GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 12 2025 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. Email inquiries to GIW@utahgeology.org. CENSUS OF CURRENTLY KNOWN SPECIMENS OF THE LATE JURASSIC SAUROPOD HAPLOCANTHOSAURUS FROM THE MORRISON FORMATION, USA Colin Boisvert, Gunnar T. Bivens, Brian Curtice, Ray Wilhite, Mathew Wedel 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 A skeletal reconstruction and life restoration of the enigmatic Late Jurassic Morrison Formation sauropod Haplocanthosaurus (scale is 2 m). 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Lund Utah Geological Survey, Emeritus 435.590.1338 williamlundugs@gmail.com Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 12 2025 1 ABSTRACT Currently known from two valid species, Haplocanthosaurus priscus and H. delfsi, the Late Jurassic sauropod Haplocanthosaurus (Morrison Formation, Western United States) has often been described as an enigmatic sauropod taxon due to its unstable phylogenetic position and paucity of specimens. Here, we quantify the number of Haplocanthosaurus specimens known from the literature and in collections. Al- though most regions of the postcranial skeleton are known, the most commonly found elements of Haplo- canthosaurus are vertebrae (dorsals and caudals) and tibiae. Our investigation identified twelve individuals of Haplocanthosaurus from ten localities across four states, Colorado, Utah, Montana (private specimen), and Wyoming, making Haplocanthosaurus spatially widespread in the central part of the Morrison Forma- tion. The existence of twelve individuals across four states indicates this genus was widely distributed and more abundant than historically thought. Haplocanthosaurus has been characterized as a ‘primitive’ sau- ropod restricted to the lower half of the Morrison Formation, but the identification of Haplocanthosaurus in the Dry Mesa Dinosaur Quarry confirms that the genus was also present within the upper part of the Morrison Formation. Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Colin Boisvert1, Gunnar T. Bivens2, Brian Curtice3, Ray Wilhite4, Mathew Wedel5 1Oklahoma State University Center for Health Sciences, Tulsa, OK 74107 USA; cdboisvert1998@gmail.com 2Mesa Community College, Mesa, AZ USA; gunnarbivens@gmail.com 3Arizona Museum of Natural History, Mesa, AZ 85202 USA; BCurtice@asu.edu 4Auburn University, Auburn, AL 36849 USA; drw0004@auburn.edu 5Western University of Health Sciences, Pomona, CA 91766 USA; mathew.wedel@gmail.com Citation for this article. Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M., 2025, Census of currently known specimens of the Late Jurassic sauropod Haplocanthosaurus from the Morrison Formation, USA: Geology of the Intermountain West, v. 12, p. 1–23, https://doi.org/10.31711/giw.v12.pp1-23. INTRODUCTION Haplocanthosaurus has long been considered among the most enigmatic of Late Jurassic Morrison Formation sauropods (McIntosh and Williams, 1988; Curtice et al., 2023). It has been recovered in sever- al disparate positions, either as a basal macronarian (Upchurch, 1995; Wilson and Sereno, 1998; Upchurch et al., 2004b), a brachiosaurid (Riggs, 1904), a derived eusauropod outside Neosauropoda (Harris, 2006), or (the most well-supported position currently) a basal di- plodocoid (Wilson 2002; Whitlock, 2011; Mannion et al., 2012; Tschopp et al., 2015a). Hatcher (1903a) used the name Haplocanthus priscus for a partial skeleton recovered from Marsh- Felch Quarry 1 in Colorado (Cañon City Quarry 1 and Felch Quarry 1 are synonymous with Marsh-Felch Quarry 1; we use the latter designation throughout) (Figure 1). Later Hatcher (1903b) incorrectly believed his name was preoccupied by the acanthodian fish Ha- placanthus (Agassiz, 1844), and proposed the replace- ment name Haplocanthosaurus. The two names were not, however, identical in either spelling or pronuncia- tion and so Haplocanthus held priority over Haplocan- thosaurus until the International Commission on Zoo- 2 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 logical Nomenclature ruled to suppress it (ICZN, 1991). A second species, Haplocanthosaurus ‘utterbacki’, was also named by Hatcher (1903c) for a second, more complete partial skeleton from the same quarry, and a third, Haplocanthosaurus delfsi, was named for a spec- imen that was both larger and stratigraphically older than either H. priscus or H. ‘utterbacki’ (McIntosh and Williams, 1988). In addition to some less complete specimens found either at or in the vicinity of Marsh-Felch Quarry 1, new Colorado material was found both near the town of Snowmass (Foster and Wedel, 2014) and at the exten- sive Dry Mesa Dinosaur Quarry (Boisvert et al., 2024). Material has also been recovered from two different sites in Wyoming (Erickson, 2014; Tschopp et al., 2019), two in Utah (Bilbey et al., 2000; the other new to the literature), and finally a possible new specimen from Montana (Ronson, 2016). Contrary to previous state- ments, Haplocanthosaurus is more common than once thought (Wilson and Sereno, 1998; Foster and Wedel, 2014; Maidment, 2024). This study provides a short description of eleven publicly held valid Haplocanthosaurus specimens from nine locales in three states and currently held in eight public collections with a note on other possible pub- lic specimens, as well as specimens that should not be identified as Haplocanthosaurus. MATERIALS AND METHODS Members of our team visited collections to exam- ine as many specimens as possible firsthand. For the Figure 1. Spatial distribution map showing all confirmed Haplocanthosaurus locales, with the area around Garden Park, Col- orado, inset zoomed in to show the comparative abundance of specimens found in this area. The states shown here contain surface exposures of the Morrison Formation. 