What factors influence our reconstructions of Morrison Formation sauropod diversity? GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 6 2019 © 2019 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. WHAT FACTORS INFLUENCE OUR RECONSTRUCTIONS OF MORRISON FORMATION SAUROPOD DIVERSITY? D. Cary Woodruff 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 Based on morphology alone, differing attributes (in- cluding body size, cranial, vertebral, and limb mor- phologies) between two specimens could result in the interpretation that they are distinct. Conversely, from just histology, only age is assessed from a growth per- spective. However, by increasing the lines of evidence, by incorporating both morphology and histology, we can more accurately account for a growth series. Growth is gradational, so we should expect to see not just the extremes, but transitional forms as well. Hu- man scale bar is Augustus Saint-Gaudens’ Diana of the Tower, depicting Diana as 1.83 m. Diplodocus sp. silhouettes originally based on art by S. Hartman available via PhyloPic (Creative Commons Attribu- tion-ShareAlike 3.0 Unported). i 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 6 2019 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. Editors President Leslie Heppler lheppler@utah.gov 801.538.5257 President-Elect Riley Brinkerhoff riley.brinkerhoff@gmail.com 406.839.1375 Program Chair Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Treasurer Greg Gavin greg@loughlinwater.com 503.509.1509 Secretary Elliot Jagniecki ejagniecki@utah.gov 801.537.3370 Past-President Peter Nielsen peternielsen@utah.gov 801.537.3359 UGA Board October 2019 – September 2020 UGA Committees 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 Outreach Greg Nielson gnielson@weber.edu 801.626.6394 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 Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Publicity Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Social/Recreation Roger Bon rogerbon@xmission.com 801.942.0533 AAPG House of Delegates 2017–2020 Term Tom Chidsey tomchidsey@utah.gov 801.537.3364 State Mapping Advisory Committe UGA Representative Jason Blake blake-j@comcast.net 435.658.3423 UGA Newsletter Newsletter Editor Bill Lund uga.newsletter@gmail.com 435.590.1338 UGA Website www.utahgeology.org Webmaster Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 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 — Utah Field House of Natural History State Park Museum Cary Woodruff — University of Toronto Octavio Mateus — Universidade Nova de Lisboa GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 6 2019 93 ABSTRACT The Upper Jurassic Morrison Formation classically represents the “Golden Age” of sauropods, and the Morrison Formation is reported to have yielded 13 genera and 24 species of sauropods. This incredible diversity has produced numerous theories attempting to reconcile the co-occurrence of such large, and similar taxa. Previously, a comparably high diversity has been proposed for the Late Cretaceous Hell Creek Formation of North America – possibly comprising nearly three dozen species from over 20 genera of an- kylosaura, caenagnathids, ceratopsians, dromaeosaurids, hadrosaurs, ornithomimids, pachycephalosaurs, thescelosaurs, and tyrannosaurs. However, much of the morphologic variation previously ascribed to taxo- nomic differences has recently been shown to be a result of stratigraphy and/or ontogeny – resulting in this rich assemblage being downsized to 13 genera and 16 species. Whereas still rich in diversity, such factors have an immediate effect towards our reconstruction of true richness. Following the example of the Hell Creek Formation, we can investigate the ontogenetic and strati- graphic origin of possible diversity inflation in other formations, and within this study, apply it to the Mor- rison Formation. New dating techniques are resulting in finer temporal resolution, and are changing the temporal position of well-known quarries. Differences in body size and ontogenetic stages can also affect diversity estimates. Plotting body size stratigraphically, it initially appears that larger specimens (interpret- ed as different species) occur higher in the section. An increase in average body size may be a legitimate trend, but there are several specimens that counter this “rule” for many genera. Likewise, dramatic allo- metric ontogenetic trajectories have led to the erection of at least three diplodocid genera – Amphicoelias, Seismosaurus, and Suuwassea – and it is suspected that many more Morrison Formation “species” could alternatively be explained as ontogimorphs. We have a long way to go towards revealing the true nature of Morrison Formation sauropod diversity. Although dietary partitioning undoubtedly occurred at the level of both the species (e.g., Brachiosaurus vs. Diplodocus) and between ontogenetic stages, a base of 24 levels of co-occurring divisions seems unlikely. The Morrison Formation may have exhibited a sauropod-rich as- semblage unlike any other in North America, and the implications of stratigraphy, ontogeny, and variation may be minor, yet these factors alter perceived “diversity.” True diversity will not be fully understood unless these factors are considered. What Factors Influence our Reconstructions of Morrison Formation Sauropod Diversity? D. Cary Woodruff1,2,3 1Royal Ontario Museum, 2Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada, 3Great Plains Dinosaur Museum & Field Station, Malta, MT, USA; sauropod4@gmail.com Citation for this article. Woodruff, D.C., 2019, What factors influence our reconstructions of Morrison Formation sauropod diversity?: Geology of the Intermountain West, v. 6, p. 93–112. © 2019 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. 94 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 INTRODUCTION The Upper Jurassic Morrison Formation of the western United States of America was home to some of the most classic of dinosaur species. Though the Morrison Formation is well known for theropods (e.g., Allosaurus), thyreophorans (e.g., Stegosaurus), and or- nithopods (e.g., Camptosaurus), the sauropods are the quintessential Morrison dinosaurs. Species such as Apatosaurus, Brachiosaurus, Camarasaurus, and Di- plodocus—discovered well over a century ago—are still famed, highly regarded, and important taxa. It is not just the species of sauropods found in the Morri- son Formation that are unique, however, but also their apparent diversity. If interpretations are correct, the Morrison Formation was home to 13 to 14 genera and 24 species of sauropods—Amphicoelias altus, A. fragil- limus (a.k.a., “Maraapunisaurus” of Carpenter, 2018), Apatosaurus ajax, A. louisae, A. excelsus, A. parvus, A. yahnahpin (the last three may alternatively belong to Brontosaurus [Tschopp and others, 2015]), Barosaurus lentus, Brachiosaurus altithorax, Camarasaurus grandis, C. lentus, C. lewisi, C. supremus, Diplodocus carnegii, D. hallorum, D. longus, Dystrophaeus viaemalae, Galeamo- pus hayi, G. pabsti, Haplocanthosaurus delfsi, H. priscus, Kaatedocus siberi, Supersaurus vivianae, and Suuwassea emilieae (figure 1). Although the Morrison Formation was geographically expansive—representing an area of 1.5 million square km (Dodson and others, 1980; Fos- ter, 2007)—how could 24 of some of the largest terres- trial herbivores ever co-exist? Niche and dietary parti- tioning could account for some of this species richness, but we can also examine the realities of such seemingly high species co-occurrence. The Late Cretaceous Hell Creek Formation rep- resents a dinosaur-bearing formation that is historically well known and extensively studied, with similarly high dinosaur diversity to that of the Morrison Formation. However, over the past decade, ontogenetic and strati- graphic assessments (Horner and Goodwin, 2006, 2008, 2009; Scannella and Fowler, 2009; Scannella and Horn- er, 2010, 2011; Campione and Evans, 2011; Horner and others, 2011; Scannella and Fowler, 2014; Scannella and others, 2014; Goodwin and Evans, 2016; Fowler 2017; Wosik and others, 2017, 2018) have reassessed the diver- sity, life development, and evolution of the Hell Creek Formation dinosaurs. These analyses alternatively sug- gest lower species richness. Using the Hell Creek For- mation diversity analysis as a guide, we can re-examine the Morrison Formation sauropod diversity, assessing the effect considering ontogeny and stratigraphy. INSTITUTIONAL ABBREVIATIONS ANS: Academy of Natural Sciences, Philadelphia, Pennsylvania; DNM: Dinosaur National Monument, Jensen, Utah; GMNH: Gunma Museum of Natural His- tory, Japan; NMMNH: New Mexico Museum of Natu- ral History, Albuquerque, New Mexico; OMNH: Sam Noble Oklahoma Museum of Natural History, Norman, Oklahoma. FACTORS INFLUENCING OUR VIEW OF DIVERSITY Ontogeny Compared to many other dinosaur groups, sau- ropod ontogeny has received relatively less attention, and has been mostly restricted to histologic analysis of relative bone maturity (including Curry, 1999; Sander, 2000; Sander and Tückmantel, 2003; Klein and Sander, 2008; Lehman and Woodward, 2008; Woodward and Lehman, 2009; Sander and others, 2011; Waskow and Sander, 2014; Rogers and others, 2016). Although no sauropod nesting sites are yet known from the Morri- son Formation, based on those from South America, such as Auca Mahuevo (Chiappe and others, 1998), the potential difference in body mass between a hatch- ling and an adult Morrison Formation sauropod is at least four orders of magnitude. Descriptions of imma- ture Morrison Formation sauropods (Gilmore, 1925; Carpenter and McIntosh, 1994; Foster, 1995; Britt and Naylor, 1996; Curtice and