Evidence for niche partitioning among ground-height browsing sauropods from the Upper Jurassic Morrison Formation of North America GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 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. EVIDENCE FOR NICHE PARTITIONING AMONG GROUND-HEIGHT BROWSING SAUROPODS FROM THE UPPER JURASSIC MORRISON FORMATION OF NORTH AMERICA Julia B. McHugh 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 ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Excavations of the Brushy Basin Member of the Mor- rison Formation (Upper Jurassic) at the Mygatt-Moore Quarry in western Colorado with a specimen of di- plodocid premaxilla fragment (MWC 8430) superim- posed. Cover photograph by Julia B. 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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 95 ABSTRACT Two tooth-bearing snout fragments from a diplodocid sauropod from the Brushy Basin Member of the Morrison Formation (Upper Jurassic) excavated from the Mygatt-Moore Quarry in Rabbit Valley, Colora- do are described. The Mygatt-Moore Quarry has produced thousands of vertebrate fossils from the Brushy Basin Member, with the diplodocid Apatosaurus cf. louisae and the tetanuran Allosaurus fragilis dominat- ing the assemblage. Additionally, remains of another diplodocid, Diplodocus sp., have been found near the quarry within Rabbit Valley. Both specimens in this study preserve eight teeth per alveolar position, as observed through broken surfaces at the gross anatomical level and also through computed tomography (CT) scans. This is inconsistent with the genus Diplodocus sp., which has been previously shown to have a maximum of six teeth per alveolus. The presence of eight replacement teeth per alveolus has previously only been reported in the Cretaceous rebbachisaurid Nigersaurus taqueti, which has been interpreted to have occupied a similar ground-height browsing feeding strategy to both Diplodocus and Apatosaurus. This is the first report of this type of high-count replacement teeth in a diplodocid sauropod from the Morrison Formation. The high number of replacement teeth in a close relative to the contemporaneous Diplodocus provides evidence for niche partitioning among the contemporary ground-height browsing diplodocid sauropods of the Late Jurassic Period in North America. Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America Julia B. McHugh1 1Museums of Western Colorado, P.O. Box 20000, Grand Junction, CO 81502; jmchugh@westcomuseum.org Citation for this article. McHugh, J.B., 2018, Evidence for niche partitioning among ground-height browsing sauropods from the Upper Jurassic Morrison Formation of North America: Geology of the Intermountain West, v. 5, p. 95–103. © 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. INTRODUCTION Apatosaurus and Diplodocus are contemporary sau- ropod genera found within the Upper Jurassic Morri- son Formation (Tschopp and others, 2015). These gen- era share a close phylogenetic history, as well as head and neck morphologies that allow for similar biome- chanical adaptions to available vegetation zones (Ste- vens and Parrish, 1999, 2005; Upchurch, 2000; Wilson, 2005; Whitlock, 2010; Stevens, 2013; Woodruff, 2017). The presence of such similar, large-bodied herbivores would have had a substantial impact on the available biomass in order to support populations of these two genera. Ecological niche partitioning in the Upper Ju- rassic Morrison Formation’s landscape has traditional- ly been proposed to circumvent this problem (Turner and Peterson, 2004). However, empirical evidence sup- porting this partitioning among coeval genera, partic- ularly between adult Apatosaurus and Diplodocus, has been limited (Fiorillo, 1998; Whitlock, 2011: D’Emic and others, 2013). Here, I describe new specimens that shed light on this ecological problem: two diplodocid 96 Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America