracks only in/on crinkly beds? Burrow at toe of dune to beach 15 cm diameter x 1.5 m vertebrate burrow single occurrence Table 1. Faunal and floral list from the Saints & Sinners Quarry (mm = millimeters, cm = centimeters, and m = meters). 18 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 June 2008, we, along with intern, Josh Finkelstein, hiked back to the location and worked our way around the up- per canyon to get to the interdunal facies in the vertical wall of the alcove. As we crossed what is now the west- ern part of the Saints & Sinners Quarry, George spied grooves in the sandstone and called out to Dan, who was farther ahead, that it looked like there were tool marks similar to those we had seen at the Cub Creek petroglyph site. At Cub Creek, grooves in the sandstone at the base of the cliff had been made by native Amer- Figure 18 caption on following page. 19 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 icans sharpening tools on the sandstone. In the alcove we examined the interdunal layers, and Dan found a tri- dactyl track. On our way out of the alcove, we checked out the groove and noticed it didn’t look right. Dan rec- ognized it as the external mold of a small tibia. Looking around, we found more external molds of bones, some with traces of bone still in them, and saw short strings of small vertebrae nearby. We followed that surface to the east, continuing to see scattered bone. When we got to the main surface where the quarry is, there was abun- dant and identifiable bone everywhere (figure 19B). We were elated with the discovery, and our excitement was obvious as we tried to assess what we had found. Josh, not being familiar with paleontologists, thought we were putting him on, on his first day in the field. In fact, it turned out to be the find of our (DJC and GFE) lives.   Collecting Following the discovery of bones in 2008, Chure and Engelmann contacted the BYU Museum of Pale- ontology to see if it was interested in collaborating with collecting and researching the site. After a visit to the site by Rodney Scheetz, BYU agreed. A permit was ap- plied for and obtained for the 2009 season. The site is unusual compared to other areas we have worked, with numerous, small, fragile bones on the surface embed- ded in structureless sandstone, largely lacking fractures. The bones are three-dimensional, with no crushing, which we attribute to burial in clean sands (now sand- stone) that does not compress because of grain-to-grain contact. The sandstone matrix is generally poorly ce- mented. The bones are not permineralized, and many can be scratched or pulverized with a fingernail. The softness of the bones is evident from the many external molds on the tops of the bonebed; the bone is usually more easily eroded than the sandstone. Fragile bones were stabilized with Vinac B-17 (polyvinyl acetate) in a low-viscosity acetone solution. Because the bone-bearing layers are low in relief, we created a grid of 1-m squares with spray paint (fig- ure 19A). Painted lines are short-lived, however, so the intersections of the easting and northing lines were marked with large washers engraved with grid coordi- nates and affixed with concrete nails (figure 19B to F).  Initially, not knowing how abundant the bones were, we made silicone rubber casts in the external molds. Our first season of collecting involved recovery of cob- ble-to-small-boulder-sized fragments of the bone-bear- ing units that had fallen downslope, and loose blocks of in situ sandstone. We also used air scribes to remove individual bones (figure 20A). Working in the field with air scribes is painfully slow, expensive to employ, and Figure 18 (on previous page). Saints & Sinners area overview, depositional environments, and facies. (A) Overview of quarry area with interdune strata bounded by solid arrowheads, sandwiched between prominently cross-bedded sandstones rep- resenting dunes. Bars with “L” = lacustrine bed, which pinch out to the right (east) at the shoreline arrow and to the north, but the latter terminus is covered. The lacustrine beds are underlain by interdune flat deposits, indicated with a ball and bar, consisting primarily of cm-scale dune and ripple sandstone (B, E). The position of the measured section (figure 1) is indicat- ed by elongate rectangle in the upper left. (B) Site-typical interdunal flat, lacustrine, and bioturbated dune facies sequence. The lacustrine facies consists of stacks of couplets of structureless sandstones overlain by green silty clays. Tracks are com- mon on the top of the lacustrine facies. Individual digits (toe infillings) are sometimes visible when the undersurface of the tracks are exposed, confirming these are tracks. (C) Oblique view of Grallator isp. Tracks on the upper surface of a lacustrine couplet. The mm-thick clay at the top of the couplet has been eroded, but the clay provided the parting plane between the two bounding sandy beds. The 3-cm-thick bed overlying the tracks is a typical wrinkly facies bed, interpreted as a silty sand interdune flat layer that was once covered by a biofilm. (D) Several interdune flat wrinkly units exposed in plan view show- ing a Grallator track in the upper left and ridges in bottom