Wilhite, 1996; Curry, 1999; Foster, 2005a; Schwarz and others, 2007; Myers and Storrs, 2007; Myers and Fiorillo, 2009; Whitlock and others, 2010; Carballido and others, 2012; Storrs and others, 2012; Tschopp and Mateus, 2013; Hedrick and others, 2014; Tschopp and others, 2015; Woodruff and others, 2015, 2017; Melstrom and others, 2016; Hanik and others, 2017, show that sauropods did not grow iso- 95 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 metrically (though some appendicular elements gener- ally appear more isometric than allometric; Woodruff and others, 2017). Sauropod ontogenetic development is still the subject of much discussion (Whitlock and Harris, 2010; Whitlock and others, 2010; Woodruff and Fowler, 2012; Wedel and Taylor, 2013; Hedrick and oth- ers, 2014; Tschopp and others, 2015; Melstrom and oth- ers, 2016; Woodruff and others, 2017), and continuing investigations and conversations are invaluable. This review adheres to the hypothesis that sauropods under- went radical ontogenetic development, but I strongly encourage readers to examine all sides of this discus- sion. The implications of ontogeny could complicate our understanding of the species richness within Morrison Formation sauropods. If these sauropods did undergo radical size and ontogenetic changes, how do we recog- nize these growth stages, and how do we assign ontogi- morphs to genera? Several studies have examined sauropod matura- tional states. Some of these studies rely on morpholo- gy (Whitlock and others, 2010; Woodruff and Fowler, 2012; Wedel and Taylor, 2013; Carballido and Sander, 2014), whereas others rely on histology (Curry, 1999; Sander, 1999, 2000; Klein and Sander, 2008; Lehman and Woodward, 2008; Griebeler and others, 2013; Mitchell and others, 2017). Whereas histology is the demonstrably proven way to assess and verify maturi- ty in dinosaurs (see Padian and Lamm [2013] and the sources therein), given the extreme changes throughout growth, understanding the development and life histo- ry of a sauropod represents a complex association with both morphology and histology. Contrary to previous speculation, sauropods did not take centuries to mature (sensu Curry, 1999; Sand- er, 1999, 2000; Erickson and others, 2001; Sander and Tückmantel, 2003; Sander and others, 2004; Rogers and Erickson, 2005; Lehman and Woodward, 2008; Wood- ward and Lehman, 2009; Griebeler and others, 2013; Waskow and Sander, 2014; Woodruff and others, 2017). Figure 1. The entirety of possible sauropod species within the Morrison Formation. Amphicoelias: A. altus (or Diplodocus altus [Woodruff and Foster, 2014]), A. fragillimus (a.k.a., “Maraapunisaurus” of Carpenter, 2018); Ap- atosaurus: A. ajax, A. louisae, A. excelsus, A. parvus, A. yahnahpin (the last three may alternatively belong to Brontosaurus [Tschopp and others, 2015]); Barosaurus lentus; Brachiosaurus altithorax; Camarasaurus: C. grandis, C. lentus, C. lewisi, C. supremus; Diplodocus: D. carnegii, D. hallorum, D. longus; Dystrophaeus viaemalae; Galea- mopus: G. hayi, G. pabsti; Haplocanthosaurus: H. delfsi, H. priscus; Kaatedocus siberi; Supersaurus vivianae; Suu- wassea emilieae. As historically portrayed in other dinosaur-bearing formations, the sauropods of the Morrison Formation are largely depicted as homogenized – an inaccurate portrayal for numerous reasons outlined in this review. Species to supposed scale. Silhouettes by S. Hartman available via PhyloPic (Creative Commons Attribu- tion-ShareAlike 3.0 Unported). 96 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Recent histologic analyses have recorded an estimat- ed maximum age-of-death in the thirties to forties in specimens of Camarasaurus and Diplodocus (Waskow and Sander, 2014; Woodruff and Foster, 2017; Wood- ruff and others, 2017). Although maximum longevity is unattainable, the majority of sauropods likely fell within the half century interval, although some exceed- ingly rare specimens may have extended this envelope (Wings and others, 2007; D.C. Woodruff, personal ob- servations). Thanks to the pioneering work of Waskow and Sander (2014), sauropod dorsal rib histology allows for numerical age estimates as opposed to maturation- al rankings (Histological Ontogenetic Stage; Klein and Sander, 2008). As paleontology relies on morphology in part to distinguish different species, these growth differences/ changes are quantifiable with radical ontogenetic trajec- tories. If we had several specimens that unbeknownst to us represented a growth series, given their “unique” and “defining” characters/combinations, one could identify each specimen as a separate taxon. How then are we to separate taxa from ontogimorphs? The case for Triceratops ontogeny and synonymy, although still debated, (Scannella and Horner, 2010, 2011; Longrich and Field, 2012; Mairoino and others, 2013) represents a good case study. This species-rich genus was previously thought to consist of over a doz- en species, all co-existing. However, analyses by Horner and Goodwin (2006, 2008) demonstrated that this high degree of morphologic variability was largely caused by ontogenetic development within two species. Thus, the Hell Creek Formation chasmosaurines were reduced to T. horridus, T. prorsus, and Torosaurus latus. Scannella and Horner (2010) later “reduced” this diversity even more by identifying that Torosaurus was an extremely mature individual of Triceratops. From the studies of Horner and Goodwin (2006, 2008) and Scannella and Horner (2010), the Hell Creek Formation “lost” over 90% of its chasmosaurine diversity. The conclusions of Horner and Goodwin (2006; 2008) were reached based on morphology that was ultimately corroborated via histology (Scannella and Horner, 2010; Horner and Lamm, 2011); thus, histol- ogy proved to be the test. Combining morphology and histology allowed for the recognition of ontogenetic change, and how these changes occurred (echoed by Hone and others, 2016). As done for Triceratops, we should be taking a similar approach towards the Mor- rison Formation sauropods. Once several specimens have been morphologically and histologically studied, then comparisons can be made regarding (1) changes through ontogeny and (2) differences among taxa. In doing so, groupings or patterns may start to occur, such as different growth stages grouping together (“juve- niles” will group separately from “adults”), and similar- ities and/or differences among taxa will become more evident (i.e., immature Diplodocus and Apatosaurus are more alike, but both are more distinct from immature Camarasaurus; Woodruff and others, 2017; figure 2). Given the incompleteness of the fossil record, many purportedly new genera may represent immature speci- mens of known taxa. The Morrison Formation sauropod Suuwassea emilieae may be one such example. Recog- nized from a single individual (specimen ANS 21122), the taxonomy of Suuwassea has been debated (Harris and Dodson, 2004; Lovelace and others, 2008; Whitlock and Harris, 2010; Whitlock, 2011a; Woodruff and Fowl- er, 2012; Wedel and Taylor, 2013; Hedrick and others, 2014; Tschopp and others, 2015; Woodruff and others, 2017). Histology shows that specimen ANS 21122 lacks an External Fundamental System (EFS) (Hedrick and others, 2014; Woodruff and others, 2017; EFS, the skele- tal indicator of osteogenesis—Padian and Lamm, 2013). Yet the morphologies have been interpreted to be valid, distinguishable characters (Harris and Dodson, 2004; Wedel and Taylor, 2013; Hedrick and others, 2014), or alternatively ontogenetic (Woodruff and Fowler, 2012; Woodruff and others, 2017). Aside from the differing taxonomic interpretations, the holotype of Suuwassea represents a relatively small-statured immature animal with intriguing morphologies (see a list of these features in Woodruff and Fowler, 2012). Currently the holotypic material of Suuwassea could equally represent an im- mature individual from a known taxon, a distinct taxon, a maturationally varied individual or taxon with pedo- morphic attributes, and possibly a combination of these conditions. At this time the holotype does not unani- mously support one distinct interpretation. Another example from the Morrison Formation is the sauropod Diplodocus hallorum. Originally known 97 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Figure 2. Caption is on the following page. 98 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 as “Seismosaurus” (specimen NMMNH P-3690; Gil- lette, 1991), this specimen likely represents one of the largest Morrison Formation sauropods (Woodruff and Foster, 2014). Whereas this taxon was originally diag- nosed mainly on postcranial proportional differences, in a reassessment of the genus, Lucas and others (2006) proposed that these differences were simply due to its extreme body size (and the “hook-like” ischium was found to be inaccurate; Lucas and others, 2006). Now indistinguishable from Diplodocus, Lucas and others (2006) synonymized “Seismosaurus” into a species of Diplodocus. In their taxonomic revision of Diplodo- cidea, Tschopp and others (2015) phylogenetically recognized several specimens now referable to D. hal- lorum; however, the question remains whether the ho- lotype of D. hallorum represents a distinct species, or an incredibly elderly Diplodocus. (Histologic analysis of specimen NMMNH P-3690 is currently underway by DCW and K. Waskow). Similar to the case of D. hal- lorum is that of Amphicoelias. The holotype material of A. fragillimus hints at a posterior dorsal vertebra in excess of 2.8 m – making it unquestionably the largest vertebrate ever (Carpenter, 2006; Woodruff and Foster, 2014; see also Carpenter, 2018). Serious doubt should be raised to the validity