McHugh, J.B. Geology of the Intermountain West 2018 Volume 5 snout fragments containing numerous teeth (Museums of Western Colorado specimens MWC 6002 and MWC 8430) from the Brushy Basin Member of the Morri- son Formation excavated from Mygatt-Moore Quarry (MMQ) in Rabbit Valley, western Colorado (figure 1). The MMQ is located within the McInnis Canyons National Conservation Area and is co-managed by the Museums of Western Colorado (MWC) and the Bureau of Land Management (BLM). Excavations at the MMQ for over thirty years by crews from the MWC, as well as the Dinamation International Society, have facilitated the recovery of over 5000 vertebrate fossils, including holotype specimens of the bony fish “Hulettia” hawesi (Kirkland, 1998) and Morrolepis schaefferi (Kirkland, 1998), and the ankylosaur Mymoorapelta maysi (Kirk- land and Carpenter, 1994). However, fossil material from the tetanuran Allosaurus fragilis and the diplodo- cid Apatosaurus cf. louisae dominate the assemblage (Foster and others, 2007, 2018). Two diplodocid taxa are positively identified from the Rabbit Valley area: Di- plodocus sp. and Apatosaurus cf. louisae. Of these two, only Apatosaurus cf. louisae has been recovered within the MMQ. Other sauropods recovered from the quar- ry include the macronarian, Camarasaurus sp., as well as several taxonomically ambiguous caudal vertebrae, which are identified as Diplodocinae indet. (Foster and others, 2018). Sauropod dinosaurs include the largest terrestrial herbivores that the Earth has ever known. Understand- ing their biology and interactions with plant biomass and other sauropods within Mesozoic ecosystems has been the subject of much research (Barrett and Upchurch, 1995; Stevens and Parrish, 1999, 2005; Upchurch, 2000; Upchurch and Barrett, 2000; Hummel and others, 2008; Yates and others, 2010; Gee, 2011; Tütken, 2011). Sau- ropods have been broadly divided into “high browsing” and “ground height” browsing ecological types based upon snout shape and neck position (Whitlock, 2011), but estimations of neck biomechanics indicate con- siderable overlap in the range of feeding between dis- parate types of sauropods (Stevens and Parrish, 1999, 2005). Additionally, co-occurring genera within these broad categories, such as Apatosaurus and Diplodocus, are known from the Morrison Formation (Tschopp and others, 2015). The refining of these broad ecological groups was attempted using snout shape morphometrics (Whit- lock, 2011). Snout shapes were quantified for six di- plodocoid genera (Apatosaurus sp., Dicraeosaurus sp., Diplodocus sp., Nigersaurus taqueti, Suuwassea emilieae, and Tornieria africana) using the ratio of the depth to and breadth in the upper jaw, the premaxillary-maxil- lary index, and the divergence angle of the premaxillae (Whitlock, 2011). Whereas some taxa were found to be mathematically distinct from each other; unfortunately, these metrics were unable to separate the broad snouted Apatosaurus and Diplodocus into disparate ecological groups (Whitlock, 2011). Tooth microwear has also been used to distinguish among ecological resource use in sauropod dinosaurs (Fiorillo, 1998; Whitlock, 2011). However, while tooth wear and microwear have been useful in differentiat- ing feeding strategies between co-occurring taxa like Camarasaurus sp. and Diplodocus sp. (Fiorillo, 1998; Whitlock, 2011), suitable teeth from Apatosaurus sp. for this type of analysis have been lacking (Whitlock, 2011). Although shed teeth are relatively common for preda- tors like Allosaurus fragilis, especially at the MMQ, shed teeth from species of Apatosaurus are less common and are often difficult to differentiate from those of Diplodo- cus. Additional dental material is required to assess the microwear in the genus Apatosaurus, and for this type of analysis to be useful in partitioning the ecological niches of Apatosaurus and Diplodocus. Observations of modern herbivores show that