3 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 remaining specimens, we relied on published sources, unpublished photographs, and assistance from muse- um staff. Measurements were taken, when possible, for comparison. Identification to genus and/or species level was made using measurements and known autapomor- phies. Diagnostic characters of the various Haplocan- thosaurus specimens are given in Table 1, and informa- tion on the multiple specimens is provided in Table 2. We follow the terminology of Wilson (1999, 2012) for vertebral laminae and Wilson et al. (2011) for vertebral fossae. Furthermore, a map of the known Haplocantho- saurus specimens was produced (Figure 1). Whereas it would be greatly beneficial to place all of the known Haplocanthosaurus specimens into a well-calibrated stratigraphic framework, unfortunately, such does not yet exist for the entirety of the Morrison Formation. On the northern part of the Colorado Pla- teau, the Morrison Formation can be divided into three distinct members (the Tidwell, Salt Wash, and Brushy Basin Members). Outside the Colorado Plateau, some local sections can be correlated to the Tidwell, Salt Wash, and Brushy Basin (e.g., the Oklahoma Panhan- dle, Richmond et al., 2020), but others show no clear correlation (e.g., the Black Hills of Wyoming, Foster et al., 2020). Long-distance correlations based on an in- ferred “clay change” (Turner and Peterson 1999) have not been supported by the absolute dates provided by radiometric dating (Trujillo 2006, Trujillo and Kowa- lis, 2015). In the absence of radiometric dates for most of the Haplocanthosaurus localities, we have placed the known specimens into relative correlation where possi- ble (Table 3). INSTITUTIONAL ABBREVIATIONS BLM – Bureau of Land Management, Utah, USA BYU – Brigham Young University, Provo, Utah, USA CM – Carnegie Museum of Natural History, Pitts- burgh, Pennsylvania, USA CMNH – Cleveland Museum of Natural History, Cleveland, Ohio, USA DMNS – Denver Museum of Nature and Science, Denver, Colorado, USA FHPR – Utah Field House of Natural History State Park Museum, Vernal, Utah, USA MWC – Dinosaur Journey, Museums of Western Colorado, Fruita, Colorado, USA MMCh – Museo Municipal ‘Ernesto Bachman’, Villa El Chocón, Neuquén, Argentina NCSM – North Carolina Museum of Natural Sci- ences, Raleigh, North Carolina, USA SMM – Science Museum of Minnesota, St. Paul, Minnesota, USA USNM – United States National Museum of Natural History, Washington, D.C., USA YPM – Yale Peabody Museum of Natural History, New Haven, Connecticut, USA SYSTEMATIC PALEONTOLOGY Dinosauria Owen, 1842 Saurischia Seeley, 1888 Sauropodomorpha Huene, 1932 Sauropoda Marsh, 1878 Neosauropoda Bonaparte, 1986 Diplodocoidea Upchurch, 1995 Haplocanthosaurus Hatcher, 1903b Type species: Haplocanthosaurus priscus Hatcher, 1903a. Revised Diagnosis The genus Haplocanthosaurus is diagnosed by the following characters (autapomorphies marked with as- terisks): (1) presacral centra pneumaticity—procamer- ate, (2) simple lateral pneumatic fossae in the cervical and dorsal vertebrae, (3) dorsal neural arches with elon- gate CPOLs* (unambiguous autapomorphy), (4) dorsal neural arches dorsoventrally heightened relative to total vertebral height, (5) mid-posterior dorsal diapophyses projecting dorsolaterally at 45° and approaching the dorsoventral height of the neural spines* (unambigu- ous autapomorphy), (6) large articular chevron facets on caudal vertebrae, (7) caudal vertebrae neural canals anteriorly inclined* (ambiguous autapomorphy), (8) scapular acromion process narrow, (9) scapular blade with a dorsally and ventrally expanded distal end, (10) large obturator foramen of pubis, (11) ambiens process absent (within Diplodocoidea, this state is only shared 4 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 with Limaysaurus tessonei), (12) proximal femur medi- al to the midshaft meridian (MMM) ratio about 75% (unique within Diplodocoidea, see Sassani and Biv- ens, 2017, Figure 9), and (13) tibia with a flared distal end. Additionally, Haplocanthosaurus can be assigned to Diplodocoidea based on the following shared apo- morphies: (1) dorsal neural spine PRSL formed by con- joined SPRL (seen in all diplodocoids except Supersau- rus vivianae), (2) cervical rib anteroposterior lengths shorter than respective centra, and (3) posterolaterally facing fibular facet of astragalus. Other Diagnoses Wilson and Sereno (1998) considered the proxi- mal end of the tibia subcircular. However, our personal observation suggests this profile is more likely due to diagenetic distortion than a legitimate feature. Wilson (2002) also considered the presence of 13 dorsal ver- tebrae an autapomorphy of Haplocanthosaurus by way of reversal. Thirteen dorsal vertebrae, however, appear to be the plesiomorphic condition for Eusauropoda, with reduced vertebral counts in some mamenchis- Character Specimen(s) Source Cervical ribs shorter than centra CM 572, 879, CMNH 10380 Hatcher (1903c), Whitlock (2011) Procamerate presacral vertebrae BYU 11506, 17530, 17689, CM 572, 879, CMNH 10380, MWC 8028 Wedel (2003, 2005), Foster and Wedel (2014), Bois- vert et al. (2024) 13 dorsal vertebrae CM 879 Wilson (2002) Long dorsal CPOLs BYU 17530, 17531, CM 572, 879, CMNH 10725, 10380 Wilson (2002) Dorsal diapophyses at 45° BYU 11506, 17530, 17531, CM 572, 879, CMNH 10725, 10380, FHPR 1106 Hatcher (1903c), McIntosh and Williams (1988), Wilson (2002), Boisvert et al. (2024) Dorsal PRSL formed by SPRLs BYU 11506, 17530, CM 572, 879, CMNH 10380 Whitlock (2011) Caudal vertebrae with large chevron facets CM 572, 879, 36034, 36036, CMNH 10380, MWC 8028, SMM P 90.37.10 McIntosh and Williams (1988), Foster and Wedel (2014) Scapular acromion narrow CM 879 Wilson (2002) Proportionally large obturator foramen CM 572, CMNH 10380 Hatcher (1903c), personal observation Scapular distal end expanded CM 879, CMNH 10380 McIntosh and Williams (1988), Wilson (2002) Tibia distal end flared BYU 12865, CM 2043, CMNH 10725, USNM V 4275 McIntosh and Williams (1988), Boisvert et al. (2024) Posterolaterally facing fibular facet of the astragalus SMM P 90.37.10, USNM V 4275 Whitlock (2011) Lack of ambiens process CM 572, CMNH 10380, FHPR 1106 Hatcher (1903c), McIntosh and Williams (1988) Dorsal neural arches dorsoventrally heightened relative to total vertebral height BYU 11506, 17530, 17531, 17689, CM 572, 879, CMNH 10380, FHPR 1106, MWC 8028 Hatcher (1903c), McIntosh and Williams (1988), Foster and Wedel (2014), Boisvert et al. (2024) Simple lateral pneumatic fossae in presacral vertebrae BYU 11506, 17530, 17531, 17689, CM 572, 879, CMNH 10380, FHPR 1106, MWC 8028 Hatcher (1903c), McIntosh and Williams (1988), Foster and Wedel (2014), Boisvert et al. (2024) Caudal vertebrae neural canals anteriorly inclined CM 572, 879, CMNH 10380, FHPR 1106, MWC 8028, SMM P 90.37.10 Hatcher (1903c), McIntosh and Williams (1988), Foster and Wedel (2014), Erickson (2014) Table 1. Diagnostic characters of Haplocanthosaurus, and the specimens in which they are visible. 