of this taxon (as all of the ho- lotype material vanished; Woodruff and Foster, 2014), whereas we now have revised autamorphies for this species (Tschopp and others, 2015), as some of the first autamorphies for D. hallorum were size related (such as a more robust pubis; Lucas and others, 2006), does “big” necessarily equal distinct? Ontogeny should not just be considered when ex- amining small-bodied individuals; large-bodied speci- mens can be just as guilty (Trujillo and others, 2011). The rarity of Torosaurus compared to the prolific Tric- eratops was proposed by Scannella and Horner (2010) to be the result of “Torosaurus” being a senescent Tric- eratops. Nature is unkind to the young and old, and at- tritional mortality shows that samples of specimens in these age ranges should be underrepresented (Lyman, 1994). Whereas immature sauropods are known from the Morrison Formation (Gilmore, 1925; Carpenter and McIntosh, 1994; Foster, 1995; Britt and Naylor, 1996; Curtice and Wilhite, 1996; Curry, 1999; Foster, 2005a; Myers and Storrs, 2007; Schwarz and others, 2007; Myers and Fiorillo, 2009; Whitlock and others, 2010; Carballido and others, 2012; Storrs and others, 2012; Tschopp and Mateus, 2013; Hedrick and others, 2014; Tschopp and others, 2015; Woodruff and oth- ers, 2015, 2017; Melstrom and others, 2016; Hanik and others, 2017), it might be possible that a few of these specimens, though identified as different species, may alternatively represent differing ontogenetic stages of known taxa. And while not yet histologically demon- strated, the case of “Elosaurus” parvus (Peterson and Gilmore, 1902) representing an immature Apatosaurus (McIntosh, 1995; or Brontosaurus by Tschopp and oth- ers, 2015), or Diplodocus “lacustris” (Marsh, 1884) being an immature Diplodocus (Upchurch and others, 2004; although see Tschopp and others, 2015), demonstrate that even previously, Morrison Formation sauropod di- versity reconstructions have been altered by ontogeny. Figure 2 is on the previous page. The interpretations that could result from single lines of evidence. Morphology: not only differing body sizes, but the differing cranial, vertebral, and limb morphologies between two specimens could result in the two being interpreted as separate taxa. Histology: from just the histology, only the individual ages of these specimens could be assessed from a growth perspective. However, by increasing the lines of evidence (here with histology and morphology), and additionally by incorporating more specimens, we see a growth series. Growth is gradational, so we should expect to see not just the extremes, but transitional forms (so “unique” char- acter combinations may instead represent hallmarks of ontogeny). Human scale bar is Augustus Saint-Gaudens’ Diana of the Tower, depicting Diana as 1.83 m. Femora and histologic images modified from Woodruff and others (2017). Diplodocus sp. silhouettes modified from Woodruff and others (2017) and originally based on art by S. Hartman available via PhyloPic (Creative Commons Attribution-ShareAlike 3.0 Unported). Immature Diplodocus sp. skull drawing in Histology and Histo. - Morph. by K. Scannella, adult Diplodocus sp. and Suuwassea emilieae skulls in Histology and Histo. - Morph. from Whitlock (2011b). 99 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Biostratigraphy The Morrison Formation represents a time interval of approximately 7 million years and spanning a geo- graphical range of about 14° latitude and to 12° longi- tude (Kowallis and others, 1998; Turner and Peterson, 1999; Foster, 2007; Trujillo and Kowallis, 2015). Thus, the Morrison Formation represents a significant geo- graphic region and temporal interval. Historically, the biota appeared to be homogeneously distributed within the formation with little if any recording of stratigraphic occurrence (especially during the “Bone Wars” of O.C. Marsh and E.D. Cope; Foster, 2007). In a pioneering at- tempt to correlate localities and compile the stratigraph- ic distribution of dinosaurs, Turner and Peterson (1999) correlated 230 Morrison Formation localities to the DNM section. From such correlations, Turner and Pe- terson (1999) claimed the formation could be better un- derstood as a single functional unit. Not only did Turn- er and Peterson (1999) claim to correlate sections over vast areas with the aid of the so-called “clay change” (the sudden shift between the lower and upper Brushy Basin Member from non-smectitic to smectitic clays, which denotes increased volcanism), but they also claimed to see diversity and species patterns change. Camarasau- rus lasted longer than Apatosaurus, Diplodocus survived longer than its sister taxon Barosaurus, and the perhaps distinct Amphicoelias appeared to be the last surviving sauropod within the formation (Turner and Peterson, 1999). The incredibly detailed and thorough work of Turner and Peterson (1999) has served as a platform for subsequent Morrison Formation studies. As demonstrated within the Hell Creek Formation, stratigraphy is an important consideration for diversi- ty estimates (Horner and others, 2011; Scannella and others, 2014; Fowler, 2017). While Horner and Good- win (2006) were able to show that Triceratops species diversity was a false artifact from ontogimorphs of two species, Scannella and others (2014) was able to refine this image by incorporating stratigraphic information. By plotting the stratigraphic position of over 50 speci- mens of Triceratops and noting their ontogenetic states, Scannella and others (2014) were able to plot evolution- ary patterns. Not only did they document morpholog- ical details (such as the nasal horn increasing in size through time), but more importantly, they were able to plot the evolution of T. horridus to T. prorsus through anagenesis (Scannella and others, 2014). From the change in Triceratops within only ~1 mil- lion years (Scannella and others, 2014), one might ex- pect to see evolutionary changes in sauropods over the course of ~7 million years (Kowallis and others, 1998; Turner and Peterson, 1999; Trujillo and Kowallis, 2015) in the Morrison Formation. By correlating species oc- currence within the formation, a body size trend may exist: Apatosaurus yahnahpin (or Brontosaurus yahnah- pin; Tschopp and others, 2015) occurs prior to A. lou- isae, Camarasaurus lentus occurs before C. supremus, and Diplodocus carnegii occurs prior to D. hallorum (Turner and Peterson, 1999; figure 3). It would seem that all of the smaller species occur before the larger; therefore, one could say that Morrison Formation sau- ropods increased in size through time. However, when plotting additional species and specimens, it becomes apparent that this is not a strict rule. An Apatosaurus sp. (Museum of the Rockies [MOR] 857) from the strati- graphically lower Salt Wash Member of the Morrison Formation equivalent may be comparable in size to the largest specimen ever collected (specimen OMNH 1670) from the higher Brushy Basin Member equivalent levels of the Morrison Formation in western Oklahoma. The smallest species of Camarasaurus, C. lewisi, occurs stratigraphically between the larger C. lentus and C. su- premus, and Diplodocus carnegii occurs before the larg- er D. hallorum. However, we should not think of this as a strict trend, as the stratigraphically highest occur- rence of Apatosaurus (within the upper meters of the Brushy Basin Member in Arches National Park, Utah), is of a size typical for the average adult lower in the for- mation (Foster, 2005b). Tentatively, whereas body size increase is not a strict rule throughout the formation, average body size increases may be a legitimate trend (sensu Foster, 2007). Plotting sauropod genera stratigraphically may even have the potential to highlight major evolu- tionary processes. In 2013, the diplodocid Kaated- ocus siberi (Tschopp and Mateus, 2013) was named from the Howe-Stevens Quarry in north-central Wy- oming. Whereas there has been some informal dis- cussion as to whether this taxon is distinct or not, 100 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Fi gu re 3 . Th e la rg es t s pe ci es o f A pa to sa ur us , C am ar as au ru s, an d D ip lo do cu s a pp ea r st ra tig ra ph ic al ly h ig he r co m pa re d to th ei r sm al le r co un te rp ar ts . Th us , o ne m ig ht in fe r t ha t M or ris on F or m at io n sa ur op od g en er a i nc re as e i n bo dy si ze th ro ug h tim e ( A ). H ow ev er , b y pl ot - tin g m or e sp ec im en s/ sp ec ie s ( B) , w e se e th at th e pr ev io us a ss um pt io n is no t a ru le n or a st ric t p at te rn . W he re as a n in cr ea se in a ve ra ge bo dy si ze a m on gs t s au ro po d ta xa m ay b e a le gi tim at e tr en d ob se rv ed w ith in th e M or ris on F or m at io n, w e ar e no t a dv oc at in g or su gg es t- in g C op e’s L aw (t ha t l in ea ge s m us t i nc re as e in si ze th ro ug h tim e) . S ilh ou et te s b y S. H ar tm an a va ila bl e vi a Ph yl oP ic (C re at iv e C om m on s At tr ib ut io n- Sh ar eA lik e 3. 