eco- logical niche partitioning is common among similar taxa within an environmental landscape (i.e., Bovidae), and that their dental and jaw morphologies are also correlated to this partitioning (Spencer, 1995). Howev- er, with considerable morphological and biomechanical overlap in sauropod dinosaurs between established eco- logical types, estimation of niche partitioning at lower taxonomic levels has proven difficult (Fiorillo, 1998; Whitlock, 2011). Tooth replacement may be an illu- minating piece of evidence in fully assessing the niche partitioning among sauropods. Since variable hardness within food sources can tend to create inconsistent wear on herbivore teeth, a higher level of tooth replacement is often associated with tougher vegetation. Tooth replacement rates among Diplodocoidea 97 Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America McHugh, J.B. Geology of the Intermountain West 2018 Volume 5 have previously been described in Camarasaurus sp., Diplodocus sp., Dicraeosaurus hansemanni, and Niger- saurus taqueti (Sereno and others, 2005; D’Emic and others, 2013; Schwarz and others, 2015). The number of teeth per alveolus within the premaxillae (including both replacement and erupted tooth positions) vary among taxa, with four reported in Camarasaurus sp., six in Diplodocus sp., five to six in Dicraeosaurus han- semanni, and eight in Nigersaurus taqueti (Sereno and others, 2005; D’Emic and others, 2013; Schwarz and others, 2015). D’Emic and others (2013) suggest the lower number of teeth in Camarasaurus sp. is correlat- ed to the relative increase in tooth volume, and thus a physiological investment in individual teeth rather than a biological investment in a lot of teeth. I present here the first description of tooth replacement in a non-Di- plodocus diplodocid from the Morrison Formation that provides empirical evidence for an ecological partition between this taxon and Diplodocus sp. GEOLOGICAL SETTING The fossil-bearing horizon at the MMQ is a 1- to 2-m-thick unit exposed within laminated to medi- um-bedded gray silty mudstone of the Brushy Basin Figure 1. Map of Mesa County in western Colorado showing the location of the Mygatt-Moore Quarry. Insert shows exca- vations at the quarry during 2014. 98 Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America McHugh, J.B. Geology of the Intermountain West 2018 Volume 5 Member of the Morrison Formation (Kirkland and Carpenter, 1994; Foster, 2003; Foster and others, 2007, 2018; Foster and Hunt-Foster, 2011). Radiometric anal- ysis of ash-fall zircons from the quarry have returned an age of 152.18 ±0.29 Ma, which places the quarry on Kimmeridgian–Tithonian boundary during the Late Jurassic Period (Trujillo and others, 2014). The MMQ is interpreted to preserve an ecosystem with abundant vegetation and a high water table, but with standing water at the surface occurring only sea- sonally (Foster, 2003; Trujillo and others, 2014; Foster and others, 2018). The site preserves a dinosaur-dom- inated assemblage with abundant plant material, with crocodylomorph, turtle, and invertebrate remains be- ing rare (Foster 2007; Foster and others, 2018). Previ- ous taphonomic work has shown the site represents an autochthonous assemblage within an attritional deposit in an overbank setting with few articulated specimens and no preferred orientation of skeletal elements (Fos- ter and others, 2018). Despite the lack of transport evi- dent in the assemblage, fossils often preserve bone sur- face modifications (e.g., breakage, abrasion, and feeding traces) that suggest a reworked death assemblage by subsequent living individuals, possibly through feeding and trampling (Foster and others, 2018). DESCRIPTION Both specimens in this study were excavated from the gray mudstone of the Brushy Basin Member of the Morrison Formation at the MMQ using hand-quarry- ing techniques. Both specimens were collected as iso- lated elements with no