right. The ridges are interpreted as pressure ridges developed as evaporites accumulated just below the surface in the Triassic interdune. (E) Detail of microdune playa flat deposit in cross section. The thin trough cross-bed sets are interpreted as microdunes (large sand ripples) migrating across the interdune. The teepee structure (upper right) may be related to curled biofilm edges and/or evaporite-related pressure ridges (Pakzad and Kulke, 2007). (F) Possible vertebrate burrow, 15 cm in diameter in the toe of a dune along the shoreline of the lake. Moderate bioturbation has destroyed all trough cross-bedding in the dune except the larger, cm-scale trough cross-bed sets. 20 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 Figure 19 caption on following page. 21 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 too dependent on weather. This quickly necessitated a shift to creating moderate-sized blocks that could be prepared in the lab.  The bones occur in three massive sandstone beds separated from each other by parting planes developed along mm-thick silty clay layers (figure 20E). At first we and the crew leader, Jeff Higgerson, worked slowly to enhance the few natural cracks to break out individu- al blocks from each bonebed stratum. However, cracks were few and some blocks defined by natural cracks were very large. Later, a demolition saw (figure 20C) was employed to cut out blocks, but the bonebeds are 25 cm thick, which was too thick to cut through with the diamond blade. We experimented with a standard pressure washer (figure 20B) and succeeded in cutting through 30 cm of sandstone, but the process was messy and the cut relatively thick and uneven. The most ef- fective cutting tool has proven to be a concrete-cutting chain saw, which can penetrate up to 60 cm in a single cut (figure 20D). The extraction system we have settled on is as follows. A block is outlined with a timber crayon, the initial guide cut made with a demolition saw (figure 20C), and the ultimate cut made with a concrete-cut- ting chainsaw (figure 20D). We have honed this method to the degree that several workers can cut, extract, and load 544 kg of sandstone blocks in a few hours by either winching the blocks upslope to a lift-gate truck (figure 20G) or driving a truck with a bed-mounted crane into the quarry. Quarry Geology Stratigraphy In the vicinity of the quarry, the true thickness of the Nugget Sandstone is difficult to measure because of the dip of the beds and the fact that it is an incomplete section. We estimate the thickness in the area to be no more, and probably substantially less, than 200 m, with the quarry being about 55 m from the base of the upper, eolian part of the formation (figure 1A). The quarry is positioned near the bottom of a large wedge-shaped unit (figure 18A, with the upper and lower boundaries indicated by solid arrowheads) dom- inated by structureless sandstone along with a stra- tum consisting of cm-scale sandstone layers separated by mm-scale mud drapes, all related to the interdunal environment (figure 18B). The maximum area within which the wedge is exposed is 320 m north-to-south and 50 m east-to-west. The wedge becomes untraceable at its north and south ends in canyons developed along faults. To the west it is obscured where it dips into the subsurface and the eastern boundary has been lost to erosion. The maximum thickness of this wedge is 10 m at its south end and it gradually thins to less than 1 m Figure 19 (on previous page). Saints & Sinners Quarry map and fossils. (A) Most of the quarry map as of mid-2016 showing the positions of over 11,500 bones. For numerical perspective, the upper right block (B84 outlined by a red rectangle) alone yielded 827 bones from the uppermost horizon, bonebed 3. Horizontal bone distribution is non-uniform, as exemplified by B84. Blocks with no bone have not been prepared. Although there are three superimposed bone layers, each approximately 25 cm thick, at the time this map was made only one layer had been prepared for most areas of the quarry. (B) Coelophysoid theropod dorsal vertebra in transverse section exemplifying how bones look as exposed by erosion. (C) Coelophysoid left maxilla in medial view after preparation showing the excellent, uncrushed nature of bones from the locality. (D) Naturally eroding bones (white) of a small, articulated sphenosuchian near the bottom of bonebed 2 horizon. Weathering reveals sub- mm-thick, irregular laminae not apparent in fresh exposures. (E) Associated and somewhat broken pelvis and hind limbs of one of the smaller coelophysoids from the top of bonebed 2. These were the only bones preserved in this area of the quarry, making it clear that they pertain to a single individual. In other areas bones of multiple individuals are mixed together, mak- ing it difficult to discern individuals. (F) Three articulated sphenosuchians from middle to top of bonebed 2. This cluster suggests they died in a burrow during estivation but at least five other specimens of the same size were found in the same area in different orientations and there was no evidence of a burrow. The voids of various colors are weathered concretion sites. (G) Plant debris covered with