0 U np or te d) . A fu ll- siz e ve rs io n of fi gu re 3 is in th e At ta ch em en ts p an e. 101 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 based on cranial and vertebral morphologies the holo- type of Kaatedocus may represent an immature individ- ual (D.C. Woodruff, personal observations; and note not a synonymous ontogimorph). What has not been discussed in-depth is the stratigraphic occurrence. Cor- related to the DNM stratigraphic section (Turner and Peterson, 1999), Kaatedocus appears to be stratigraph- ically lower than any reported Diplodocus. Accord- ing to Turner and Peterson (1999), all Diplodocus and Barosaurus specimens occur within the upper part of the Salt Wash or Brushy Basin Members or their equiv- alents, whereas Kaatedocus may occur equivalently in the lower regions of the Salt Wash Member. More work and better resolution is needed—both ontogenetic and stratigraphic—but the apparent stratigraphic distinct- ness could be used as evidence in favor for the validity of Kaatedocus. Furthermore, given that Kaatedocus is geologically one of oldest Morrison Formation diplodo- cids known, it could represent part of a diplodocine cladogenic or anagenetic lineage (figure 4). As the Mor- rison Formation temporally represents an increasing terrestrial space (Foster, 2007), and as fauna migrate into this newly acquired space through time, could we expect to document evolutionary changes? Anagene- sis (potentially with Kaatedocus and the co-occurring diplodocid Galeamopus) and cladogenesis (Apatosau- rus vs. Brontosaurus?) are evolutionary processes that should be considered. Fortunately, due to the wealth of time spent exploring the Morrison Formation and the vast number of specimens collected (Foster, 2007; Brinkman, 2010), these kinds of questions have the po- tential to be examined. Finally, the assumptions and inferences regarding Morrison Formation stratigraphy could be derived from a false signal. As presented herein, stratigraphic distribution of sauropods within the Morrison has been based around the framework created by Turner and Peterson (1999). Within the past decade, the reliabili- ty of Turner and Peterson (1999) has been questioned. Starting with Trujillo (2006), it was demonstrated that the “clay change” was not supported by clay mineralogy nor X-ray diffraction (XRD) data. According to Trujil- lo (2006), this marker unit was not a consistently de- veloped horizon, and within a given section numerous clay-type alterations could occur. These findings meant that correlating sections in this manner, particular- ly over such long distances, was not reliable (Trujillo, 2006). After the Trujillo (2006) “clay change” study, there were a series of seminal papers reporting on U-Pb dates and recalibrated 40Ar/39Ar dates for several Morrison Formation localities (Trujillo and Chamberlain, 2013; Trujillo and others, 2014; Trujillo and Kowallis, 2015). Radiometric dating (such as K/Ar or 40Ar/39Ar) can rely on silicate (or potassium feldspar) minerals from over- lying or underlying volcaniclastic deposits to determine a relative temporal interval (Olsson, 1986). However, by using non-detrital zircon crystals from smectitic mud- stones, some of these newer studies were able to deter- mine dates for localities previously dated with detrital material, in addition to new, and geographically distant localities (Trujillo and Chamberlain, 2013; Trujillo and others, 2014; Trujillo and Kowallis, 2015). As the stan- dard used for 40Ar/39Ar dating and decay constants be- came over time more refined, these necessitated recal- ibration; and these recalibrated dates now correspond more favorably to the same derived from U-Pb (Trujillo and Kowallis, 2015). With these refined dates, while rel- ative stratigraphic position of many localities remains the same, their position within a chronostratigraph- ic context has changed. From 40Ar/39Ar dating, Turner and Peterson (1999) had the Mygatt-Moore Quarry in western Colorado correlated within the upper portion of the upper Brushy Basin Member between 147.8 ± 0.6 and 150.3 ± 0.3 Ma. However, new studies conducted using zircon U-Pb chemical abrasion (CA-TIMS), still place this locality within the lower portion of the upper Brushy Basin Member, but with a date of 152.18 ± 0.29 Ma (Trujillo and others, 2014; figure 4). In fact, based on recalibrated 40Ar/39Ar dates, the entire temporal po- sitioning has likewise changed. From 40Ar/39Ar dating, Turner and Peterson (1999) chronostratigraphically placed the correlated dinosaur quarries between 148.1 ± 0.5 and 154.8 ± 0.6 Ma, yet 40Ar/39Ar recalibration by Trujillo and Kowallis (2015) refined this range to 150.00 ± 1.03 and 156.84 ± 1.18 Ma, respectively. Newer magnetostratigraphic and sequence strati- graphic studies are also changing the interpretation of the northern extent of the formation. Following the correlated section of Turner and Peterson (1999), lo- 102 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Figure 4. (A) The stratigraphic section of the Morrison Formation (from Turner and Peterson, 1999) with the recalibrated dates of Trujillo and Kowallis (2015). Note the locations of the Mygatt-Moore Quarry (CO-21; red) and the O’Hair Quarries (MT-2; blue). In Turner and Peterson (1999) the Mygatt-Moore Quarry was correlated within the upper portion of the upper Brushy Basin Member between 147.8 ± 0.6 and 150.3 ± 0.3 Ma. With the recalibrated 40Ar/39Ar dates of Trujillo and others (2014), this locality still remains within the upper portion of the upper Brushy Basin Member, but now at 152.18 ± 0.29 Ma. The outlined blue box represents the correlated posi- tion of O’Hair Quarries (MT-2) from Turner and Peterson (1999). The solid blue box represents this locality’s new correlated position based on new magnetostratigraphic (Maidment and Muxworthy, 2016) and sequence strati- graphic studies (McMullen and others, 2014; McMullen, 2016). The undulating gray line is to cross out the “clay change” of Turner and Peterson (1999). Portions of this image – particularly those pertaining to the palynomorph zones and the recalibrated 40Ar/39Ar dates – are modified from Trujillo (2016). (B) Possible trends in Camarasau- rus body size throughout the formation. (C) Possible explanation of Kaatedocus within the evolutionary trajectory of Diplodocus. Silhouettes by S. Hartman available via PhyloPic (Creative Commons Attribution-ShareAlike 3.0 Unported). 103 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 calities from Montana were placed equivalently within the upper portion of the Salt Wash Member (specifi- cally the O’Hair Quarries; Cooley and Schmitt, 1998; Schimelfening and others, 2014). In addition to correla- tion with the DNM section, association with the under- lying marine Swift Formation and the predominance of sandstone to mudstone beds, all supported a Salt Wash Member-equivalent position (Cooley and Schmitt, 1998; Turner and Peterson, 1999; Schimelfening and others, 2014). However, new magnetostratigraphic (Maidment and Muxworthy, 2016) and sequence strati- graphic analyses (McMullen and others, 2014; McMul- len, 2016) are changing the position of these localities. The magnetostratigraphic analyses of Maidment and Muxworthy (2016) proposes that the O’Hair Quarries are temporally equivalent to the upper Brushy Basin Member in Dinosaur National Monument (figure 4). How then can stratigraphic correlation and mag- netostratigraphy produce such different results? In the beginning of the deposition of the Morrison Formation, the terrestrial area in the southwest was adjacent to the Sundance Sea. Later this same region was seasonally arid, or savanna-like (Foster, 2007). But as the Sundance Sea kept retreating northwards, the contiguous terrestrial deposits would continue to have a coastal composition. Systems and environments do vary latitudinally, but the lower stratigraphic placement of the Montana localities could be due to an inaccurate stratigraphic signal. A time-transgressive Morrison Formation has been previ- ously proposed by Harris and Dodson (2004) and Har- ris (2005), and perhaps the geographical extremes of the formation are the better locations to study this phe- nomenon. If this is correct, then due to the regression of the Sundance Sea, by the time Montana had become terrestrial, it would have been temporally equivalent to Dinosaur National Monument. But the coastal environ- ment in Montana would result in depositional systems similar to those recognized in the lower and earlier por- tions of the formation. In light of these newer and ongoing studies, the work of Turner and Peterson (1999) should still be com- mended. Prior to Turner and Peterson (1999), Morrison Formation stratigraphic research had little, if any, cohe- sion or unity. Workers outside of the Colorado Plateau region were unable to place and understand their local- ities within the context of the entire formation. The cor- relatable “clay change” of Turner and Peterson (1999) at the time seemed to change that. Even in consideration of these studies incorporating new techniques/recali- bration (Trujillo and Chamberlain, 2013; Trujillo and others, 2014; Trujillo and Kowallis, 2015), the Morrison Formation may be too large to correlate, and perhaps geographical regions are not a part of the same deposi- tional system (S. McMullen, Hess Corporation, written communication, 2018). Certainly, regional correlation may still possible, but the aforementioned radiometric, magnetostratigraphic, and sequence stratigraphic stud- ies collectively suggest that incorporating all of this new information should result in a better and more accu- rate reconstruction of these localities. An initial blend of this new information is already being performed by Tschopp and others (2016). By taking Morrison Forma- tion diplodocid occurrences and incorporating them into the magnetostratigraphy of Maidment and Mux- worthy (2016), Tschopp and others (2016) may be able to reanalyze these basic Morrison Formation sauropod questions, and I greatly await these results and future works. Furthermore, while we continue to relocate, recal- ibrate, and reanalyze specimens within a stratigraphic context, we can thus far identify—even at coarse resolu- tion—some levels of biozones (sensu Foster, 2003, 2007; figure 5). Due to the inherent inaccuracies in assuming formational