5 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 Specimen Number Quarry Strat Age Species Year Described State Known Elements USNM V 4275 Marsh-Felch Quarry 1 Morrison, age unknown H. priscus 1896 CO Left femur, right femur, both ischia, left tibia, left fibula, left astragalus CM 572 Marsh-Felch Quarry 1 Morrison, age unknown H. priscus 1903 CO Two posterior cervicals, ten dorsals, five sacrals, both ilia, both ischia, and both pubes, nineteen anterior caudals, two chevrons, a nearly complete series of ribs, and a left femur CM 879 Marsh-Felch Quarry 1 Morrison, age unknown H. priscus 1903 CO Ten cervical, thirteen dorsal, five sacral, and seven cau- dal vertebrae, several ribs, left scapula, right coracoid CM 2043 Marsh-Felch Quarry 1 Morrison, age unknown H. priscus 1981 CO Right tibia, fibula, and astragalus CM 2046 Marsh-Felch Quarry 1 Morrison, age unknown H. priscus 1981 CO Left tibia and fibula CM 33995 Marsh-Felch Quarry 1 Morrison, age unknown H. priscus 1981 CO Left scapula CMNH 10380 Cleveland-Delfs Quarry Morrison, age unknown H. delfsi 1988 CO Four anterior cervicals, nine posterior dorsals with ribs on left side, five sacrals, fourteen anterior caudals, several chevrons, partial left scapula, possible fragmentary cora- coid, right sternal plate, partial left radius and ulna, both ilia, left pubis, left femur, and left ischium SMM P 90.37.10 Poison Creek Quarry Morrison, age unknown H. sp. 2014 WY Left tibia, fibula, astragalus, calcaneum, and foot minus proximal phalanx of digit one, along with 30 articulated caudal vertebrae SMM D14-77 Poison Creek Quarry Morrison, age unknown H. sp. undescribed WY Tibiae, caudal vertebrae P 78.21.20 P 78.21.36 P 84.15.72 P 87.14.6 FHPR 1106 William’s Slow Eagle Quarry Salt Wash (upper) H. sp. 2000, paper in prep UT Cervical, dorsal, sacral, and caudal vertebrae, scapulo- coracoid, humerus, radius, ulna, metapodial elements, femur, tibia, fibula, ribs, and pelvic material MWC 8028 Gordon-Bramson- Brothers Quarry Morrison, age unknown H. sp. 2014 CO Four dorsal centra, five partial ribs, sacrum, five caudal vertebrae, three chevrons, five partial neural spines, and bone fragments. CM 312 RFPRB Morrison, age unknown H. sp. 2019 WY Anterior caudal vertebrae, chevrons, partial ischium CM 36034 RFPRB Morrison, age unknown H. sp. 2019 WY Two middle caudal vertebrae CM 36036 RFPRB Morrison, age unknown H. sp. 2019 WY Middle caudal vertebra BYU 9194, 11506, 12865, 17530-1, 17689 Dry Mesa Dino- saur Quarry Brushy Basin (middle) H. sp. 2024 CO Seven dorsal vertebrae (three articulated, four disarticu- lated), right tibia BLM field specimen Tal Site Salt Wash (upper) H. sp. undescribed UT Dorsal vertebra, ribs CMNH 10725 Oil Creek Quarry Morrison, age unknown H. sp. undescribed CO Two partial dorsal vertebrae, right tibia, and bone pieces Table 2. Confirmed Haplocanthosaurus specimen information including stratigraphic age, location (discovery and curation), paper mention/description, and material available. RFPRB = Red Fork of the Powder River Quarry B. 6 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 aurids (e.g., “Omeisaurus” tianfuensis, He et al., 1988; Xinjiangtitan shanshanesis, Zhang et al., 2022), all mac- ronarians, and Flagellicaudata. Within Diplodocoidea, 13 dorsal vertebrae are seen in Rebbachisauridae (e.g., Nigersaurus taqueti, the unnamed specimen MMCh-Pv 49, see Vidal, 2019). Haplocanthosaurus priscus Hatcher, 1903a =Haplocanthus priscus Hatcher, 1903a =Haplocanthosaurus utterbacki Hatcher, 1903c Holotype: specimen CM 572. Revised Diagnosis Haplocanthosaurus priscus can be diagnosed by the following characters: (1) middle dorsal vertebrae mildly opisthocoelous, and (2) distal end of ischia fused (see discussion below). Referred specimens: CM 2043, CM 2046, CM 879, CM 33995. Haplocanthosaurus delfsi McIntosh and Williams, 1988 Holotype: specimen CMNH 10380. Revised Diagnosis Haplocanthosaurus delfsi can be diagnosed based on the following characters (autapomorphies marked with an asterisk): (1) middle dorsal vertebrae strongly opist- hocoelous, (2) V-shaped anterolaterally projecting lam- inae present on mid-dorsal vertebral neural spines*, (3) posterior dorsal vertebral neural spines with greater de- velopment of median laminae compared to H. priscus*, (4) pelvis and femur more robust than H. priscus, and (5) distal end of ischia unfused. Other Diagnoses McIntosh and Williams (1988) considered H. delfsi distinct from H. priscus based on its larger body size (roughly a third again as large, see below). However, size is variable within adult taxa of other dinosaur popula- Table 3. Stratigraphic position of the known Haplocanthosaurus specimens within the Late Jurassic Morrison Formation. The asterisks refer to a potential Haplocanthosaurus specimen. Stratigraphic Unit Quarry Name State Brushy Basin Member (youngest strata) Dry Mesa Quarry Colorado Salt Wash Member William’s Slow Eagle Quarry Tale Site Utah Utah Tidwell Member (oldest strata) None Morrison Formation, age unknown Cleveland-Delfs Quarry Como Bluff* Gordon-Bramson-Brothers Quarry Marsh-Felch 1 Quarry Oil Creek Quarry Poison Creek Quarry Red Fork of the Powder River Quarry B Colorado Wyoming Colorado Colorado Colorado Wyoming Wyoming 7 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 tions (e.g., Allosaurus, Edmontosaurus, Plateosaurus). Additionally, at least one other specimen attributable to Haplocanthosaurus (SMM P 90.37.10) is comparable in size to the H. delfsi holotype CMNH 10380 and does not appear to be referable to this species. Haplocanthosaurus sp. Referred specimens: BYU 9194, BYU 11506, BYU 12865, BYU 17530, BYU 17531, BYU 17689, CM 312, CM 36034, CM 36036, CMNH 10725, FHPR 1106, MWC 8028, SMM P 90.37.10, unnumbered BLM spec- imen, USNM V 4275. Issues with