efflorescent, puffy evaporites on modern playa flat bordered by, and overlain by dunes in Utah’s West Desert. (H) Top of bonebed 2 with what appear to be sandstone casts of plant debris based on their straightness and bundled morphology. Compare with G, to the left. 22 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 thick over some 200 m to the north. The wedge contin- ues another 120 m in that direction before being termi- nated by a fault. The wedge also thins eastward. Above and below this wedge-shaped stratum is sandstone with Figure 20. Collecting methods. (A) Air scribes were used for the first season before we realized the bones were abundant and often closely spaced. (B) A high pressure washer with a stream nozzle was used successfully to cut to depths of over 30 cm but the cut was wide (15 mm) and rough. (C) A demolition saw is used to make the guide cuts after block locations are demarcated with timber crayons and the block numbers marked on the sandstone. We often followed pre-existing joints for the cuts. (D) A concrete chain saw with a potential cut depth of 60 cm is used to cut through up to two bonebeds simultaneously. Water to cool the chain is sourced from standard barrel equipped with a gas-powered pump. (E) Cut blocks ready to collect showing the three discreet bonebeds separated by mm-thick silty clay beds. Quarry grid lines marked in orange spray paint. (F) Grid coordinates are engraved on washers at grid intersections held in place by concrete nails. (G) A truck-mounted crane is used to lift and load larger blocks. 23 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 high-angle cross-beds, a facies that overwhelmingly dominates the Nugget Sandstone and represents dune fields (figure 18A). Interdune Strata The three bone-bearing strata (figure 20E) consti- tute only part of a complex of strata deposited in an interdunal flat that at times included a lake (Shumway and Britt, 2015, 2016). The base of the interdunal wedge is marked by a planar deflation surface on which there are centime- ter-scale, cross-bedded and convolute sandstone beds, whose contacts are accentuated by post-depositional hematite laminae (figure 18E). These sandstones rep- resent alternately dry to damp (Kocurek and Fielder, 1982) interdunal flats, sometimes with biofilm-covered surfaces (Hagadorn and Bottjer, 1993; Eisenberg, 2003). The small-scale, trough cross-bedding represents small dunes (Ahlbrandt and Fryberger, 1981). This interdunal flat stratum extends across the entire base of the inter- dunal wedge, and to the north, it is the only facies of the wedge preserved. Largely structureless sandstone beds cm to dm thick and overlain by green, silty mudstones (figure 18B) are indicative of a flooded interdune (Ry- ang and Chough, 1997), and were deposited in a small lake. The lacustrine units pinch out only 30 m north of the southern end of the wedge. It is on the northeastern shoreline of this lake that the bones are preserved. At the north end of the lake, cm-scale beds exhib- iting wavy textures in cross section, a wrinkly surface, and pressure ridges in plan view (figure 18D) some- times overlie lacustrine, clay-sand couplets. Some of these wrinkly beds preserve Grallator tracks. Often, these beds exhibit pressure ridges, and alternate with the finer lacustrine beds (figure 18). These beds repre- sent short-lived interdune flats sometimes blanketed with biofilms or biomats that developed as the lake level dropped (Hagadorn and Bottjer, 1993; Eisenberg, 2003). These organic-rich beds are thought to play a role in the preservation of vertebrate tracks (Carvalho and others, 2013). The pressure ridges formed as the surface dried and growth of evaporite crystals increased the surface area of the bed surface (Pakzad and Kulke, 2007). Overlying the uppermost lacustrine unit are struc- tureless sandstone beds with some relict cross-bedding and sporadic deflation surfaces (figure 18A, 18B, and 18C) that are now accentuated by post-depositional he- matite. This unit is as much as 7 m thick and represents eolian dunes that migrated across the flats, covering the shoreline and what was left of the lake. These dune sands were often wetted by a high water table. The moist sands encouraged vegetation and burrowing invertebrates, ul- timately resulting in the structureless texture (Thomas, 1984; Loope and Rowe, 2003). The intense burrowing all but destroyed the trough cross-bedding. On top of these structureless sandstone beds rest thick, high-an- gle, trough cross-bedded sandstone representing struc- turally intact (not bioturbated) dunes, marking the end of the local interdunal phase (figure 18A). The interdune story is summed up as follows: (1) deflation and development of an interdunal flat with de- flation being controlled by the water table – specifically by the capillary zone above the water table, (2) depo- sition of ~1 m of sand and silt on the interdunal flat in the form of thin layers of sand via microdunes/sand ripples (some trapped by moist ground) with minor de- velopment of biofilms, (3) flooding of the southern part of the interdunal flat, and development of a lake, with fluctuating lake and shoreline levels, (4) development of cm-scale sand/biofilm