homogeneity, Foster (2003, 2007) suggest- ed that differing paleoenvironments throughout the for- mation would consist of differing groups of dinosaurs. The benefits of biozone demarcations are that they re- flect environmental groupings throughout the course of the formation (i.e., environments + time versus strict- ly time). Until we have a better understanding of the time component, which admittedly could significantly alter the biozone signals, grouping via biozones may be a more neutral way to group or demarcate Morrison Formation sauropod taxa. There are four tentative bio- zone signals that we may see within the Morrison For- mation. (1) The lowermost occurrence of Barosaurus, Dystrophaeus, and Haplocanthosaurus in Zone 1 may indicate that these forms could represent or be relics of pre-Morrison Formation sauropod genera. (2) The low sauropod diversity of Zones 1, 3, and 6 could indicate 104 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Fi gu re 5 . B io zo ne d em ar ca tio ns o f s au ro po ds w ith in th e M or ris on F or m at io n. B io zo ne s a re fr om F os te r ( 20 03 ) w ith ad di tio na l s pe ci m en s pl ot te d ac co rd in g to st ra tig ra ph ic p la ce m en t v ia T ur ne r a nd P et er so n (1 99 9) . S tr at ig ra ph ic se ct io n is fr om F os te r ( 20 03 ). C lo se d sil ho u- et te s r ep re se nt g en er a w ith id en tifi ed sp ec ie s, op en si lh ou et te s r ep re se nt o nl y ge ne ric le ve l r ef er ra l. Si lh ou et te s b y S. H ar tm an a va ila bl e vi a Ph yl oP ic (C re at iv e C om m on s A ttr ib ut io n- Sh ar eA lik e 3. 0 U np or te d) . S ilh ou et te s t o re la tiv e sc al e. A fu ll- siz e ve rs io n of fi gu re 5 is in th e At ta ch em en ts p an e. 105 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 negative events/environments, or this signal could in- dicate poor sampling. (3) The ecology of Zones 5 and 6 may have been more conducive to speciation—these zones exhibit the most genera and the greatest number of species. (4) Out of all of the sauropod genera that appear in at least two zones, only Barosaurus and Bra- chiosaurus have a single species—such could indicate something special about their biology (being longer oc- curring species), or their diversity (number of species) may be incorrect (note that Woodruff and others, 2017 suggest that based on possible morphologic differences seen stratigraphically, Barosaurus may represent more than a single species). Ecological Capacity In addition to ontogeny and stratigraphy, there are several other factors at play that could equally be inter- fering with our signal of Morrison Formation sauropod diversity. As previously mentioned, 24 different kinds of multi-ton herbivores across a single landscape would theoretically be ecologically taxing. Many studies have proposed ecological segregation among the Morrison Formation sauropods including feeding height (Bakker, 1971; Dodson and others, 1980; Bakker, 1986; Martin, 1987; Stevens and Parrish, 1999, 2005; Upchurch and Barrett, 2000; Christian, 2002; Foster, 2003, 2007; Dz- emski and Christian, 2007; Christian, 2010; Hummel and Clauss, 2011), dietary niche partitioning (Gal- ton, 1986; Fiorillo, 1998; Upchurch and Barrett, 2000; Christian, 2002; Foster, 2003, 2007; Engelmann and others, 2004; Stevens and Parrish, 2005; Carpenter, 2006; Whitlock and others, 2010; Hummel and Clauss, 2011; Young and others, 2012; D’Emic and others, 2013; Button and others, 2014; Woodruff and others, 2015), or ontogenetic segregation (Dodson and others, 1980; Foster, 2003; Myers and Storrs, 2007; Myers and Fioril- lo, 2009; Woodruff and others, 2015). Such ecological segregation undoubtedly had to occur, and likely sev- eral of these forms co-occurred—such as young sauro- pods feeding on different plant material than their adult forms (sensu Whitlock and others, 2010 and Woodruff and others, 2015). Not only are feeding-related factors important, but so too is their respect to body size. The African savanna today is made up a multitude of co-oc- curring herbivores ranging from the dik-dik (up to 6 kg; Grubb, 2005) to the African elephant (up to 10,400 kg; Larramendi, 2016). Several exemplary studies have ex- amined the relationship between body size and foraging height to explain the extreme prevalence of herbivores in this ecosystem (Du Toit, 1990, 2003; Woolnough and Du Toit, 2001; Fritz and others, 2003; Cameron and Du Toit, 2006; Anderson and others, 2016), and possibly a similar body size to feeding height/vegetation type stratification occurred within the sauropods of the Mor- rison Formation. Perhaps the predominance of small- er body sized Morrison Formation sauropods, such as Camarasaurus (approximately 12,530 kg for C. grandis; specimen GMNH 101 – this analysis) versus the rarity of larger body sized taxa, such as Supersaurus (36,287 kg; Lovelace and others, 2008) may be analogous to the pattern seen in the African savanna. Individual Morphological Variation within Species Another possible confounding factor is individual variation. Unlike the degree of variation observed with- in hadrosaurs (Campione and Evans, 2011; Fowler and Horner, 2015; Woodward and others, 2015; McFeeters and others, 2018; Takasaki and others, 2018) and cera- topsians (Scannella and Horner, 2010; Frederickson and Tumarkin-Deratzian, 2014; Scannella and others, 2014; Campbell and others, 2016, 2018), the exacting degrees of morphologic variation in Morrison Formation sauro- pods have not been precisely quantified. Morrison sau- ropod workers have colloquially referred to observed variation, but as morphologic variation in sauropods can derive from taxonomy or body size (among others), denoting whether a character is taxonomically distinct or within the range of variation can become more neb- ulous (Foster, 2015). Also, treating what may be contin- uous variation as discrete characters from low sample sizes is likely to be problematic (J. Foster, Utah Field House of Natural History State Park Museum, written communication, 2018). As an example, let us examine cranial openings. One could presume that the number of cranial openings would be a significant taxonomic signal. Therefore, said number or presence/absence of specific openings would seemingly be a strong phyloge- 106 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 netic indicator. And yet, this is absolutely not the case in Camarasaurus. The frontal aperture (Woodruff and Foster, 2017) is variably present among Camarasaurus specimens of various species and body sizes (echoed in Madsen and others, 1995). If the presence of an extra cranial opening is demonstrably not a taxonomically vi- able character in a Morrison Formation sauropod, then what characters are significant, and how do we begin to measure and quantify such variability? CONCLUSIONS The information presented herein and throughout this themed set of Morrison Formation papers, begin- ning in 2016 in the Geology of the Intermountain West, will hopefully contribute to a shift in our understanding of the true nature and complexities within the Morri- son. In regard to the sauropod diversity, the Morrison Formation may have legitimately exhibited unparal- leled sauropod diversity. However, stratigraphy and on- togeny, among others, have been demonstrated to have repercussions on diversity estimates. Previously, the Hell Creek Formation was thought to harbor nearly three dozen dinosaurian species. However, subsequent analyses examining the critical variables of ontogeny, stratigraphy, and variation have shown that while still highly diverse, this diversity is in actuality constructed by less than half of the perceived species (Carr and Williamson, 2004; Horner and Good- win, 2009; Scannella and Horner, 2010; Campione and Evans, 2011). Certainly, there are unique attributes of the Morrison Formation, one such is clade represen- tation. Within the Hell Creek Formation, species rich- ness seems fairly evenly distributed across the different clades. Yet, in the Morrison Formation, the dominating clade constitutes almost an equal number of species to all the other clades combined (26 versus 24, respec- tively). As in the Hell Creek Formation, the Morrison Formation was likely very species rich. This review is not meant to admonish against richness, merely to con- template the multifaceted factors that could affect said richness. In consideration of the expanse that is the Morrison Formation, approximately 7 million years, and a geo- graphical range of 14 degrees of latitude and 12 degrees of longitude (Kowallis and others, 1998; Turner and Pe- terson, 1999; Foster, 2007; Trujillo and Kowallis, 2015), stratigraphy, ontogeny, variation, evolutionary patterns, and environments should have some effect on sauropod diversity. Currently the degree or significance of these factors towards Morrison Formation sauropod diversi- ty is unresolved; yet in another formation it has been demonstrated that these factors do alter diversity recon- structions. Therefore, until these factors are accounted for, we may not be accurately reconstructing the true diversity of sauropods within the Morrison Formation. ACKNOWLEDGMENTS Thanks to the Society of Vertebrate Paleontology for hosting the “An Ecosystem We Thought We Knew: The Emerging Complexities of the Morrison Formation” symposium during the 76th annual meeting, and to all of the presenters for such an informative, engaging, and thought-provoking symposium. My presentation co-author Denver Fowler (Dickinson Museum Center) provided numerous encouraging and lively discussions that culminated in this work. Finally, thanks to V. Díez Díaz (Universidad del Pais/Euskal Herriko