Diagnosing Haplocanthosaurus Species Whereas various specimens have been attributed to Haplocanthosaurus in the past, little has been done in the way of diagnosing the lower-level taxonomy of this genus. When initially described, Haplocanthosau- rus delfsi was distinguished from H. priscus (McIntosh and Williams, 1988); however, little to no effort has been made in distinguishing the latter from the former or from other Haplocanthosaurus specimens. At least one character used by McIntosh and Williams (1988) to distinguish H. delfsi from H. priscus—the lack of fused ischia in the former—could represent ontogenetic or in- dividual variation. Specimen USNM V 4275 was found in close proximity to the specimens of H. priscus and possessed fused ischia as in that taxon, yet the femo- ral least breadth to length measurements, as currently defined, imply closer affinities to H. delfsi. It is unclear which, if any, of these characters are more taxonomical- ly informative. Some characters that have been previously used to distinguish the species of Haplocanthosaurus, such as the level of co-ossification of sacral neural spines, are likely a result of individual variation, whereas others may be attributable to ontogeny or other factors (Hone et al., 2016). For example, specimen CM 572 exhibits a pneumatic fossa at least in the first caudal vertebra (Up- church and Mannion, 2009; Wedel, 2009); although at this time it is unclear whether this is a result of individu- al variation or is more taxonomically significant. Other characters, such as a proportionally large obturator fo- ramen for the genus, may be phylogenetically informa- tive but have yet to be extensively discussed (Figure 2). Ontogeny and individual variation have long plagued sauropod taxonomy and phylogeny (Woodruff, 2019) and it is unclear how much these factors affect Haploc- anthosaurus. Additional study and reevaluation, both of the original type specimens and new specimens, such as specimen FHPR 1106, will hopefully further elucidate the interrelationships of this genus. SPECIMEN DESCRIPTIONS Specimen CM 572 This specimen represents the holotype for the ge- nus Haplocanthosaurus and of the species H. priscus (Hatcher 1903a, 1903b, 1903c). Specimen CM 572 was recovered from the Marsh-Felch Quarry 1 (Turner and Peterson, 1999) and includes two posterior cervical and Figure 2. Comparative photographs of the large obturator fo- ramen (OF) of Haplocanthosaurus based on specimens CM 572 (A) and Apatosaurinae indet. BYU 681-12915 (B). Image of CM 572 (not to scale) is modified from an unpublished image by John S. McIntosh (Wesleyan University), courtesy of Dan Chure (Dinosaur National Monument). 8 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 ten dorsal vertebrae, the sacrum and ilia, ischia and pu- bes, nineteen anterior caudal vertebrae, two chevrons, a nearly complete series of dorsal ribs, and a left femur (Figure 3). As the genoholotype of this taxon, it forms the basis for the generic diagnosis, including the tall neural arch pedicles, non-bifurcated neural spines, dor- sally angled transverse processes, and simple amphicoe- lous caudal centra with large chevron facets (Hatcher, 1903c). A right tibia, fibula, and astragalus cataloged as specimen CM 2043 likely belongs to Haplocanthosau- rus (McIntosh, 1981) and possibly to the genoholotypic individual of H. priscus (contra McIntosh and Williams, 1988) due to the least breadth/length measurement of the tibia, tibial morphology, overall limb proportions, and its proximity to other bones in the quarry assigned to Haplocanthosaurus. There is also no overlap in ele- ments with specimen CM 572, which further suggests such an association (Figure 3). The limb proportions also do not match specimen CM 879, providing evi- dence against it being assigned to that Haplocantho- saurus specimen. A left scapulocoracoid specimen CM 33995 and left tibia and fibula specimen CM 2046 also match the cross-scaled limb proportions for CM 572 and very likely also belong to this individual. Specimen CM 879 Specimen CM 879 was originally designated the holotype of Haplocanthosaurus ‘utterbacki’ (Hatcher, 1903c). Also recovered from Marsh-Felch Quarry 1, this specimen consists of ten cervicals (likely repre- senting cervical vertebrae two, three, and five through twelve), thirteen dorsal vertebrae, several dorsal ribs, the sacrum, the anterior seven caudal vertebrae, the left scapula, and right coracoid (Hatcher, 1903c). This individual was referred to Haplocanthosaurus based on tall neural arch pedicles, the simple non-bifurcated neural spines, and dorsally angled transverse processes (Hatcher, 1903c, and Figure 4). Haplocanthosaurus ‘utterbacki’ was distinguished by Hatcher (1903c) solely based on the level of sacral neural spine fusion. However, this character was later recognized as ontogenetically variable, and as a result, this species was synonymized with H. priscus (McIn- tosh and Williams, 1988). Figure 3. Skeletal reconstruction of composite type individual (A), with specimen CM 572 cervical eleven in right lateral (B), dorsal thirteen in posterior and right lateral views (C), fused ischia in posterior view (D), right tibia specimen CM 2043 in posterior view (D), and fused left scapulocoracoid specimen CM 33995 in left lateral view (F). Scale bars equal 2 m (A), 20 cm (B) and 50 cm (C, D, E, F). Scale individual for this and all skeletals is co-author GTB at his natural height of 165 cm. Photographs of ischia and scapulocoracoid modified from unpublished images by John S. McIntosh (Wesleyan University), courtesy of Dan Chure (Dinosaur National Momument). 