layers along the lake shore- line resulting in the preservation of theropod tracks, (5) partial drying of the lake and dune migration across the interdunal flat (these dunes are bioturbated by inverte- brates during damp phases), and (6) complete burial of the interdunal flat by large dunes. The bonebeds are part of the lacustrine phase and are described and interpret- ed below. The Quarry Bonebeds Description The quarry is small, with only 19 m2 (figure 19A) excavated to date, and in most areas only one of the three bone-bearing layers has been collected. The quar- ry is located in the lacustrine facies, along the paleo- shoreline, which consists of generally structureless sandstones separated by green silty clays as described above. In the main area of the quarry, there are three su- perposed bone-bearing strata (figures 1B and 20E). The 24 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 sediments are often mottled, with light, off-white blebs among a more pervasive tan matrix. This mottling may be a function of invertebrate bioturbation. All three lay- ers are sandstone that appear to be essentially devoid of sedimentary structure, except for sub-mm-thick green clay layers. Deep weathering, however, reveals thin, ir- regular sandstone laminae that may represent relict rip- ple laminae, near the bases of bonebeds 2 and 3 (figure 19D). It is the thicker or closely spaced mud drapes that on weathering separate the three layers and permit their extraction (figure 20E). Bones in the quarry are not evenly distributed, neither vertically nor horizon- tally (figure 19A). Vertically, there is a high predictive value from the level of occurrence to both the size of bones and degree of articulation. The articulated small skeletons are found on the very top of bonebed 1 (the lowest bone layer) and the bottom to middle of bonebed 2. Bones of larger taxa are common upward from the middle of bonebed 2 and abundant in bonebed 3 (fig- ure 19A). The orientation of bones/skeletons ranges from roughly random, from the bottom of bonebed 1 to bonebed 2 (figure 20E), to substantially oriented in bonebed 3 (Britt and others, 2011; Chambers and oth- ers, 2011). Interpretation The three nearly structureless sandstone beds that contain bone are interpreted to represent sand blown into standing water during haboobs or local wind storms. The capping green clay laminae on top of each sandstone bed marks a quiescence that allowed silt and clays to settle. The large number of animals and the taxonomic diversity of the fauna suggest a mass die- off, likely during drought. The animals may have gath- ered from across the interdunal flat to the last oasis and died because the waters were not potable, or the oasis completely dried up. In the drought, the carcasses may have dried and littered the landscape. Then when the lake level rose, the small carcasses of the drepanosaurs and sphenosuchians were buried by wave action or by sands blown in during a haboob. Larger carcasses were not immediately buried, and macerated in the shallows, where wave action reworked them, oriented them, and finally largely buried them. The increased orientation of bones in the uppermost bonebed, which consists of the disarticulated bones of the coelophysoid, suggests substantial reworking but only very minor transport by wave action. The Quarry Flora and Fauna Flora The only plant fossils identifiable to a reasonable de- gree are extremely faint external molds of compressed cycadeoid fronds (William Tidwell, BYU, personal communication, 2012). Usually only the rachis or pet- iole is found but one specimen preserves pinnae (leaf- lets) that extend at right angles from the rachis.  We attribute the dearth of plants to postdeposition- al oxidizing groundwaters and/or alkaline waters. The latter is common in desert environments and can break down plant tissues before or soon after burial (Retal- lack, 2001). Despite the rarity of plant fossils, it is likely that the interdune supported a substantial floral mass and that growing conditions were favorable for relative- ly long periods. The same conditions that destroyed the plants could have destroyed organic components of the bones, resulting in their “bleached” white color. The ab- sence of relic organics explains, in part, the friable con- dition of the bones.   Fauna Invertebrate: No invertebrate body fossils have been observed within the quarry nor in the vicinity of the quarry. Whereas this could be a preservational bias, the absence of invertebrate trace fossils in the lacustrine de- posits suggests that no complex invertebrates of any size resided in sediments at the bottom of the lake. The abun- dance of small vertebrates that were probably at least partly insectivorous, along with the occurrence of Sko- lithos and Planolites traces throughout the dune sands where slipface surfaces are well exposed, indicate that invertebrates were common. The near complete disrup- tion of sedimentary structure by bioturbation of dunes above and lateral to the lake indicates that invertebrates thrived in such environments in the interdune area. The mottled sediments in the bonebeds is best interpret- ed as evidence of moderate bioturbation by invertebrates. 