Unibert- sitatea), F. Holwerda (Bayerische Staatssammlung für Paläontologie und Geologie), and J. Richard (Delgado Community College) for review comments on the man- uscript; J. Foster (Utah Field House of Natural History State Park Museum), and K. Trujillo (Laramie County Community College) for editorial assistance and com- ments on an earlier version of the manuscript; and K. Nordén (Princeton University) and D. Fowler (Dickin- son Museum Center) for fruitful discussions and en- couragement. REFERENCES Anderson, T.M., White, S., Davis, B., Erhardt, R., Palmer, M., Swanson, A., Kosmala, M., and Packer, C., 2016, The spa- tial distribution of African savannah herbivores—species associations and habitat occupancy in a landscape context: Philosophical Transactions of the Royal Society B, v. 371, 20150314, p. 1–14, http://dx.doi.org/10.1098/rstb.2015.0314. Bakker, R.T., 1971, Ecology of the brontosaurs: Nature, v. 229 (5281), p. 172–174. Bakker, R.T., 1986, The dinosaur heresies: New York, William 107 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Morrow, 481 p. Brinkman, P.D., 2010, The second Jurassic dinosaur rush—mu- seums and paleontology in America at the turn of the twen- tieth century: Chicago, Illinois, University of Chicago Press, 345 p. Britt, B.B., and Naylor, B.G., 1996, An embryonic Camarasaurus (Dinosauria, Sauropoda) from the Upper Jurassic Morrison Formation (Dry Mesa Quarry), in Carpenter, K., Hirsch, K.F., and Horner, J.R., editors, Dinosaur eggs and babies: New York, Cambridge University Press, p. 256–264. Button, D.J., Rayfield, E.J., and Barrett, P.M., 2014, Cranial biome- chanics underpins high sauropod diversity in resource-poor environments: Proceedings of the Royal Society B, v. 281, 20142114, p. 1–9, https://doi.org/10.1098/rspb.2014.2114. Cameron, E.Z., and Du Toit, J.T., 2006, Winning by a neck—tall giraffes avoid competing with shorter browsers: The Ameri- can Naturalist: v. 169, p. 130–135. Campbell, J.A., Ryan, M.J., Holmes, R.B., and Schröder-Adams, C.J., 2016, A re-evaluation of the chasmosaurine ceratopsid genus Chasmosaurus (Dinosauria: Ornithischia) from the Upper Cretaceous (Campanian) Dinosaur Park Formation of western Canada: PLoS ONE, v. 11, no. 1, e0145805, p. 1–26, https://doi.org/10.1371/journal.pone.0145805. Campbell, J.A., Ryan, M.J., Schröder-Adams, C.J., Evans, D.C., and Holmes, R.B., 2018, New insights into chasmosaurine (Dinosauria: Ceratopsidae) skulls from the Upper Creta- ceous (Campanian) of Alberta, and an update on the distri- bution of accessory frill fenestrae in Chasmosaurinae: PeerJ 6:e5194, p. 1–39, https://doi.org/10.7717/peerj.5194. Campione, N.E., and Evans, D.C., 2011, Cranial growth and vari- ation in edmontosaurs (Dinosauria: Hadrosauridae)—im- plications for latest Cretaceous megaherbivore diversity in North America: PLoS ONE, v. 6, no. 9, e25186, 12 p. Carballido, J.L., Marpmann, J.S., Schwarz‐Wings, D., and Pabst, B., 2012, New information on a juvenile sauropod specimen from the Morrison Formation and the reassessment of its systematic position: Palaeontology, v. 55, p. 567–582. Carballido, J.L., and Sander, P.M., 2014, Postcranial axial skeleton of Europasaurus holgeri (Dinosauria, Sauropoda) from the Upper Jurassic of Germany—implications for sauropod on- togeny and phylogenetic relationships of basal Macronaria: Journal of Systematic Palaeontology, v. 12, p. 335–387. Carpenter, K., 2006, Biggest of the big—a critical re-evaluation of the mega-sauropod Amphicoelias fragillimus Cope, 1878, in Foster, J.R., and Lucas, S.G., editors, Paleontology and ge- ology of the Upper Jurassic Morrison Formation: New Mex- ico Museum of Natural History and Science Bulletin 36, p. 131–137. Carpenter, K., 2018, Maraapunisaurus fragillimus, N.G. (formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the Morrison Formation (Upper Jurassic) of Colorado: Geology of the Intermountain West, v. 5, p. 227–244. Carpenter, K., and McIntosh, J., 1994, Upper Jurassic sauropod babies from the Morrison Formation, in Carpenter, K., Hirsch, K.F., and Horner, J.R., editors, Dinosaur eggs and babies: New York, Cambridge University Press, p. 265–278. Carr, T.D., and Williamson, T.E., 2004, Diversity of late Maas- trichtian Tyrannosauridae (Dinosauria: Theropoda) from western North America: Zoological Journal of the Linnean Society, v. 142, p. 479–523. Chiappe, L.M., Coria, R.A., Dingus, L., Jackson, F., Chinsamy, A., and Fox, M., 1998, Sauropod dinosaur embryos from the Late Cretaceous of Patagonia: Nature, v. 396, p. 258–261. Christian, A., 2002, Neck posture and overall body design in sau- ropods: Fossil Record, v. 5, no. 1, p. 271–281. Christian, A., 2010, Some sauropods raised their necks—evi- dence for high browsing in Euhelopus zdanskyi: Biology Let- ters, v. 6, p. 823–825. Cooley, J.T., and Schmitt, J.G., 1998, Sedimentology and stra- tigraphy—an anastomosed fluvial system in the Morrison Formation (Upper Jurassic) of southwest Montana: Modern Geology, v. 22, p. 171–208. Curry, K.A., 1999, Ontogenetic histology of Apatosaurus (Dino- sauria: Sauropoda)—new insights on growth rates and lon- gevity: Journal of Vertebrate Paleontology, v. 19, p. 654–665. Curtice, B.D., and Wilhite, D.R., 1996, A re-evaluation of the Dry Mesa Dinosaur Quarry sauropod fauna with a description of juvenile sauropod elements, in Huffman, A.C., Jr., Lund, W.R., and Godwin, L.H., editors, Geology and Resources of the Paradox Basin: Utah Geological Association Publication 25, p. 325–338. D’Emic, M.D., Whitlock, J.A., Smith, K.M., Fisher, D.C., and Wilson, J.A., 2013, Evolution of high tooth replacement rates in sauropod dinosaurs: PLoS ONE, v. 8, no. 7, e69235, doi:10.1371/journal.pone.0069235. Dodson, P., Behrensmeyer, A.K., Bakker, R.T., and McIntosh, J.S., 1980, Taphonomy and paleoecology of the dinosaur beds of the Jurassic Morrison Formation: Paleobiology, v. 6, p. 208– 232. Du Toit, J.T., 1990, Feeding‐height stratification among African browsing ruminants: African Journal of Ecology, v. 28, p. 55–61. Du Toit, J.T., 2003, Large herbivores and savanna heterogeneity— the Kruger experience, in Du Toit, J.T., Rogers, K.H., and Biggs, H.C., editors: Ecology and management of savanna heterogeneity: Washington, D.C., Island Press, p. 292–309. 108 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Dzemski, G., and Christian, A., 2007, Flexibility along the neck of the ostrich (Struthio camelus) and consequences for the re- construction of dinosaurs with extreme neck length: Journal of Morphology, v. 268, p. 701–714. Engelmann, G.F., Chure, D.J., and Fiorillo, A.R., 2004, The impli- cations of a dry climate for the paleoecology of the fauna of the Upper Jurassic Morrison Formation: Sedimentary Geol- ogy, v. 167, p. 297–308. Erickson, G.M., Rogers, K.C., and Yerby, S.A., 2001, Dinosaurian growth patterns and rapid avian growth rates: Nature, v. 412, p. 429–433. Fiorillo, A.R., 1998, Dental micro wear patterns of the sauropod dinosaurs Camarasaurus and Diplodocus—evidence for re- source partitioning in the Late Jurassic of North America: Historical Biology, v. 13, p. 1–16. Foster, J.R., 1995, Allometric and taxonomic limb bone robust- ness variability in some sauropod dinosaurs [abs.]: Journal of Vertebrate Paleontology, v. 15, supplement to number 3, p. 29A. Foster, J.R., 2003, Paleoecological analysis of the vertebrate fauna of the Morrison Formation (Upper Jurassic), Rocky Moun- tain region, USA: New Mexico Museum of Natural History and Science Bulletin 23, 95 p. Foster, J.R., 2005a, New juvenile sauropod material from western Colorado, and the record of juvenile sauropods from the Up- per Jurassic Morrison Formation, in Carpenter, K., and Tid- well, V., editors, Thunder-lizards—the sauropodomorph di- nosaurs: Bloomington, Indiana University Press, p. 141–153. Foster, J.R., 2005b, New sauropod dinosaur specimens found near Moab, Utah, and the sauropod fauna from the Morrison Formation: Canyon Legacy, v. 55, p. 22–27. Foster, J.R., 2007, Jurassic west—the dinosaurs of the Morrison Formation and their world: Bloomington, Indiana Universi- ty Press, 389 p. Foster, J.R., 2015, Dangers of low sample size in studies of sauro- pod dinosaur species diversity—a Morrison Formation case study [abs.]: Journal of Vertebrate Paleontology, Program and Abstracts, 2015, p. 126. Fowler, D.W., 2017, Revised geochronology, correlation, and di- nosaur stratigraphic ranges of the Santonian-Maastrichtian (Late Cretaceous) formations of the Western Interior of North America: PLoS ONE, v. 12, no. 11, e0188426, 20 p., https://doi.org/10.1371/journal.pone.0188426. Fowler, E.A.F., and Horner, J.R., 2015, A new brachylophosauran hadrosaur (Dinosauria: Ornithischia) with an intermediate nasal crest from the Campanian Judith River Formation of northcentral Montana: PLoS ONE, v. 10, no. 11, e0141304, p. 1–35, https://doi.org/10.1371/journal.pone.0141304. Frederickson, J.A., and Tumarkin-Deratzian, A.R., 2014, Cra- niofacial ontogeny in Centrosaurus apertus: PeerJ, 2, e252, p. 1–32, https://doi.org/10.7717/peerj.252. Fritz, H., Duncan, P., Gordon, I.J., and Illius, A.W., 2002, Mega- herbivores influence trophic guilds structure in African un- gulate communities: Oecologia, v. 131, p. 620–625. Galton, P.M., 1986, Herbivorous adaptations of Late Triassic and Early Jurassic dinosaurs, in Padian, K., editor, The beginning of the age of dinosaurs: New York, Cambridge University Press, p. 203–221. Gillette, D.D., 1991, Seismosaurus halli, gen. et sp. nov., a new sauropod dinosaur from the Morrison Formation (Upper Jurassic/Lower Cretaceous) of New Mexico, USA: Journal of Vertebrate Paleontology, v. 11, p. 417–433. Gilmore, C.W., 1925, A nearly complete articulated skeleton of