additional associated cranial or postcranial material. These specimens were mechani- cally prepared at the MWC fossil preparation lab, re- paired, and stabilized using polyvinyl acetate dissolved in acetone, as well as cyanoacrylate (PaleoBONDTM 750) adhesives, and then imaged using two-dimensional (2- D) photography and a 64-slice computed tomography (CT) scanner at Colorado Canyons Hospital in Fruita, Colorado. MWC 6002—Diplodocid Maxilla Specimen MWC 6002 is a partial maxilla that pre- serves eight sets of teeth, with each set representing a single alveolus. The mesial suture between the premax- illa and maxilla is poorly preserved and the ventrodistal margin is incomplete. Therefore, it is doubtful that all maxillary alveoli are present in the specimen, and alve- olar positions are instead referred to as mx1-mx8 and not to their exact maxillary placement (figure 2). The number of replacement teeth per alveolus de- creases from medial to distal. The mesial-most four alveolar positions (mx1-mx4) each preserve seven to eight replacement teeth, similar to the numbers pre- served in specimen MWC 8430. The next two alveoli (mx5-mx6) preserve six and five replacement teeth, re- spectively, whereas the distal-most positions preserve only four and three replacement teeth (mx7, mx8). This reduction in the number of replacement teeth reflects the decreasing volume in maxillary space distally. Tooth replacement positions vi-viii are labial-linugally com- pressed. Erupted crown morphology and the lingual inflection at the separation of root and crown are both distinguishable by position v and become more pro- nounced towards the erupted tooth position (i) (figures 3 and 4). MWC 8430—Diplodocid Premaxilla Fragment Specimen MWC 8430 is a well-preserved, isolated premaxilla fragment that preserves four sets of replace- ment teeth, each representing a single alveolus in the upper jaw. However, lack of preservation on the exte- rior surface has not preserved the alveoli themselves. The lingual face of the premaxilla is preserved as a thin, trapezoidal fragment with a smooth surface morpholo- gy. The fragmental nature of specimen MWC 8430 al- lows for determination of lingual/labial and superior/ inferior orientations, but not mesial/distal and it pro- hibits assignment to precise tooth row placement. The alveolar positions are therefore referred to as px1-px4 (figure 5). The maximum number of replacement teeth (eight) in an individual set are found within px1, with px2 and px3 each preserving seven teeth, and px4 with four teeth. Teeth in the replacement rows incrementally increase in length towards the erupted tooth position. Tooth replacement positions vi-viii are labial-lingual- ly compressed. Erupted crown morphology is distin- 99 Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America McHugh, J.B. Geology of the Intermountain West 2018 Volume 5 guishable by position v and becomes more pronounced towards the erupted tooth position (i). Also visible at position v is the lingual inflection at the separation of root and crown (figures 5 and 6). DISCUSSION Despite the lack of association with more diagnostic material, some taxonomic assessments for specimens MWC 6002 and MWC 8430 are possible. These speci- mens both bear the characteristic peg-like teeth of Di- plodocoidea. However, they are not consistent with the known pattern of tooth replacement in Diplodocus sp. or the African taxon Dicraeosaurus hansemanni which have been shown to support no more than five to six tooth positions per alveolus in the premaxilla, including the erupted tooth (D’Emic and others, 2013; Schwarz, 2015). The newly described specimens MWC 6002 and MWC 8430, although incomplete, preserve a maximum of eight teeth (both replacement teeth and erupted tooth) per alveolus. The premaxilla exhibits the max- imum number of replacement teeth, as supported by the mesial increase in the number of tooth replacement position seen