25 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3  Vertebrate: Aside from the sheer number of indi- viduals (>76), the most surprising aspect of the site is the faunal diversity. The minimum number of individ- uals (MNI) presented here is a conservative estimate because of a backlog of about 9000 specimens that have yet to be entered into the database. The estimates for MNI are most accurate for the larger taxa, the bones of which were the first recovered and entered into the database. We recognize eight vertebrate taxa at Saints & Sinners, which are enumerated in table 1 and briefly described here. This diversity is far greater than at any other site in the Triassic to Jurassic ergs of the American Southwest. Most, if not all, of these taxa are new and most are now in various stages of study, preparatory to their formal descriptions. Sphenosuchians – The most common taxon in the quarry is a crocodylomorph, sphenosuchian A. Over 41 individuals are present based on articulated and as- sociated skeletons. The taxon is represented by a range of sizes, with skulls ranging from 25 to >50 mm and body lengths to >500 mm. Many, if not all, are juveniles. Articulated, complete and partial skeletons are appar- ently randomly distributed between bonebeds 1 and 2, and about halfway up into bonebed 2 (figure 20E). Some skeletons occur in discrete clusters, with one clus- ter consisting of three overlapping skeletons all facing the same direction (figure 19F), suggesting they may have died in an estivation burrow. The armor is simple, limited to the parasagittal region, and dermal plates are unornamented save for a single ridge. The greatly elon- gated legs (figure 19F), a common feature in sphenosu- chians, indicate it was fleet of foot, an advantage in the open spaces of the desert environs.   A second crocodylomorph, sphenosuchian B, is represented solely by a partial braincase and a couple of scutes of a 1.5-m-long individual(s). This may simply be an adult of sphenosuchian A but the morphology of the skull differs more than we expect for a single taxon.   Sphenodonts – Based solely on mandibular rami, there are two sphenodontian genera, one form with a mandible more typical for the group, and one other with a more gracile mandible, both similar in size. Unlike the other small taxa in the fauna, no articulated specimens have been recovered. Counting both forms, only six in- dividuals have been recovered.   Drepanosaur – Numerous individuals are present including at least six articulated individuals of varying degrees of completeness plus numerous other partial skeletons (Engelmann and others, 2012, 2013; Chure and others, 2013, 2015). All portions of the skeletons save for the last half of the tail are known. The head is bird-like with large orbits and an expanded braincase; the arms and shoulder are adapted to scratch digging with a hypertrophied ungual on digit II (Chure and oth- ers, 2015); the chest is barrel-like; and proximal caudal vertebrae have bifid chevrons. There appear to be two morphs, distinguished primarily by differences of the pes, which we attribute to sexual dimorphism. The teeth are rectangular in cross section and high-crowned. The Saints & Sinners taxon differs from Drepanosaurus in a number of characters, including elongate straight un- guals on digits III, IV, V of the manus; slender and thin digit I; an opposable hallux on the male morph; unex- panded distal end of the scapula; and more. Theropods – There are two distinct theropods, a coelophysoid and much larger taxon. The larger taxon is represented only by few shed, robust, tooth crowns up to 40 mm long, suggesting a body length of approximate- ly 7 m. The coelophysoid MNI is 20, with skulls rang- ing from 100 to 350 mm long and an estimated maxi- mum body length of 3 m. Overall the taxon is similar to Coelophysis in build and morphology (Britt and others, 2010). A diagnostic difference is seen in that the shaft of metatarsal II is reduced to a splint, but with a function- al distal articulation. No specimen is articulated, aside from short strings of vertebrae and partial limbs, but the skulls are typically closely associated and the skeletons of several individuals can be discerned based on relat- ed elements of bone pertaining to exceptionally large or small individuals. The large number of bones of inter- mediate sizes are not easily differentiated into individu- als because the bones are so common and closely spaced.   Pterosaur – A dimorphodontid pterosaur is rep- resented by a single individual consisting of a partial- ly articulated and closely associated skull and a single 26 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 postcranial element, the distalmost wing phalanx (Britt and other, 2015a, 2015b). Most of the skull is known, and it shares a number of diagnostic characters with Dimorphodon (e.g., supernumerary small teeth in the mandible along with two rostral fangs, needle-like na- sal process in lateral view), but there are substantial dif- ferences in the Saints & Sinners taxon (e.g., large and bladiform maxillary teeth and a small orbit). Triassic pterosaurs are extraordinarily rare, with only 27 report- ed finds worldwide, and the majority of the finds consist of isolated elements, all of which are from marine or lacustrine deposits. The discovery of a desert-dwelling, Triassic pterosaur was quite unexpected.   