Camarasaurus, a saurischian dinosaur from the Dinosaur National Monument, Utah: Memoirs of the Carnegie Muse- um, no. 10, p. 347–384. Goodwin, M.B., and Evans, D.C., 2016, The early expression of squamosal horns and parietal ornamentation confirmed by new end-stage juvenile Pachycephalosaurus fossils from the Upper Cretaceous Hell Creek Formation, Montana: Journal of Vertebrate Paleontology, v. 36, no. 2, article e1078343. Griebeler, E.M., Klein, N., and Sander, P.M., 2013, Aging, matu- ration and growth of sauropodomorph dinosaurs as deduced from growth curves using long bone histological data—an assessment of methodological constraints and solutions: PloS ONE, v. 8, no. 8, e67012, p. 1–17, https://doi.org/10.1371/ journal.pone.0067012. Grubb, P., 2005, Genus Madoqua, in Wilson, D.E., and Reeder, D.M., editors, Mammal species of the world—a taxonom- ic and geographic reference (3rd edition): Baltimore, Johns Hopkins University Press, p. 683–684. Hanik, G.M., Lamanna, M.C., and Whitlock, J.A., 2017, A juve- nile specimen of Barosaurus Marsh, 1890 (Sauropoda: Di- plodocidae) from the Upper Jurassic Morrison Formation of Dinosaur National Monument, Utah, USA: Annals of Carn- egie Museum, v. 84, p. 253–263. Harris, J.D., 2005, A review of Morrison Formation paleogeogra- phy and tests for time-transgression and faunal provincial- ism [abs.]: Geological Society of America Abstracts with Programs, v. 37, no. 6, p. 13. Harris, J.D., and Dodson, P., 2004, A new diplodocoid sauro- pod dinosaur from the Upper Jurassic Morrison Formation of Montana, USA: Acta Palaeontologica Polonica, v. 49, p. 197–210. Hedrick, B.P., Tumarkin-Deratzian, A.R., and Dodson, P., 2014, Bone microstructure and relative age of the holotype speci- 109 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 men of the diplodocoid sauropod dinosaur Suuwassea emi- lieae: Acta Palaeontologica Polonica, v. 59, p. 295–304. Hone, D.W., Farke, A.A., and Wedel, M.J., 2016, Ontogeny and the fossil record—what, if anything, is an adult dinosaur?: Biology Letters, v. 12, no. 2, 20150947, p. 1–9, http://dx.doi. org/10.1098/rsbl.2015.0947. Horner, J.R., and Goodwin, M.B., 2006, Major cranial changes during Triceratops ontogeny: Proceedings of the Royal So- ciety of London B, Biological Sciences, v. 273, p. 2757–2761. Horner, J.R., and Goodwin, M.B., 2008, Ontogeny of cranial epi-ossifications in Triceratops: Journal of Vertebrate Paleon- tology, v. 28, p. 134–144. Horner, J.R., and Goodwin, M.B., 2009, Extreme cranial ontog- eny in the Upper Cretaceous dinosaur Pachycephalosaurus: PLoS ONE, v. 4, no. 10, e7626, 11 p., https://doi.org/10.1371/ journal.pone.0007626. Horner, J.R., Goodwin, M.B., and Myhrvold, N., 2011, Dinosaur census reveals abundant Tyrannosaurus and rare ontogenetic stages in the Upper Cretaceous Hell Creek Formation (Maas- trichtian), Montana, USA: PloS ONE, v. 6, no. 2, e16574, 9 p., https://doi.org/10.1371/journal.pone.0016574. Horner, J.R., and Lamm, E.T., 2011, Ontogeny of the parietal frill of Triceratops—a preliminary histological analysis: Comptes Rendus Palevol, v. 10, p. 439–452. Hummel, J., and Clauss, M., 2011, Sauropod feeding and diges- tive physiology, in Klein, N., Remes, K., Gee, C.T., and Sand- er, P.M., editors, Biology of the sauropod dinosaurs—under- standing the life of giants: Bloomington, Indiana University Press, p. 11–33. Klein, N., and Sander, M., 2008, Ontogenetic stages in the long bone histology of sauropod dinosaurs: Paleobiology, v. 34, p. 247–263. Kowallis, B.J., Christiansen, E.H., Deino, A.L., Peterson, F., Turn- er, C.E., Kunk, M.J., and Obradovich, J.D., 1998, The age of the Morrison Formation: Modern Geology, v. 22, p. 235–260. Larramendi, A., 2016, Shoulder height, body mass and shape of proboscideans: Acta Palaeontologica Polonica, v. 61, no. 3, p. 537–574, doi:10.4202/app.00136.2014. Lehman, T.M., and Woodward, H.N., 2008, Modeling growth rates for sauropod dinosaurs: Paleobiology, v. 34, p. 264–281. Longrich, N.R., and Field, D.J., 2012, Torosaurus is not Tricer- atops—ontogeny in chasmosaurine ceratopsids as a case study in dinosaur taxonomy: PloS ONE, v. 7, no. 2, e32623, p. 1–10, doi:10.1371/journal.pone.0032623. Lovelace, D.M., Hartman, S.A., and Wahl, W.R., 2008, Morphol- ogy of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evalua- tion of diplodocid phylogeny: Arquivos do Museu Nacional, v. 65, p. 527–544. Lucas, S.G., Spielmann, J.A., Rinehart, L.F., Heckert, A.B., Herne, M.C., Hunt, A.P., Foster, J.R., and Sullivan, R.M., 2006, Tax- onomic status of Seismosaurus hallorum, a Late Jurassic sau- ropod dinosaur from New Mexico, in Foster, J.R., and Lucas, S.G., editors, Paleontology and geology of the Upper Jurassic Morrison Formation: New Mexico Museum of Natural His- tory and Science Bulletin 36, p. 149–162. Lyman, R.L., 1994, Vertebrate taphonomy (Cambridge Manuals in Archaeology): New York, Cambridge University Press, 552 p., doi:10.1017/CBO9781139878302. Madsen, J.H., MacIntosh, J.S., and Berman, D.S., 1995, Skull and atlas-axis complex of the Upper Jurassic sauropod Camara- saurus Cope (Reptilia: Saurischia): Bulletin of the Carnegie Museum of Natural History, v. 31, p. 1–115. Maidment, S.C.R., and Muxworthy, A., 2016, A chronostrati- graphic framework for the Morrison Formation, and the lat- itudinal biodiversity gradient in Morrison dinosaurs [abs.]: Journal of Vertebrate Paleontology Program and Abstracts, p. 181–181. Maiorino, L., Farke, A.A., Kotsakis, T., and Piras, P., 2013, Is To- rosaurus Triceratops? Geometric morphometric evidence of late Maastrichtian ceratopsid dinosaurs: PLoS One, v. 8, no. 11, e81608, p. 1–15, doi:10.1371/journal.pone.0081608. Marsh, O.C., 1884, Principal characters of American Jurassic di- nosaurs, part VII, on the Diplodocidae, a new family of the Sauropoda: American Journal of Science, series 3, p. 160– 168. Martin, J., 1987, Mobility and feeding of Cetiosaurus (Saurischia: Sauropoda)—why the long neck?, in Currie, P.J., and Koster, E.H., editors, Fourth symposium on Mesozoic terrestrial ecosystems, short papers: Occasional papers of the Tyrrell Museum of Palaeontology, Drumheller, v. 3, p. 154–159. McFeeters, B., Evans, D., and Maddin, H., 2018, Variation in the braincase and cranial ornamentation of Maiasaura peeble- sorum (Ornithischia, Hadrosauridae) from the Campanian Two Medicine Formation of Montana—implications for brachylophosaurin ontogeny and evolution [abs.]: 6th An- nual Meeting, Canadian Society of Vertebrate Palaeontology, May 14–16, 2018, Ottawa, Ontario, p. 37. McIntosh, J.S., 1995, Remarks on the North American sauropod Apatosaurus Marsh: Sixth Symposium on Mesozoic Terres- trial Ecosystems and Biota, Short Papers, p. 119–123. McMullen, S.K., 2016, Controls on the stratigraphic distribution of non-marine fossils—a case study in the Jurassic Morrison Formation, western USA [abs.]: Journal of Vertebrate Pale- ontology Program and Abstracts, p. 187. 110 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 McMullen, S.K., Holland, S.M., and O’Keefe, F.R., 2014, The oc- currence of vertebrate and invertebrate fossils in a sequence stratigraphic context—the Jurassic Sundance Formation, Bighorn Basin, Wyoming, USA: Palaios, v. 29, p. 277–294. Melstrom, K.M., D’Emic, M.D., Chure, D., and Wilson, J.A., 2016, A juvenile sauropod dinosaur from the Late Jurassic of Utah, USA, presents further evidence of an avian style air- sac system: Journal of Vertebrate Paleontology, v. 36, no. 4, e1111898, DOI: 10.1080/02724634.2016.1111898. Mitchell, J., Sander, P.M., and Stein, K., 2017, Can secondary os- teons be used as ontogenetic indicators in sauropods? Ex- tending the histological ontogenetic stages into senescence: Paleobiology, v. 43, p. 321–342. Myers, T.S., and Fiorillo, A.R., 2009, Evidence for gregarious be- havior and age segregation in sauropod dinosaurs: Palaeoge- ography, Palaeoclimatology, Palaeoecology, v. 274, p. 96–104. Myers, T.S., and Storrs, G.W., 2007, Taphonomy of the Mother’s Day Quarry, Upper Jurassic Morrison Formation, south-cen- tral Montana, USA: Palaios, v. 22, p. 651–666. Olsson, I.U., 1986, Radiometric dating, in Berglund, B.E., editor, Handbook of Holocene palaeoecology and palaeohydrology: Chichester, U.K., Wiley and Sons, p. 273–312. Padian, K., and Lamm, E.T., 2013, Bone histology of fossil tet- rapods—advancing methods, analysis, and interpretation: Berkeley, University of California Press, 298 p. Peterson, O.A., and Gilmore, C.W., 1902, Elosaurus parvus; a new genus and species of the Sauropoda: Annals of the Carnegie Museum I, p. 490–499. Rogers, K.C., and Erickson, G.M., 2005, Sauropod histology, in Rogers, K.C., and Wilson, J.A., editors, The sauropods—evo- lution and paleobiology: Berkeley, University of California Press, p. 303–326. Rogers, K.C., Whitney, M., D’Emic, M., and Bagley, B., 2016, Pre- cocity in a tiny titanosaur from the Cretaceous of Madagas- car: Science, v. 352, p. 450–453. Sander, P.M., 1999, Life history of Tendaguru sauropods as in- ferred from long bone histology: Fossil Record, v. 2, p. 103– 112. Sander, P.M., 2000, Longbone histology of the Tendaguru sauro- pods—implications for growth and biology: Paleobiology, v. 26, p. 466–488. Sander, P.M., and Tückmantel, C., 2003, Bone lamina thickness, bone apposition rates, and age estimates in sauropod humeri and femora: Palaontologische Zeitschrift, v. 77, p. 161–172. Sander, P.M., Klein, N., Buffetaut, E., Cuny, G., Suteethorn, V., and Le Loeuff, J., 2004, Adaptive radiation in sauropod di- nosaurs—bone histology indicates rapid evolution of giant body size through acceleration: Organisms Diversity and Evolution, v. 4, p. 165–173. Sander, P.M., Klein, N., Stein, K., and Wings, O., 2011, Sauropod