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. STRATIGRAPHIC AND ANATOMICAL EVIDENCE FOR MULTIPLE TITANOSAURID DINOSAUR TAXA IN THE LATE CRETACEOUS (CAMPANIAN-MAASTRICHTIAN) OF SOUTHWESTERN NORTH AMERICA Gregory S. Paul 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 The color rendering of Utetitan zellaguymondeweyae in its habitat. Same scale comparison of mature (scaled to USNM 15560) and juvenile (scaled to TMM 43621) skeletal restorations of generalized composite North American Maastrichtian titanosaurids. Scale bar equals 2 m. Illustrattions by Gregory S. Paul. 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 $30 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 12 2025 Geology of the Intermountain West (GIW) is an open-access journal in which the Utah Geological As- sociation permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. 2024–2025 UGA Board President Keilee Higgs keileeann@utah.gov 801.678.3683 President-Elect Rob Buehring robbuehring@yahoo.com 713.412.9269 Program Chair Mike Arnoff marnoff@utah.gov 385.303.0431 Treasurer Will Hurlbut wdhurlbut@gmail.com 860.733.3190 Secretary Trae Boman tboman@teanues.com 801.648.5206 Past President Eugene Syzmanski eugenes@utah.gov 801.537.3364 UGA Committees Environmental Affairs Craig Eaton eaton@ihi-env.com 801.633.9396 Geologic Road Sign Greg Gavin greggavin@gmail.com 513.509.1509 Historian Paul Anderson paul@pbageo.com 801.364.6613 Outreach Greg Nielsen gnielsen@weber.edu 801.626.6394 Public Education Zach Anderson zanderson@utah.gov 801.537.3300 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 801.580.1331 AAPG House of Delegates 2023–2026 Term David A. Wavrek dwavrek@petroleumsystems.com 801.322.2915 State Mapping Advisory Committee UGA Representative Bill Loughlin bill@loughlinwater.com 435.649.4005 UGA Newsletter Newsletter Editor William Lund uga.newsletter@gmail.com 435.590.1338 UGA Website — www.utahgeology.org Webmaster Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Scholarship Golf Tournament Co-Chair Rick Ford rford@weber.edu 801.915.3188 Co-Chair John South jsouth@utah.gov 385.266.2113 Earthquake Safety Committee Chair Grant Willis gwillisgeol@gmail.com 801.537.3355 Douglas A. Sprinkel Azteca Geosolutions 801.391.1977 GIW@utahgeology.org dsprinkel@gmail.com Thomas C. Chidsey, Jr. Utah Geological Survey, Emeritus 801.824.0738 tomchidsey@gmail.com Bart J. Kowallis Brigham Young University 801.380.2736 bkowallis@gmail.com Steven Schamel GeoX Consulting, Inc. 801.583.1146 geox-slc@comcast.net John R. Foster Utah Field House of Natural History State Park Museum 435.789.3799 johnfoster@utah.gov William R. 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 201 ABSTRACT After the return of giant sauropod dinosaurs in the form of titanosaurids to North America in the Cam- panian of the Late Cretaceous, Alamosaurus sanjuanensis is generally considered to have been the sole taxon on the continent over a few million years. The possibility of one species existing that long is very low because sauropods often exhibit taxonomic diversity in the same habitat. The fossils from the southwest- ern states and northern Mexico are all incomplete, overlapping elements are often scarce, and sometimes differ in ontogenetic development. The fragmentary New Mexican A. sanjuanensis material from the ear- ly Maastrichtian lower Ojo Alamo Formation shows significant distinctions from the much later partial skeletons from the late Maastrichtian lower North Horn Formation of Utah. The latter is therefore made the holotype of Utetitan zellaguymondeweyae. Some late Maastrichtian Texas fossils can be assigned to U. zellaguymondeweyae, others cannot. Fossils from the middle Campanian cannot be assigned to either genus. Southwestern North America supported a diversity of titanosaurids, which may have formed a Utetitan miniclade as they evolved in semi-isolation from the global titanosaurid fauna. Past calculations that these titanosaurids were among the most massive in the group are not borne out by scaling of skeletal restorations. Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Gregory S. Paul Baltimore, MD 21218 USA; GSP1954@aol.com Citation for this article. Paul, G.S., 2025, Stratigraphic and anatomical evidence for multiple titanosaurid dinosaur taxa in the Late Cretaceous (Campanian-Maastrichtian) of southwestern North America: Geology of the Intermountain West, v. 12, p. 201–220, https://doi.org/10.31711/giw.v12.pp201-220. INTRODUCTION After an absence from the fossil record of North America over most of the Late Cretaceous, sauropod dinosaurs returned to the southwestern part of the con- tinent in the Campanian in the form of the only sauro- pod clade extant by then, titanosaurids (D’Emic, 2010, see below). It has long been widely presumed to varying degrees of confidence that all fossil juvenile to adult tita- nosaurid specimens (Figure 1) from New Mexico, Utah, Texas, and Chihuahua (Figures 2 and 3), none of which preserved the majority of the individual, represent one species, Alamosaurus sanjuanensis (Gilmore, 1922, 1946; Lawson, 1972; Mateer 1976; Wolberg et al., 1986; Lucas and Hunt, 1989; Lehman and Coulson, 2002; Woodward, 2005; Lehman et al., 2006; Rivera-Sylva et al., 2006; D’Emic et al., 2010, 2011; Fowler and Sulli- van, 2011; Jasinski et al., 2011; Wick and Lehman, 2014; Carrano and D’Emic, 2015; Curtice, 2016; Tykoski and Fiorillo, 2017)—or at least the one genus, the exact tax- onomic intent is not always clear in these papers. The holotype of the taxon is an incomplete scapula, support- ed by a paratype pubis that probably does not belong to the same individual. A number of other fossil remains ranging from fragmentary to a major minority of a skel- eton have been assigned to the other species. A rare expression of skepticism of the single taxon hypothe- sis has been Lucas and Sullivan (2000) whose analysis 202 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 Figure 1. Same scale comparison of postcranial elements of Maastrichtian North American titanosaurids, entire bar equals 1 m (this figure is more accurate to a common scale than Figure 5 in D’Emic et al., 2011), appendicular elements are from the left side (reversed when necessary), dotted lines indicate missing bone, arrows point to taxonomic characters discussed in text and listed in diagnoses as numbered; traced from published images as indicated. (A) Scapulae in lateral view: left, early Maastrichtian Alamosaurus sanjuanensis holotype USNM 10846 (Plate 1 in Gilmore, 1922, Plate 10 in Gilmore, 1946; Figure 5C in D’Emic et al., 2011, bone is not as complete); middle, latest Maastrichtian Utetitan zellaguymondeweyae referred BIBE 45958 juvenile (Figure 12A in Tykoski and Fiorillo, 2017); right, late Maastrichtian U. zellaguymondeweyae holotype USNM 15560 (Figure 5D in D’Emic et al., 2011, contrast to Figure 6 in Gilmore, 1946). (B) Ischia in anterior view: top, A. sanjuanensis paratype USNM 10847 (Plate 2 in Gilmore 1922); middle left, late Maastrichtian titanosaurid incertae sedis TMM 41541-1 (Figure 11B in Tykoski and Fiorillo, 2017); middle right, late Maastrichtian titanosaurid incertae sedis TMM 43621-1 juvenile (Figure 9 in Lehman and Coulson, 2002); bottom, USNM 15560 (Figure 11 in Gilmore, 1946). (C) Mid cer- vicals in left lateral view left, A. sanjuanensis provisionally referred SMP VP-1850 (Figure 1A in Fowler and Sullivan, 2011); right, BIBE 45854 (Figure 3A in Tykoski and Fiorillo, 2017). (D) Anterior caudals in posterior view; left, A. sanjuanensis provisionally referred SMP VP-2104 (Figure 2B in Fowler and Sullivan, 2011); middle, TMM 41541-1 (Figure 10C in Tykoski and Fiorillo, 2017); right, USNM 15560 (Plate 8 in Gilmore, 1946). 