Vertebrate trace fossils: The quarry horizon is bor- dered by a ledge that protrudes from a ledge/cliff along one side and sandstone hills or ledges on the other three sides. On one of these sides, erosion has been sufficient to show the toe of a dune interfingered with the shore- line lacustrine facies of the bonebeds. Thus, at least on one side of the lake, dunes migrated into the lake. The toe of this dune preserves several relics of larger slip-face laminae, but is otherwise nearly structureless. A gently curved, cylindrical trace (figure 18F), about 15 cm in diameter and about 1.5 m long, follows the slope of the slipface down toward the lake margin. There are no pos- itively discernable scratch or other marks nor branches on the surface of the trace, and it does not branch. Simi- lar burrows in other parts of the erg have been interpret- ed to have been made by reptiles or synapsids (Odier and others, 2004, 2007; Hasiotis and others, 2007). Such burrows indicate the sand was moist when burrowed (Loope, 2006). Wilkens (2008), however, argued that the branching traces described by those authors were made by plant roots, not vertebrates. The 15-cm-diam- eter trace at Saints & Sinners does not branch, and we attribute it to a relatively small, unknown vertebrate. Interpretation of Fossils The diversity of fossils at Saints & Sinners Quarry is unexpectedly diverse for a desert-dwelling biota. Else- where in the Nugget Sandstone, and at most fossil lo- calities in the balance of the Late Triassic to Early Juras- sic sand erg, ichnofossils are the only fossils, with body fossils being extraordinarily rare, and those occurring as isolated specimens (Engelmann and others, 2016). Happily, the Saints & Sinners, where bones are abun- dant and well preserved, representing eight taxa, is an exception to this general characterization. Little can be said about the flora as it contains only poorly preserved compressions, molds, and casts. How- ever, the presence of a cycadeoid and the sizes of the molds and casts of unknown plant parts indicate the area provided adequate growing conditions for at least years to decades. In addition to vascular plants, the biofilm covering wet to damp interdune surfaces likely consisted of photosynthesizing and non-photosynthe- sizing microbes, and played a role in vertebrate track preservation. Invertebrates are represented only by trace fossils assignable to Skolithos and Planolites, and these are only discernible immediately below and in the lightly bioturbated dune sands east of the lake, overlying the interdune wedge. Intense bioturbation by invertebrates, however, is responsible for the structureless sandstones, indicating favorable conditions for a number of inver- tebrates. The diversity of the vertebrate fauna indicates hab- itable conditions (abundant food in the form of arthro- pods/plants and fresh water) were stable for some time, allowing the area to become populated with a number of taxa, ranging from small, lizard-sized reptiles up to a medium-sized theropod. The presence of vertebrates with low vagility, such as drepanosaurs and sphenodon- tids, supports the idea of the lake/oasis being a perma- nent feature of long duration. The taxonomic diversity and large number of individuals seems too large for the size of the preserved portion of the interdune, suggest- ing either that the interdune area was much more ex- pansive or that animals from other interdunes migrated to this area in response to drought or other conditions. One of the great mysteries of the Saints & Sinners fauna is that is it composed entirely or almost entirely of carnivorous and insectivorous taxa. Were insects the dominant primary consumers? Through continuing re- search, we will be able to paint a better picture of this rare desert biota as we determine the taxonomy/phylog- eny and taphonomy of this spectacular site in the near future. 27 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3  Summary The Saints & Sinners Quarry, including sphenosu- chians, drepanosaurs, theropods, and a pterosaur, rep- resents the most diverse vertebrate assemblage known from the Late Triassic-Early Jurassic erg of North Amer- ica. It provides insights into the diversity and adapta- tions of vertebrates early in the age of dinosaurs. The bones are uncrushed and reveal previously unknown osteological details of pterosaurs and drepanosaurs that have been obscured by crushing at other Triassic local- ities. The drepanosaur and basal sphenosuchian indi- cate that much more of the Nugget Sandstone is Triassic than previously suspected.  