in the maxilla, specimen MWC 6002. However, this mesial increase in tooth replacement po- sitions may not be characteristic of all sauropods, for example in the titanosaur Euhelopus zdanskyi, the max- imum number of tooth replacement positions is located within the maxilla and not the premaxilla (Poropat and Figure 2. Diplodocid maxilla (spec- imen MWC 6002). (A) dorsal view. (B) ventral view. Scale bar equal 10 cm. 100 Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America McHugh, J.B. Geology of the Intermountain West 2018 Volume 5 Kear, 2013). Minimally, specimens MWC 6002 and MWC 8430 can be assigned as a non-Diplodocus diplodocid. The only other known diplodocid taxa from the Rabbit Val- ley consists of the very abundant Apatosaurus cf. louisae and the ambiguous vertebrae not positively identified beyond Diplodocinae indet. The MMQ in Rabbit Val- ley has been explored in scientific earnest for more than three decades, producing thousands of fossils, none of which were positively assigned to any diplodocid taxon other than Apatosaurus cf. louisae. And although the possibility of future diplodocoid discoveries at the site cannot be discounted, given the present data available, it is most parsimonious for specimens MWC 6002 and MWC 8430 to be tentatively referred to Apatosaurus sp., pending the recovery of more diagnosable cranial ma- terial. Despite a closer phylogenetic, geographic, and stratigraphic relationship with Diplodocus sp., the spec- imens in this study that are referred to Apatosaurus sp. are more similar to the tooth replacement counts seen in the Cretaceous African form Nigersaurus taqueti. A higher number of replacement teeth than the co- eval Diplodocus sp. indicates that these large-bodied, “ground height” browsing herbivores were orally pro- cessing different types of vegetation, with Apatosaurus sp. potentially adapted for tougher, more abrasive veg- etation than Diplodocus sp. The Morrison Formation is interpreted as representing a predominantly riparian landscape with limited botanical biomass (Foster, 2003; Engelmann and others, 2004; Rees and others, 2004; Turner and Peterson, 2004; Hummel and others, 2008; Gee, 2011). This partitioning of resources between large herbivores would have been critical for survival in this limiting environment, and may have been the key to the success and diversity of the sauropods in the Late Juras- sic Period. CONCLUSIONS Populations of co-occurring herbivores in modern landscapes often partition plant biomass resources to facilitate survival in competitive environments. Using modern analogs, paleontologists have long hypothe- Figure 3. CT scan of specimen MWC 6002 in and superior view. Alveolar positions are indicated with mx1-mx8 labels. Figure 4. CT scan of specimen MWC 6002 in and mesial view. Roman numerals indicate tooth positions for the mx2 alveolus. 101 Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America McHugh, J.B. Geology of the Intermountain West 2018 Volume 5 Figure 5. Specimen MWC 8430 in (A) ?mesial, (B) ?distal, (C) labial, (D) lingual, (E) superior, and (F) inferior views. Roman numerals indicate tooth posi- tions for an individual alveolus. All scale bars equal 1 cm. Figure 6. CT scan of specimen MWC 8430 in and inferior view. Roman numerals indicate tooth positions for individual alveoli. Alveolar positions are indicated with px1-px4 labels. 