9 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 Specimen CM 36034 Specimen CM 36034 was collected from the Red Fork of the Powder River Quarry B in Wyoming (Tschopp et al., 2019). CM 36034 consists of two mid-caudal vertebrae (Figure 5). Tschopp et al. (2019) considered this specimen to belong to an indeterminate neosauropod, as the prezygapophyses of these caudals are shorter proportionally than can be seen in other Haplocanthosaurus specimens (e.g., specimens CM 572, CM 879, CMNH 10380, SMM P 90.37.10). It is possible, however, that this is an artifact of taphonomic processes rather than a legitimate feature; and besides this mor- phological discrepancy, the remainder of the vertebral morphologies are consistent with Haplocanthosaurus. For example, the large chevron facets exceed those of Camarasaurus, but are consistent with those of Haploc- anthosaurus; therefore, we recognize CM 36034 as Hap- locanthosaurus sp. Additionally, specimens CM 36036 and CM 312, also from the Red Fork of the Powder River Quarry B, preserve additional anterior and mid-caudal verte- brae, several chevrons, and a partial ischium. Whereas originally being assigned to separate individuals, this material likely belongs to the same individual as speci- men CM 36034, based on the comparable size and mor- phology present in all the vertebrae. There may be ad- ditional Haplocanthosaurus material from this quarry as well (originally all cataloged as specimen CM 1256), although given the disarticulated nature of this quarry and lack of a quarry map (see Tschopp et al., 2019), it requires further study that is outside the scope of this review. Specimen CMNH 10380 Specimen CMNH 10380 represents the holotype of Haplocanthosaurus delfsi (Mcintosh and Williams, 1988). CMNH 10380 comes from the Cleveland-Delfs Quarry (or “Delfs’ Quarry” of Turner and Peterson, 1999). This specimen is represented by cervical vertebrae one through four, nine posterior dorsal vertebrae with left dorsal ribs, five sacral vertebrae, caudal vertebrae one through four- teen, several chevrons, shaft and distal end of left scap- ula, a possible fragmentary coracoid, right sternal plate, proximal ends of the left radius and ulna, both ilia, left pubis, left femur, and left ischium (McIntosh and Wil- liams, 1988, and Figure 6). This specimen was referred to Haplocanthosaurus based on the tall neural arch pedicles, dorsally angled transverse processes, and a flared distal end of the tibia (McIntosh and Williams, 1988). Figure 4. Skeletal reconstruction of specimen CM 879 (A), with fifth cervical in left lateral view (B), unfused sacral neural spines in right lateral view (C), and first caudal in right lateral view (D). Scale bars equal 2 m (A), 20 cm (B, D), and 50 cm (C). Images of cervical and sacrals modified from ones by John S. McIntosh (Wesleyan University), courtesy of Dan Chure (Dinosaur National Monument). 10 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 Specimen CMNH 10725 Specimen CMNH 10725 comes from the Oil Creek Quarry in Colorado. While CMNH 10725 awaits de- scription, the specimen is represented by two dorsal vertebrae, a right tibia, and some unidentified frag- ments. The specimen was referred to Haplocanthosau- rus based on the tall neural arch pedicles, dorsally an- gled transverse processes, and flared distal end of the tibia (Hatcher, 1903c, McIntosh and Williams, 1988, and Figure 7). CMNH 10725 was found in close prox- imity to the specimens from the Cleveland-Delfs and Marsh-Felch Quarries in Colorado (Figure 1), suggest- ing populations of Haplocanthosaurus were more nu- merous here than in other areas. Specimen MWC 8028 Specimen MWC 8028 comes from the Gor- don-Bramson-Brothers Quarry in undifferentiated Morrison strata in Colorado (Foster and Wedel, 2014), and includes four dorsal centra, five partial dorsal ribs, sacrum, five caudal vertebrae, three chevrons, and many unidentified fragments (Figure 8). MWC 8028 was identified as Haplocanthosaurus based on the am- phicoelous caudal vertebrae with large chevron facets. Further studies have indicated that this individual had caudal vertebrae with unusually large neural canals and deeply amphicoelous centra (Wedel et al., 2021), and that these morphologies, in combination with other differences in the centra and neural spines, may indi- cate enough morphological disparity to warrant a new species within the genus. Specimen USNM V 4275 Specimen USNM V 4275 comes from Marsh-Felch Quarry 1 in Colorado and is represented by a left fe- mur, right femur, both ischia, left tibia, left fibula, and left astragalus. USNM V 4275 was referred to Haplocan- thosaurus based on the least breadth to length measure- ment, flared distal end of the tibia, and fused ischia – as in H. priscus (McIntosh and Williams, 1988, and Figure 9). Of historical note, specimen USNM V 4275 was described seven years before specimen CM 572. Marsh (1896; Pl. XVIII) originally figured the ischia of this specimen and assigned them to Diplodocus longus. However, Gilmore (1907) recognized that they were very different from ischia referred to Diplodocus, and reassigned these elements to “Morosaurus.” Figure 5. Skeletal reconstruction of the Red Fork of the Powder River Quarry B individual (A), with specimen CM 36034 twelfth caudal in left lateral view (B), and specimen CM 312 fused posterior caudals in left lateral view (C). Scale bars equal 2 m (A) and 20 cm (B, C). Photograph of fused caudals modified from Tschopp et al. (2019) 11 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 Specimen FHPR 1106 Specimen FHPR 1106 is the most complete Haploc- anthosaurus known to date, with over 60% of the skele- ton recovered (Bilbey et al., 2000). While currently un- der study, FHPR 1106 was recovered from the William’s Slow Eagle Quarry in Utah within the upper Salt Wash Member of the Morrison Formation (Turner and Peter- son, 1999; Maidment, 2024; Boisvert et al., 2024). FHPR 1106 is represented by a majority of the cervical, dorsal, sacral, and caudal vertebral series, a scapulocoracoid, numerous stylopodial, zeugopodial, and metapodial el- ements, dorsal ribs, and pelvic material (Bilbey et al., 2000, and Figure 10). The specimen has been tentatively Figure 6. Skeletal reconstruction of specimen CMNH 10380 (A), with twelfth dorsal in posterior view (B), and first caudal in right lateral and posterior views (C). Scale bars equal 4 m (A) and 50 cm (B, C). Figure 7. Skeletal reconstruction of specimen CMNH 10725 (A), with middle dorsal in anterior and left posteroventral views (B), and right tibia in medial and posterior views (C). Scale bars equal 2 m (A) and 50 cm (B, C). 