bone histology and its implications for sauropod biology, in Klein, N., Remes, K., Gee, C.T., and Sander, P.M., editors, Bi- ology of the sauropod dinosaurs—understanding the life of giants: Bloomington, Indiana University Press, p. 276–302. Scannella, J.B., and Fowler, D.W., 2009, Anagenesis in Tricer- atops—evidence from a newly resolved stratigraphic frame- work for the Hell Creek Formation: Ohio, Cincinnati Muse- um Center Science Contributions, v. 3, p. 148–149. Scannella, J.B., and Fowler, D.W., 2014, A stratigraphic survey of Triceratops localities in the Hell Creek Formation, northeast- ern Montana (2006–2010): Geological Society of America Special Paper 503, p. 313–332. Scannella, J.B., Fowler, D.W., Goodwin, M.B., and Horner, J.R., 2014, Evolutionary trends in Triceratops from the Hell Creek Formation, Montana: Proceedings of the National Academy of Sciences, p. 111, p. 10245–10250. Scannella, J.B., and Horner, J.R., 2010, Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae)— synonymy through ontogeny: Journal of Vertebrate Paleon- tology, v. 30, p. 1157–1168. Scannella, J.B., and Horner, J.R., 2011, ‘Nedoceratops’—an exam- ple of a transitional morphology: PLoS ONE, v. 6, no. 12, e28705, 9 p., https://doi.org/10.1371/journal.pone.0028705. Schimelfening, A.G., Woodruff, D.C., and Nordén, K.K., 2014, Description of a lower Morrison Formation dinosaur quarry from south-western Montana [abs.]: Geological Society of America Abstracts with Programs, v. 46, no. 5, p 13. Schwarz, D., Ikejiri, T., Breithaupt, B.H., Sander, P.M., and Klein, N., 2007, A nearly complete skeleton of an early juvenile di- plodocid (Dinosauria: Sauropoda) from the lower Morrison Formation (Late Jurassic) of north central Wyoming and its implications for early ontogeny and pneumaticity in sauro- pods: Historical Biology, v. 19, p. 225–253. Stevens, K.A., and Parrish, J.M., 1999, Neck posture and feeding habits of two Jurassic sauropod dinosaurs: Science, v. 284, p. 798–800. Stevens, K.A., and Parrish, J.M., 2005, Neck posture, dentition, and feeding strategies in Jurassic sauropod dinosaurs, in Carpenter, K., and Tidwell, V., editors, Thunder-lizards—the sauropodomorph dinosaurs: Bloomington, Indiana Univer- sity Press, p. 212–232. Storrs, G.W., Oser, S.E., and Aull, M., 2012, Further analysis of a Late Jurassic dinosaur bone-bed from the Morrison Forma- tion of Montana, USA, with a computed three-dimensional reconstruction: Earth and Environmental Science Transac- 111 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 tions of the Royal Society of Edinburgh, v. 103, p. 443–458. Takasaki, R., Chiba, K., Kobayashi, Y., Currie, P.J., and Fiorillo, A.R., 2018, Reanalysis of the phylogenetic status of Nippon- osaurus sachalinensis (Ornithopoda: Dinosauria) from the Late Cretaceous of southern Sakhalin: Historical Biology, v. 30, p. 694–711. Trujillo, K.C., 2006, Clay mineralogy of the Morrison Formation (Upper Jurassic-?-Lower Cretaceous), and its use in long dis- tance correlation and paleoenvironmental analysis, in Foster, J.R., and Lucas, S.G., editors, Paleontology and geology of the Upper Jurassic Morrison Formation: New Mexico Museum of Natural History and Science Bulletin 36, p. 17–23. Trujillo, K.C., 2016, Tying it all together—using radiometric ages to correlate fossil localities across the Upper Jurassic Morri- son Formation, Western Interior, USA [abs.]: Journal of Ver- tebrate Paleontology, Program and Abstracts, p. 238. Trujillo, K.C., and Chamberlain, K.R., 2013, The Morrison For- mation U-Pb dating project—using high-precision, chemical abrasion (CA-TIMS), single zircon, ashfall dates for chro- nostratigraphic correlations [abs.]: Journal of Vertebrate Pa- leontology, Program and Abstracts. v. 33, p. 227A. Trujillo, K., Demar, D., Foster, J., and Bilbey, S.A., 2011, An ex- ceptionally large juvenile Camarasaurus from the Morrison Formation (Upper Jurassic) of Albany County, WY, USA [abs.]: Journal of Vertebrate Paleontology, Program and Ab- stracts, v. 31, p. 205. 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, p. 107–114. Trujillo, K.C., and Kowallis, B.J., 2015, Recalibrated legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, USA: Geology of the Intermountain West, v. 2, p. 1–8. Tschopp, E., Giovanardi, S., and Maidment, S.C.R., 2016, Tem- poral distribution of diplodocid sauropods across the Upper Jurassic Morrison Formation (USA) [abs.]: Journal of Verte- brate Paleontology, Program and Abstracts, v. 2016, p. 239. Tschopp, E., and Mateus, O., 2013, The skull and neck of a new flagellicaudatan sauropod from the Morrison Formation and its implication for the evolution and ontogeny of diplodo- cid dinosaurs: Journal of Systematic Palaeontology, v. 11, p. 853–888. Tschopp, E., Mateus, O., and Benson, R.B., 2015, A specimen-level phylogenetic analysis and taxonomic revision of Diplodocid- ae (Dinosauria, Sauropoda): PeerJ 3:e857, p. 1–298, https:// doi.org/10.7717/peerj.857. Turner, C.E., and Peterson, F., 1999, Biostratigraphy of dinosaurs in the Upper Jurassic Morrison Formation of the Western In- terior, USA, in Gillette, D.D., editor, Vertebrate paleontology in Utah: Utah Geological Survey Miscellaneous Publication 99-1, p. 77–114. 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: New York, Cambridge University Press, p. 79–122. Upchurch, P., Barrett, P.M., and Dodson, P., 2004, Sauropoda, in Weishampel, D.B., Dodson, P., and Osmólska, H., editors, The dinosauria (2nd edition): Berkeley, University of Cali- fornia Press, p. 259–322. Waskow, K., and Sander, P.M., 2014, Growth record and histolog- ical variation in the dorsal ribs of Camarasaurus sp. (Saurop- oda): Journal of Vertebrate Paleontology, v. 34, p. 852–869. Wedel, M.J., and Taylor, M.P., 2013, Neural spine bifurcation in sauropod dinosaurs of the Morrison Formation—ontogenet- ic and phylogenetic implications: Palarch’s Journal of Verte- brate Palaeontology, v. 10, p. 1–34. Whitlock, J.A., 2011a, A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda): Zoological Journal of the Lin- nean Society, Article first published online: 12 Jan 2011. doi:10.1111/j.1096-3642.2010. Whitlock, J.A., 2011b, Inferences of diplodocoid (Sauropoda: Di- nosauria) feeding behavior from snout shape and microwear analyses: PLoS ONE, v. 6, no. 4, e18304, p. 1–20, https://doi. org/10.1371/journal.pone.0018304. Whitlock, J.A., and Harris, J.D., 2010, The dentary of Suuwassea emilieae (Sauropoda: Diplodocoidea): Journal of Vertebrate Paleontology, v. 30, p. 1637–1641. Whitlock, J.A., Wilson, J.A., and Lamanna, M.C., 2010, De- scription of a nearly complete juvenile skull of Diplodocus (Sauropoda: Diplodocoidea) from the Late Jurassic of North America: Journal of Vertebrate Paleontology, v. 30, p. 442– 457. Wings, O., Sander, P.M., Tütken, T., Fowler, D.W., and Sun, G., 2007, Growth and life history of Asia’s largest dinosaur [abs.]: Journal of Vertebrate Paleontology, Program and Abstracts, v. 27, p. 167A. Woodruff, D.C., and Foster, J.R., 2014, The fragile legacy of Am- phicoelias fragillimus (Dinosauria: Sauropoda; Morrison Formation – latest Jurassic): Volumina Jurassica, v. 12, p. 211–220. Woodruff, D.C., and Foster, J.R., 2017, The first specimen of Ca- marasaurus (Dinosauria: Sauropoda) from Montana—the northernmost occurrence of the genus: PLoS ONE, v. 12, no. 5, e0177423, p. 1–34, https://doi.org/10.1371/journal. pone.0177423. 112 What factors influence our reconstructions of Morrison Formation sauropod diversity? D. Cary Woodruff Geology of the Intermountain West 2019 Volume 6 Woodruff, D.C., and Fowler, D.W., 2012, Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda)—a critical phylogenetic character: Journal of Morphology, v. 273, p. 754–764. Woodruff, D.C., Fowler, D.W., and Horner, J.R., 2017, A new multi-faceted framework for deciphering diplodocid ontog- eny: Palaeontologia Electronica, article number 20.3.43A, p. 1–53. Woodruff, D.C., Storrs, G., Curry-Rogers, K.A., Carr, T.D., and Wilson, J.P., 2015, The smallest known diplodocid skull— new insights into sauropod cranial development [abs.]: Jour- nal of Vertebrate Paleontology, Program and Abstracts, v. 2015, p. 241. Woodward, H.N., Freedman Fowler, E.A., Farlow, J.O., and Horner, J.R., 2015, Maiasaura, a model organism for extinct vertebrate population biology—a large sample statistical as- sessment of growth dynamics and survivorship: Paleobiolo- gy, v. 41, p. 503–527. Woodward, H.N., and Lehman, T.M., 2009, Bone histology and microanatomy of Alamosaurus sanjuanensis (Sauropoda: Ti- tanosauria) from the Maastrichtian of Big Bend National Park, Texas: Journal of Vertebrate Paleontology, v. 29, p. 807–821. Woolnough, A., and Du Toit, J., 2001, Vertical zonation of browse quality in tree canopies exposed to a size-structured guild of African browsing ungulates: Oecologia, v. 129, p. 585–590. Wosik, M., Goodwin, M.B., and Evans, D.C., 2017, A nest- ling-sized skeleton of Edmontosaurus (Ornithischia, Had- rosauridae) from the Hell Creek Formation of northeastern Montana, USA, with an analysis of ontogenetic limb allome- try: Journal of Vertebrate Paleontology, v. 37, no. 6, doi:10.10 80/02724634.2017.1398168. Wosik, M., Goodwin, M.B., and Evans, D.C., 2018, Nestling-sized hadrosaurine cranial material from the Hell Creek Forma- tion of northeastern Montana, USA, with an analysis of cra- nial ontogeny in Edmontosaurus annectens: PaleoBios, v. 36, p. 1–18 p. Young, M.T., Rayfield, E.J., Holliday, C.M., Witmer, L.M., Button, D.J., Upchurch, P., and Barrett, P.M., 2012, Cranial biome- chanics of Diplodocus (Dinosauria, Sauropoda)—testing hy- potheses of feeding behaviour in an extinct megaherbivore: Naturwissenschaften, v. 99, p. 637–643.