203 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 indicated that Alamosaurus sanjuanensis had become a wastebasket taxon for the Late Cretaceous titanosau- rids from the region. Fronimos and Lehman (2014) and Lehman et al. (2018) declined to refer some Texas ti- tanosaurid elements to A. sanjuanensis while doing so with others. Lozinsky et al. (1984) noted the common tendency to refer Late Cretaceous North American sau- ropods to Alamosaurus without justification, while do- ing it themselves. Because of the taxonomically deficient nature of the fragmentary A. sanjuanensis holotype USNM 10486, and the lack of sufficient anatomical overlap between the limited titanosaurid fossils from the early Maas- trichtian lower Ojo Alamo Formation in northwestern New Mexico, the species has largely been defined by the Utah specimen USNM 15560 partial skeleton, it being the best single specimen available. Diagnosing a species based on fossils from a different geographic and strati- graphic location is inherently problematic and must be considered provisional until the systematic unification is verified, or perhaps more probably refuted, on mor- phological and stratigraphic grounds. While preparing a new edition of a field guide (Paul, 2024), it became ap- parent that a solid verification has not been established regarding Alamosaurus, in part because the deep time evolutionary issues have not been adequately addressed in the literature. This brief study is largely limited to as- sessing whether the pertinent fossils can be confidently assigned to one species, or if they more probably belong to at least two taxa at the species and perhaps genus lev- el. This is not a major examination of larger issues of ti- tanosaurid systematics, phylogeny, or biogeography, the investigation being only that necessary to achieve the basic aims. Also examined is the issue of the maximum masses indicated by the larger fossils of these titanosau- rids. ABBREVIATIONS AMNH – American Museum of Natural History, New York City, New York. BYU – Brigham Young Uni- versity – Provo, Utah. BIBE – Big Bend National Park, Texas. NMMNH – New Mexico Museum of Natural History and Science, Albuquerque, New Mexico. PMNS – Perot Museum of Nature and Science, Dallas, Texas. Figure 2. Map of the southwestern states and bordering Mexico show- ing the locations of formations con- taining Late Cretaceous titanosau- rid fossils, and town of Huntington, Utah; entire scale bar equals 500 km; UT = Utah, NM = New Mexico, TX = Texas, and CH = Chihauhau State (Mexico). 204 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 PMU – Paleontological Museum, Uppsala University, Uppsala, Sweden. SMM – Science Museum of Minneso- ta, Saint Paul, Minnesota. SMP – State Museum of Penn- sylvania, Harrisburg, Pennsylvania. TMM – Texas Me- morial Museum, Austin, Texas. TTU – Museum of Texas Tech University, Lubbock, Texas. USNM – United States National Museum of Natural History, Washington, D.C. UTEP – Centennial Museum at the University of Texas at El Paso, Texas. PREVIOUS WORK The titanosaurid fossils from the New Mexico, Utah, Texas and the Chihuahua Provinces, Mexico, have been extensively described and illustrated including with Figure 3. Stratigraphic chart for North American Late Cretaceous titanosaurids. Ages indicated by vertical dashed lines with placement of selected specimens also indicated—among the latter, those that are oriented vertically are less well dated than those that are oriented horizontally Based on references listed in the main text. All formation boundaries (horizontal dotted lines) and levels of most specimens are approximate to varying degrees, sometimes very much so, such as the CHH specimen that is from either the upper Aguja or lower Javelina Formations, which apparently have substantial missing sediments be- tween them. The most precisely located fossils are TMM 43621, which is close to Cretaceous/Paleogene (K/Pg) boundary. A Javelina Formation, from which an uncollected femur (T.M. Lehman, Texas Tech University, written communication, 2025) has an isotopic (U/Pb) age of 69 ± Ma (Lehman et al., 2006). In some locations the top of the Javelina is close to the K/Pg boundary. In the lower Ojo Alamo, Hall Lake, Javelina, and North Horn Formations, the vertical dashed lines incorporate multiple specimens of uncertain exact level. Dalman et al. (2024) has obtain an isotopic age of 73 Ma (late Campanian) from the Hall Lake Formation. 