102 Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America McHugh, J.B. Geology of the Intermountain West 2018 Volume 5 sized similar niche partitioning among large sauropod dinosaurs in the Mesozoic. New specimens from the MMQ in Rabbit Valley, Colorado, reveal higher counts of tooth replacement in Apatosaurus sp. than in Diplod- ocus sp. This indicates that species of Apatosaurus may have been selectively feeding on tougher vegetation (e.g., cycads and conifers) than species of Diplodocus and provides the strong empirical evidence for niche partitioning among these coeval and morphologically similar taxa. ACKNOWLEDGMENTS Thank you to Rob Gay (Colorado Canyons Associ- ation), Chris Racay (Museums of Western Colorado), John Foster (Museum of Moab), ReBecca Hunt-Foster (Bureau of Land Management, Canyon Country Dis- trict), and Michael D’Emic (Adelphi University) for their helpful discussions on this project. I thank Doro- thy Stewart and the volunteer corps at Dinosaur Journey Museum, as well as Dinosaur Journey field coordina- tor Chris Racay and field assistant Mitchell Lukens for assistance in recovery and preparation of specimens. I sincerely thank the Bureau of Land Management Grand Junction Field Office for support and site access, as well as Dr. Michael Neste, M.D. (Family Health West) and the Colorado Canyons Hospital in Fruita for the use of CT imaging facilities. Thank you to Stephen K. Poro- pat (Swinburne University of Technology) and Femke Holwerda (Bayerische Staatssammlung für Paläontolo- gie und Geologie) for their thoughtful comments on an earlier version of this manuscript, and I also thank Cary Woodruff (Great Plains Dinosaur Museum), John Fos- ter (Museum of Moab), and Daniela Schwarz (Museum für Naturkunde) for their time and comments on this manuscript. REFERENCES Barrett, P., and Upchurch, P., 1995, Sauropod feeding mecha- nisms—their bearing on palaeoecology, in Sun, A., and Wang, Y., editors, Proceeding 6th symposium, Mesozoic terrestrial ecosystems and biotas: Beijing, China Ocean Press, p. 107–110. D’Emic, M.D., Whitlock, J.A., Smith, K.M., Fisher, D.C., and Wil- son, J.A., 2013, Evolution of high tooth replacement rates in sauropod dinosaurs: PLoS ONE, v. 8, no. 7, p. e69235. Engelmann, G.F., Chure, D.J., and Fiorillo, A.R., 2004, The implica- tions of a dry climate for the paleoecology of the fauna of the Upper Jurassic Morrison Formation: Sedimentary Geology, v. 167, no. 3-4, p. 297–308. Fiorillo, A.R., 1998, Dental micro wear patterns of the sauropod di- nosaurs camarasaurus and diplodocus—evidence for resource partitioning in the Late Jurassic of North America: Historical Biology, v. 13, no. 1, p. 1–16. Foster, J.R., 2003, Paleoecological analysis of the vertebrate fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain region, USA: New Mexico Museum of Natural History and Science Bulletin 23, 95 p. Foster, J.R., Hunt, R.K., and King, L.R., 2007, Taphonomy of the Mygatt-Moore Quarry, a large dinosaur bonebed in the Up- per Jurassic Morrison Formation of western Colorado [abs.] Geological Society of America Abstracts with Programs, v. 39, no. 6, p. 400. Foster, J.R., and Hunt-Foster, R.K., 2011, New occurrences of dinosaur skin of two types (Sauropoda? and Dinosauria in- det.) from the Late Jurassic of North America (Mygatt-Moore Quarry, Morrison Formation): Journal of Vertebrate Paleon- tology, v. 31, no. 3, p. 717–721. 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 sed- imentology 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. 23–89. Gee, C.T., 2011, Dietary options for the sauropod dinosaurs from an integrated botanical and paleobotanical perspective, in Klein, N., Remes, R., Gee, C.T., and Sander, P.M., editors, Bi- ology of the sauropod dinosaurs—understanding the life of giants: Bloomington, Indiana University Press, p. 34–56. Hummel, J., Gee, C.T., Südekum, K-H., Sander, P.M., Nogge, G., and Clauss, M., 2008, In vitro digestibility of fern and gymno- sperm foliage—implications for sauropod feeding ecology and diet selection: Proceedings of the Royal Society of London B: Biological Sciences, v. 275, no. 1638, p. 1015–1021. Kirkland, J.I., 1998, Morrison fishes: Modern Geology, v. 22, p. 503–533. Kirkland, J.I., and Carpenter, K., 1994, North America’s first pre-Cretaceous ankylosaur (Dinosauria) from the Upper Ju- rassic Morrison Formation of western Colorado: Brigham Young Univerity Geological Studies, v. 40, p. 25–42. Poropat, S.F., and Kear, B.P., 2013, Photographic atlas and three-di- mensional reconstruction of the holotype skull of Euhelopus zdanskyi with description of additional cranial elements: PLoS ONE, v. 8, no. 11, p. e79932. 