12 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 identified as Haplocanthosaurus based on the tall neu- ral arch pedicles, simple non-bifurcated neural spines in the dorsal vertebrae, and dorsally angled transverse processes (Hatcher, 1903c). The caudal vertebrae pos- sess enlarged chevron facets. Due to its completeness, this specimen will be important in future phylogenetic and taxonomic analyses of Haplocanthosaurus. Specimens SMM D14-77, P 78.21.20, P 78.21.36; P 84.15.72, P 87.14.6, P 90.37.10 Specimen SMM P 90.37.10 was discovered in Wy- oming at the Poison Creek Quarry, and is represented by a lower left hind limb with a tibia, fibula, astragalus, calcaneum, pes (minus the proximal phalanx of digit 1), Figure 8. Skeletal reconstruction of specimen MWC 8028 (A), with sacrum in right lateral view (B), and third caudal in an- terior and right lateral views (C). Scale bars equal 2 m (A) and 20 cm (B, C). Figure 9. Skeletal reconstruction of specimen USNM V 4275 (A), with left femur in anterior view (B), fused ischia in posteri- or view (C), and left fibula, and fused tibia and astragalus in posterior view (D). Scale bars equal 2 m (A) and 50 cm (B, C, D). 13 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 and thirty articulated caudal vertebrae (Erickson, 2014). It was referred to Haplocanthosaurus based on the flared distal end of the tibia and large posterior chevron fac- ets on the caudal vertebrae (McIntosh and Williams, 1988; Erickson, 2014; and Figure 11). This specimen marks the northernmost occurrence of the genus Hap- locanthosaurus within a museum collection. Additional SMM specimens examined by the authors indicate at least three additional individuals of Haplocanthosaurus in the quarry, based on tibiae (specimens SMM field no. D14-77, SMM P 78.21.20, SMM P 84.15.72) and caudal vertebrae (specimens SMM P78.21.36, SMM P87.14.6). Figure 11. Skeletal reconstruction of specimen SMM P 90.37.10 (A), with caudals 5 and 6 in right lateral view (B), and left crus and pes in anterior view (C). Scale bars equal 4 m (A), 50 cm (B), and 1 m (C). Photograph of left limb modified from Erickson (2014). Figure 10. Skeletal inventory of specimen FHPR 1106. Scale bar equals 2 m. 14 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 Specimens BYU 9194, 11506, 12865, 17530, 17531, and 17689 These specimens, probably representing a single in- dividual, come from the Dry Mesa Dinosaur Quarry in the middle Brushy Basin Member of the Morrison For- mation in Colorado (Turner and Peterson, 1999; Cur- tice et al., 2023; Boisvert et al., 2024; Maidment, 2024; and Figure 12). The BYU specimens are represented by three anterior dorsal vertebrae, four disarticulated posterior dorsal vertebrae, and a right tibia (Curtice et al., 2023; Boisvert et al., 2024). The specimens were referred to Haplocanthosaurus based on the tall neural arch pedicles, dorsally angle transverse processes, and the flared end of the distal tibia (Hatcher, 1903c; McIn- tosh and Williams, 1988). These BYU specimens are also the geologically youngest Haplocanthosaurus specimens (Turner and Peterson, 1999), and suggest that Haplocanthosaurus may have temporally spanned a much longer part of the Morrison Formation than has traditionally been thought (Boisvert et al., 2024). BLM Specimen The BLM specimen was discovered on BLM land, known as the Tal Site, near the Brachiosaur Gulch Quarry (Foster et al., 2021, and Figure 1). It is still in the ground at the time of this paper’s publication. The only material found was a single dorsal vertebra and as- sociated partial ribs. It is identified as Haplocanthosau- rus by the tall neural arch pedicles and dorsally angled transverse processes (Hatcher 1903c, and Figure 13). The locality is in the upper Salt Wash Member of the Morrison Formation (Foster et al., 2021; Boisvert et al., 2024). The specimen is currently under study for possi- ble collection by FHPR. Specimens Potentially Referable to Haplocanthosaurus There is a large partial skeleton from Spindletop Dome Quarry 3 in central Montana that has been pre- viously suggested to be Haplocanthosaurus (Woodruff and Foster, 2017; Richmond, 2023). This specimen is currently in private hands so further information was not gleaned from it (Ronson, 2016). Figure 12. Skeletal reconstruction of the composite BYU individual (A), with conjoined anterior dorsals BYU 17531 in right lateral (B), dorsal thirteen in posterior and right lateral views (C), and right tibia from specimen BYU 12865 in anterior view (D). Scale bars equal 2 m (A), and 50 cm (B, C, D). 15 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 Specimen NCSM 28359 Specimen NCSM 28359 consists of a radius, ulna, manual ungual, left femur, phalanx, metapodial, and a fragmentary element that may belong to either a cora- coid or sternal plate. It was found at Como Bluff in Wy- oming (no more precise locality information is current- ly available). Much of the material is in poor condition and difficult to identify to a lower phylogenetic level. The femur is not incongruent with other Haplocan- thosaurus femora but cannot reliably be distinguished from Camarasaurus either. Further research is needed on this specimen, which should be revisited once spec- imen FHPR 1106 is described, as more of the skeleton of Haplocanthosaurus will be known (including the ra- dius and ulna, which cannot be compared to any con- firmed Haplocanthosaurus specimens at the moment). If referable to Haplocanthosaurus, this would document the first occurrence of this genus within southern Wy- oming. Specimen USNM V 4264 Specimen USNM V 4264 consists of a partial left humerus, radius, ulna, and five left metacarpals from Marsh-Felch Quarry 1, which is cataloged in the USNM database as “Haplocanthosaurus? sp.” These elements were not found in association with the partial skeletons of Haplocanthosaurus from this quarry. To date no asso- ciated Haplocanthosaurus forelimb has been described, so USNM V 4264 cannot be referred to Haplocantho- saurus. Specimen USNM V 4267 Specimen USMN V 4267, consisting of a right hu- merus, radius, and ulna, is from Marsh-Felch Quarry 1 and is cataloged in the USNM database as “Haploc- anthosaurus? sp.” It is quite probable, given the similar ratios and overall size of the elements, that this is the opposite limb of specimen USNM V 4264. Specimen