The bones preserved at the Saint & Sinners Quar- ry represent part of a biocoenosis inhabiting an inter- dune flat and perhaps the surrounding dunes during a climatic wet phase, when the interdune hosted a long- lived playa lake abutted by sand dunes. There was likely a moderately developed plant community that includ- ed cycadeoids. The plants and the diversity of the small vertebrates indicate that, although no body fossils are preserved, invertebrates were common. The diversity of the vertebrate thanatocoenosis sug- gests that the death assemblage is the result of a single drought, perhaps as the lake dried out and animals gath- ered at the last standing water. Carcasses accumulated in the lowest portion of the interdune. As the lake rose, the smaller carcasses were buried by wind-blown sands settling into the shallow margins of the lake. Larger car- casses were later entombed as the waters continued to rise, macerating the carcasses and facilitating disartic- ulation. The remainder of the bones were buried after slight-to-moderate reworking by wave action along the shoreline combined with silt and sand blown into the waters from nearby dunes. Ultimately, the interdune was buried by continued or renewed migration of the dunes.  Unlike the other interdune sites visited on this field trip, there are no carbonates at the Saints & Sinners lo- cality. Even though the dominant cement in the sand- stones is calcite, cementation is weak. It is clear that the lakes in the carbonate mound area and the lake at Saints & Sinners differed substantially. This implies a different source of water, or different positions of the interdunes relative to the water source as a result of geographic or temporal variation. STOP 6. THE MUSEUM OF PALEONTOLOGY, BRIGHAM YOUNG UNIVERSITY Specimens from the Saints & Sinners Quarry are housed at the BYU Museum of Paleontology. BYU is a private research university owned and operated by the Church of Jesus Christ of Latter-day Saints. With about 30,000 students, the campus is located in Provo, Utah, along the eastern boundary of the Basin and Range Province. It is nestled against the Wasatch Range, which is part of the Cordilleran thrust belt west of the Colora- do Plateau. On the edge of campus, we will visit the BYU Muse- um of Paleontology. This museum began as a small ge- ology department museum that housed a collection pri- marily of minerals and invertebrates. It started to thrive in 1960 when James A. Jensen was hired as the new cu- rator. Prior to coming to BYU, Jensen worked as one of Al Romer’s preparators at Harvard’s Museum of Com- parative of Zoology. Jensen’s lack of academic degrees was made up for by his uncanny artistic and tradesman skills. His earlier careers as a card-carrying machinist, welder, and longshoreman came in handy when collect- ing giant sauropods, where hands-on knowledge was more valuable than academic experience. His ability to gain the trust of rockhounds, however, was his skill that paid the highest dividends. With the added input of rockhounds combing the Upper Jurassic Morrison For- mation of the intermountain region, many significant finds were discovered that were passed on to Jensen. Jensen then often used heavy equipment, such as bull- dozers, to open large quarries, many of which produced thousands of bones (e.g., Dry Mesa, Dalton Wells, Cac- tus Park). Some of these specimens represented new taxa, ranging from the diminutive, a pterosaur, Mesad- actylus, or two small ornithopods such as Othneliasau- rus, to giant sauropods including Supersaurus and the massive theropod Torvosaurus. In the 1960s and most of the 1970s, there were very few dinosaur workers. But in a little more than a decade after Jensen’s arrival at BYU, the museum began growing exponentially. In 1972, with his opening (by bulldozer) 28 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 of the Dry Mesa Quarry, BYU’s dinosaur program re- ceived world-wide coverage in the media and the col- lections grew rapidly. Largely because of the Dry Mesa collection and its giant sauropods, the university spun the museum off from the Geology Department and a new museum was built specifically for vertebrate pale- ontology. The museum was completed in 1976. Jensen was awarded an honorary doctorate (perhaps his great- est source of pride), and became known world-wide as “Dinosaur Jim.” The collections now consist of over 20,000 cataloged specimens, primarily dinosaurs. The collections are housed in three buildings: (1) one for fossil mammals (primarily amassed by Wade E. Miller (retired BYU vertebrate paleontologist) between the 1970s and the end of the millennium), (2) one for unprepared spec- imens (with pallet racking and a forklift), and (3) the main museum with exhibit halls, an 167-m2 prepara- tion lab with a 5-ton ceiling crane, and two collections areas, one with two forklifts. The crane, forklifts, and 6-m-high pallet racking are crucial in the handling and storage of the many sauropod bones. The field trip will begin with a quick tour of the mu- seum’s main buildings and dinosaur collections. The fo- cus, however, is the Saints & Sinners Quarry collection, which currently consists of over 11,500 mapped spec- imens. These specimens range from broken bones, to isolated and associated bones, to dozens of articulated skeletons of drepanosaurs and sphenosuchians. We will lay out some of the most spectacular specimens for ex- amination by participants. In the lab, we will watch as students use a combi- nation of several sizes of pneumatic scribes to conduct the gross preparation. Moderate-level preparation is done with sharpened carbide rods and the preparation of small bones is conducted with a needle