103 Evidence for Niche Partitioning Among Ground-Height Browsing Sauropods from the Upper Jurassic Morrison Formation of North America McHugh, J.B. Geology of the Intermountain West 2018 Volume 5 Rees, P.M., Noto, C.R., Parrish, J.M., and Parrish, J.T., 2004, Late Ju- rassic climates, vegetation, and dinosaur distributions: Journal of Geology, v. 112, no. 6, p. 643–653. Sereno, P.C., and Wilson, J.A., 2005, Structure and evolution of a sauropod tooth battery, in Curry Rogers, K.A., and Wilson, J.A., editors, The sauropods—evolution and paleobiology: Berkeley, University of California Press, p. 157–177. Spencer, L.M., 1995, Morphological correlates of dietary resource partitioning in the African Bovidae: Journal of Mammalogy, v. 76, no. 2, p. 448–471. Stevens, K.A., 2013, The articulation of sauropod necks—method- ology and mythology: PLoS ONE, v. 8, no. 10, p. e78572. Stevens, K.A., and Parrish, J.M., 1999, Neck posture and feeding habits of two Jurassic sauropod dinosaurs: Science, v. 284, no. 5415, p. 798–800. Stevens, K.A., and Parrish, J.M., 2005, Neck posture, dentition, and feeding strategies in Jurassic sauropod dinosaurs, in Tid- well, V., and Carpenter, K., editors, Thunderlizards—the sau- ropodomorph dinosaurs: Bloomington, Indiana University Press, p. 212–232. Schwarz, D., Kosch, J.C.D., Fritsch, G., and Hildebrandt, T., 2015, Dentition and tooth replacement of Dicraeosaurus hansem- anni (Dinosauria, Sauropoda, Diplodocoidea) from the Tend- aguru Formation of Tanzania: Journal of Vertebrate Paleontol- ogy, v. 35, no. 6, p. e1008134. Trujillo, K.C., Foster, J.R., Hunt-Foster, R.K., and Chamberlain, K.R., 2014, A U/Pb age for the Mygatt-Moore Quarry, Upper Jurassic Morrison Formation, Mesa County, Colorado: Volu- mina Jurassica, v. 12, no. 2, p. 107–114. Tschopp, E., Mateus, O., and Benson, R.B.J., 2015, A specimen-lev- el phylogenetic analysis and taxonomic revision of Diplodoci- dae (Dinosauria, Sauropoda): PeerJ, v. 3, p. e857. Turner, C.E., and Peterson, F., 2004, Reconstruction of the Upper Jurassic Morrison Formation extinct ecosystem—a synthesis: Sedimentary Geology, v. 167, no. 3-4, p. 309–355. Tütken, T., 2011, The diet of sauropod dinosaurs—implications from carbon isotope analysis of teeth, bones, and plants, in Klein, N., Remes, R., Gee, C.T., and Sander, P.M., editors, Bi- ology of the sauropod dinosaurs—understanding the life of giants: Bloomington, Indiana University Press, p. 57–79. Upchurch, P., 2000, Neck posture of sauropod dinosaurs: Science, v. 287, no. 5453, p. 547–547. Upchurch, P., and Barrett, P.M., 2000, The evolution of sauropod feeding mechanisms, in Sues, H-D., editor, Evolution of her- bivory in terrestrial vertebrates—perspectives from the fossil record: Cambridge, England, Cambridge University Press, p. 79–122. Whitlock, J.A., 2010, Paleoecology and systematics of the diplodo- coid sauropods: Ann Harbor, The University of Michigan, Ph.D. dissertation, 369 p. Wilson, J.A., 2005, Overview of sauropod phylogeny and evolu- tion, in Curry Rogers, K.A., and Wilson, J.A., editors, The sau- ropods—evolution and paleobiology: Berkeley, University of California Press, p. 15–49. Whitlock, J.A., 2011, Inferences of diplodocoid (Sauropoda: Di- nosauria) feeding behavior from snout shape and microwear analyses: PLoS ONE, v. 6, no. 4, p. e18304. Woodruff, D.C., 2017, Nuchal ligament reconstructions in di- plodocid sauropods support horizontal neck feeding postures: Historical Biology, v. 29, no. 3, p. 308–319. Yates, A.M., Bonnan, M.F., Neveling, J., Chinsamy, A., and Black- beard, M.G., 2010, A new transitional sauropodomorph dino- saur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism: Proceedings of the Royal Society of London B: Biological Sciences, v. 277, no. 1682, p. 787–794.