YPM VP.001906 Specimen YPM VP.001906 consists of the left ra- dius, ulna, metacarpals I-V, and manual phalanx V-1, and was recovered from Marsh-Felch Quarry 1. YPM VP.001906 was originally considered to belong to “Mo- rosaurus”, then Diplodocus longus (Marsh, 1896), be- fore being tentatively excluded by McIntosh and Car- penter (1998), who considered it possibly belonging to either Apatosaurus or Haplocanthosaurus. Tschopp et al. (2018) only assigned it as far as Diplodocoidea in- det., noting it differed from both Apatosaurinae and Diplodocinae. The first metacarpals of specimen FHPR 1106 (the only confirmed Haplocanthosaurus specimen with a manus) are both incomplete, thus precluding comparison here to determine if it was proximodistally longer than metacarpal IV as in most apatosaurines (but see specimen NSMT-Pv 20375; Upchurch et al., 2004a). We agree with Tschopp et al. (2018) in tentatively iden- tifying this specimen Diplodocoidea indet. Figure 13. Mid-posterior dorsal of the unnumbered BLM specimen in situ. Scale bar equals 10 cm. 16 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 Specimens Not Referable to Haplocanthosaurus Several additional specimens have been previous- ly referred to the genus Haplocanthosaurus. Here, we briefly review these specimens and discuss issues with these referrals. Specimen DMNS EPV.2909 Specimen DMNS EPV.2909 was recovered from DMNH Locality 582, the “Haplocanthosaurus Quarry,” in Colorado. McIntosh and Carpenter (1998) consid- ered DMNS EPV.2909, which consists of a dorsal ver- tebra, caudal vertebrae, and two femora, to belong to Haplocanthosaurus, and additionally referred specimen USNM V 4273 to this taxon as well (see below). The dorsal vertebral transverse processes are dorsoventrally short compared to the very dorsoventrally tall neural spines, and they do not project dorsally. Additionally, the chevron facets are small, and the femoral shafts are also slenderer than seen in other Haplocanthosaurus specimens (Figure 14). Taking all of these characters into account, we consider DMNS EPV.2909 to represent an indeterminate diplodocid. Specimen USNM V 4723 Specimen USNM V 4723 consists of a series of mid-posterior caudals from Marsh-Felch Quarry 1, and was originally referred to Diplodocus longus by Marsh at the advice of Marshal Felch. McIntosh and Carpenter (1998) questioned this assignment based on compari- son to the caudal sequence CM 307 from the Red Fork of the Powder River Quarry B, and assigned USNM V 4723 to Haplocanthosaurus based on favorable compar- ison to that of specimen DMNS EPV.2909. As discussed above, the latter specimen is clearly not referable to Haplocanthosaurus, and as a result, we exclude USNM V 4723 as well, and consider it to be an indeterminate diplodocid. Specimen USNM PAL 337859 Specimen USNM PAL 337859 includes several partial cervical vertebrae from Marsh-Felch Quarry 1. Regarding one of the middle cervical vertebrae, McIntosh and Carpenter (1998) wrote it “...has an undivided neural spine and probably belongs to Haplocanthosaurus.” This specimen is listed as “cf. Brachiosaurus sp.” in the USNM collections database. Personal observation by Mathew J. Wedel (Western University of Health Sciences, October 31, 2023) shows an anteroposteriorly long cervical centrum and pre- and post-zygapophyses that are set forward of their respective centrum ends, consistent with brachiosaurids and unlike those seen in Haplocanthosaurus, corroborating the USNM’s database assignment. Specimen YPM VP.004688 Specimen YPM VP.004688 consists of a right scapu- la, humerus, radius, and ulna from Marsh-Felch Quarry 1. It is currently cataloged as “?Haplocanthosaurus sp.” in the collections database and was identified as such Figure 14. Diplodocinae indet. specimen DMNS EPV.2909 mid-posterior dorsal vertebra in posterior (A), right lateral (B), and anterior (C) views. Haplocanthosaurus priscus spec- imen CM 572 dorsal thirteen in posterior (D), right lateral (E), and anterior (F) views. Scale bars equal 20 cm. 17 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 by Carpenter (2019, Figure 1c). However, Tschopp et al. (2018) note that the radius possesses a distinct medi- al projection on the proximal articular surface, a mor- phology consistent in known brachiosaurid radii (e.g., Cedarosaurus, Giraffatitan, Lusotitan). The radius:hu- merus ratio of YPM VP.004688 also compares favorably to Brachiosauridae, although some diplodocoids pos- sess a similar ratio (Table 4). The scapular morphology of YPM VP.004688 is similar to other brachiosaurids in the widely expanded distal end and slender shaft. Collectively, we find it most parsimonious that YPM VP.004688 represents Brachiosauridae indet. Specimen YPM VP.059137 Specimen YPM VP.059137 consists of five right metatarsals from Marsh-Felch Quarry 1, was originally cataloged as part of specimen YPM VP.001920 (McIn- tosh and Carpenter, 1998), and was considered by them to belong to either Brachiosaurus or Haplocanthosaurus. As noted by Tschopp et al. (2018), this pes shares the synapomorphic brachiosaurid beveling of the distal end of metatarsal IV relative to its proximodistal long axis (Mannion et al., 2013; Maltese et al., 2018), and our cur- rent analysis agrees with their assignment of the speci- men to Brachiosauridae. DISCUSSION Despite the proportionally low number of speci- mens compared to the coeval sauropods Apatosaurus, Diplodocus, or Camarasaurus, all of which are known from dozens or even hundreds of specimens (Foster, 2020), Haplocanthosaurus appears to be both geograph- Table 4. Measurements ( in mm) and radius:humerus ratios of various sauropod forelimbs. Taxon Specimen Humerus Radius Ratio Reference Brachiosauridae indet. YPM VP.004688 1700 985 0.58 Personal Observation Haplocanthosaurus? sp. USNM V 4264/4267 816/805 537/525 0.66 Personal Observation Brachiosaurus altithorax BYU 4744/USNM 21903 2060 1270 0.61 D’Emic and Carrano (2019) Brachiosaurus sp. Gyeryongsan Museum specimen 1530 940 0.61 Personal Observation Giraffatitan brancai MB.R.2181 2130 1240 0.58 Janensch (1961) Cedarosaurus weiskopfae DMNS 39045 1380 812 0.59 Tidwell et al. (1999) Camarasaurus lewisi NMZ 1000002 705 464 0.66 Tschopp et