under a mi- croscope. We will demonstrate how specimens are mapped in the lab. This begins with marking blocks with a 10 or 20-cm grid system using permanent markers. Then each bone is labeled with the block and field number and photographed. The bones in the photographs are traced using Wacom pen-on-screen monitors and dig- itally mapped using Avenza System’s Map Publisher, a geographic information system (GIS) package that runs in Adobe Illustrator. With so many bones in a single quarry (one sandstone block 0.75 m2 and 25 cm thick yielded 834 bones), a link between field and specimen databases is crucial. Using the special database analysis features of GIS we have been able to discover an associ- ated skeleton in what appears to the eye to be an inco- herent mass of bones. The wide size range of the bones, from a few mm to about 400 mm, requires different storage methods. Few bones are fully extracted from the matrix; most are prepared in bas-relief. Both are stored in standard spec- imen drawers. Some bones are left in blocks of sand- stone to preserved taphonomic and geologic informa- tion and are stored on pallets such that the blocks can be reassembled on the floor. The bone is soft, sometimes not much harder than graham crackers, and only rarely can bones be com- pletely extracted from the matrix. Important bones pre- pared only in bas relief have been scanned using a micro CT (micro Computed Tomography), segmented, and printed on high resolution 3D printers. All the bones of the Saints & Sinners dimorphodontid pterosaur have been printed. Holding the printed bones to the light al- lows one to peer inside and discern, for example, how the bones of the mandible articulate and how they are pneumatized. This can be done on a computer using 3D images, but it is infinitely more satisfying to hold in your hand printed bones of bizarre Late Triassic creatures. Following the visit to the BYU Museum of Pale- ontology, we head 45 minutes north to the host hotel where the field trip ends. THE DUNE DEPOSITS Not unexpectedly, an erg deposit such as the Nugget Sandstone is dominated by dune sediments. These beds can contain significant fossils, all in the form of trace fos- sils. Although ichnofossils are generally rare, they can be locally abundant. The only vertebrate trace fossils found in the dune deposits themselves are assignable to Bra- silichnium, which is a track believed to have been made by an advanced non-mammalian synapsid such as a tri- tylodont (Engelmann and Chure, in press). Engelmann and others (2010) reported a site with approximately 400 Brasilichnium footprints on a 10 m2 surface (figure 21). 29 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 Figure 21. Ichnofossils in the dune deposits of the Nugget Sandstone in and around Dinosaur National Monument. (A) Dune slip surface with hundreds of Brasilichnium isp. tracks traveling upslope. (B) Close up of single Brasilichnium isp. pes track from (A). (C) Paleohelcura isp. trackway. (D) Octopodichnus isp. trackway. (E) Dune slip surface with abundant Planolites beverleyensis. (F) Possible small vertebrate burrow. Scale bar in cm. 30 Rise of the Erg—Paleontology and Paleoenvironments of the Triassic–Jurassic Transition in Northeastern Utah Britt, B.B., Chure, D.J., Engelmann, G.F., and Shumway, J.D. Geology of the Intermountain West 2016 Volume 3 Invertebrate traces are more widespread and diverse than vertebrate ichnofossils in the dune sediments. Pa- leohelcura and Octopodichnus are trackways made by scorpions and spiders (figure 21C and D). Entradich- nus, Planolites, and Taenidium are invertebrate bur- rows and trails both parallel and perpendicular to the bedding surfaces of the dune slip face (figure 21E). In modern dune environments, such traces are made by larval dipterans and coleopterans. In addition, there are a number of unnamed forms that are of uncertain af- finities and/or behavior (Good, 2013, 2014). Large bur- rows far out on the toe of the dune face (figure 21F) may have been made by large scorpions or small vertebrates (Engelmann and others, 2014). ACKNOWLEDGMENTS Work in the Chinle Formation by Irmis (University of Utah) was funded by the National Park Service (NPS) Co-operative Ecosystem Studies Unit TA J8R07100009. Work in the Nugget Sandstone by Engelmann (Uni- versity of Nebraska at Omaha) was funded by the NPS Co-operative Ecosystem Studies Unit TA J1404094676. Dan Chure is supported by DINO. We thank Robin L. Hansen (Bureau of Land Management Vernal, Utah) for supporting our work at the Saints & Sinners Quarry which is operated under permit UT08-025E. We are grateful to reviewers Douglas Sprinkel (Utah Geolog- ical Survey) and Judith Parrish (University of Idaho) whose comments and suggestions improved this paper. We thank Melissa Morgan for her input and Douglas Sprinkel for formatting the document. We also thank the Geology of the Intermountain West editors for their work to help bring this paper to publication. REFERENCES Ahlbrandt, T.S., and Fryberger, S.G., 1981, Sedimentary features and significance of interdune deposits: Society for Sedimenatry Ge- ology (SEPM) Special Publication no. 31, p. 293–314. 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