al. (2015b) Camarasaurus grandis YPM VP.001901 890 615 0.69 Vidal (2019) Camarasaurus lentus WDC B 1140 795 0.69 Ikejiri (2004) Camarasaurus lewisi BYU 9047 1018 720 0.71 McIntosh et al. (1996) Apatosaurus louisae CM 3018 1150 800 0.69 Gilmore (1936) Brontosaurus excelsus YPM VP.001980 1101 765 0.69 Personal Observation Diplodocus hallorum USNM V 10865 1010 690 0.68 Gilmore (1932) Galeamopus pabsti NMZ 1000011 870 601 0.69 Tschopp and Mateus (2017) Barosaurus lentus SDSM 25217 735 571 0.77 Foster (1996) 18 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 ically and temporally widespread, and potentially eco- logically present across a wide expanse of the Morrison Formation based on latitudinal occurrences. Although rare demographically, it is possible that Haplocantho- saurus may be a part of the cosmopolitan Morrison Formation sauropod fauna represented by Apatosau- rus, Diplodocus, and Camarasaurus, as these taxa are also found in Wyoming, Utah, Colorado, and Montana (Dodson et al., 1980; Woodruff and Foster, 2017; Fos- ter, 2020; Boisvert et al., 2024). If Haplocanthosaurus contributed to this cosmopolitan fauna, it raises fur- ther ecological questions of how this basal sauropod survived throughout and across the Morrison Forma- tion. As one example, co-occurrence with more numer- ous and derived sauropod taxa highlights a potentially complex series of ecological mechanisms necessary to concurrently sustain a high diversity and abundance of large-bodied terrestrial herbivores (Button et al., 2014). The specimens listed above represent a census of known Haplocanthosaurus remains. The scarcity of published material and lack of complete skeletons (e.g., no skull material) have impacted nearly every aspect of our understanding of this genus. Hopefully as more specimens are identified and described, this will answer many of the questions surrounding this genus. Currently, there is support for at least three species within the genus; two are the previously described and valid H. priscus and H. delfsi, in addition to one currently under study (specimen MWC 8028 by Mathew J. Wedel). This raises other ecological questions regarding the species turnover rate in the Morrison Formation, as many taxa found in the Late Jurassic of North America have multiple species to a genus, especially the sauropods (Table 5). Genus Species Taxonomic authority Amphicoelias A. altus Cope (1877c) Apatosaurus A. ajax, A. louisae Marsh (1877), Holland (1915) Ardetosaurus A. viator van der Linden et al. (2024) Barosaurus B. lentus Marsh (1890) Brachiosaurus B. altithorax Riggs (1903) Brontosaurus B. excelsus, B. parvus, B. yahnapin Marsh (1879), Peterson and Gilmore (1902), Filla and Redman (1994) Camarasaurus C. grandis, C. lentus, C. lewisi, C. supremus Cope (1877b), Marsh (1877), Marsh (1889), Jensen (1988) Diplodocus D. carnegii, D. hallorum, D. longus Marsh (1878), Hatcher (1901), Gillette (1994) Dystrophaeus D. viaemalae Cope (1877a) Galeamopus G. hayi, G. pabsti Holland (1924), Tschopp et al. (2015), Tschopp and Mateus (2017) Haplocanthosaurus H. delfsi, H. priscus Hatcher (1903a), Hatcher (1903b), McIntosh and Williams (1988) Kaatedocus K. siberi Tschopp and Mateus (2013) Maraapunisaurus M. fragillimus Cope (1878), Carpenter (2018) Smitanosaurus S. agilis Marsh (1889), Whitlock and Wilson (2020) Supersaurus S. vivianae Jensen (1985) Suuwassea S. emilieae Harris and Dodson (2004) Table 5. Valid Morrison Formation sauropod species as recognized at the time of writing. 19 Census of Currently Known Specimens of the Late Jurassic Sauropod Haplocanthosaurus from the Morrison Formation, USA Boisvert, C., Bivens, G.T., Curtice, B., Wilhite, R., and Wedel, M. Geology of the Intermountain West 2025 Volume 12 CONCLUSION From the current paleobiological data, the sauro- pod genus Haplocanthosaurus is restricted to the north- western and central regions of the Morrison Formation. Temporally, Haplocanthosaurus is known from the mid- dle and upper parts of the defined Morrison Formation and potentially, based on undifferentiated strata (i.e., not able to be correlated within the Tidwell, Salt Wash, or Brushy Basin Members), may be found throughout nearly the entirety of the formation stratigraphically. Whereas more detailed description and identification of some of the aforementioned specimens are needed, the current evidence illustrates that Haplocanthosaurus was more geographically and stratigraphically widespread than previously known, and there is currently support for high species diversity within this genus. ACKNOWLEDGMENTS We thank Rod Scheetz and Brooks Britt (Brigham Young University) for permitting us to study and pho- tograph the specimens in the BYU collections. We thank Michael Brett-Surman and Matthew Miller (Na- tional Museum of Natural History) and Vicki Yarbor- ough (Yale Peabody Museum) for collections access. We thank Caitlin Colleary (Cleveland Museum of Natural History) for photographs of specimens and Matt La- manna and Amy Henrici (Carnegie Museum of Natu- ral History) for access to the Carnegie collections. We thank Sean Moran and Vince Schneider (North Caro- lina Museum of Nature and Sciences) for helping with specimens from their collection. We thank Dan Chure (Dinosaur National Monument) for the use of photo- graphs from the John S. McIntosh Archives. We thank Emanuel Tschopp (Freie Universität, Berlin) and Cary Woodruff (Phillip & Patricia Frost Museum of Science) for constructive reviews that greatly improved the qual- ity of this work. REFERENCES Agassiz, L., 1844, Monographie des poissons fossiles du vieux grés rouge, ou système Dévonien (Old Red Sandstone) des Iles Britanniques et de Russie: Neuchatel, XXVI, 171 p. Bilbey, S.A., Hall, J., and Hall, D.A., 2000, Preliminary results on a new haplocanthosaurid sauropod dinosaur from the low- er Morrison Formation of northeastern Utah [abs.]: Jour- nal of Vertebrate Paleontology, Programs with Abstracts, v. 2000, p. 30A. Boisvert, C., Curtice, B., Wedel, M., and Wilhite, R., 2024, De- scription of a new specimen of Haplocanthosaurus from the Dry Mesa Dinosaur Quarry: The Anatomical Record, 19 p., https://doi.10.1002/ar.25520. 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