205 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 photographs (Gilmore, 1922, 1946; Lawson, 1972; Ma- teer, 1976; Lucas and Hunt, 1989; Lucas and Sullivan, 2000; Lehman and Coulson, 2002; Rivera-Sylva et al., 2006; Williamson and Weil, 2008; D’Emic et al., 2011; Fowler and Sullivan, 2011; Jasinski et al., 2011; Fronimos and Lehman, 2014; Wick and Lehman, 2014; Tykoski and Fiorillo, 2017). The author has seen and examined much of the North American titanosaur material over the decades. Element illustrations (Figure 1) are traced at large scale directly from published photographs. Note that the illustrations of the USNM 15560 scapula in Gilmore (1946, Figure 6) differs from the actual fos- sil (Figure 5D in D’Emic et al., 2011). Two photographs of the scapulae are available, but I chose not to include them here because they are medial views of the bones showing their very poorly preserved inner surfaces that no one uses because they are very badly preserved on that side, and therefore useless. There is no point pub- lishing these since high-quality photographs of the lateral surfaces are already in the literature, and there are no good resolution photographs in the files of the Smithsonian Institution’s Museum of Natural History in Washington, D.C. Recent photographs of the USNM 10486 and USNM 15560 scapulae (Figures 5C and5D in D’Emic et al., 2011) show the bones have been some- what degraded over the decades (compared to Plate 1 in Gilmore, 1922, and Figure 6 in Gilmore, 1946). The new illustrations are the most accurate images of the fossils yet produced. Evidence has grown that herbivorous dinosaur spe- cies were prone to turning over rapidly, with species typ- ically not lasting more than a few hundreds of thousands of years (Ryan and Evans, 2005; Paul, 2006, 2016, 2024, in press; Gates, 2012; Scannella et al., 2014; Tschopp et al., 2015; Mallon, 2017, 2019). The same studies also of- ten found that a given herbivorous dinosaur family can exhibit diversity at the same level in a particular for- mation, including sauropods. Even taking possible over splitting of taxa into account, differing stratigraphic lev- els each feature an array of diplodocid, camarasaurid, and brachiosaurids, and the same is true of titanosau- rids in the Adamantina, lower Allen, lower and upper Anacleto, lower Bajo Barreal, Huincul, Portezuelo and Rio Neuguen Formations of South America (Weisham- pel et al., 2004; Tschopp et al., 2015; Paul, 2016, 2024, in press). The possibility that just one titanosaur taxon inhabited the entirety of the North American southwest during most of the Maastrichtian is correspondingly quite low and nonparsimonious. A factor in the fast and diverse evolution of dinosaur taxa may have a genetic basis due to high chromosome numbers that remain operative in the many thousands of species of modern birds (O’Connor et al., 2018). The rapid, R-strategy re- production of giant dinosaurs depositing large numbers of their small eggs may have also favored swift evolution and diversification compared to calf dropping K-strat- egy breeding big mammals. It is therefore presumed that if fossils are separated by more than a few hundred thousand years that they probably are different species, unless strong evidence indicates otherwise. The latter includes near identical morphology of overlapping el- ements as further discussed below. Nor can it be pre- sumed that incomplete specimens found at the same stratigraphic level of a formation from a given family represent one species—unless found in very close asso- ciation—rather than being from more than one taxon. Any conclusion one way or another on North American titanosaur diversity needs to be based on the prepon- derance of the data. Consisting of multiple species, gen- era can last over long stratigraphic periods. Yet gradis- tic anatomical differences may favor generic separation, even if the taxa are closely related to one another rela- tive to other known fossil members of the taxonomic family. There is a good probability that gaps in the fossil record make two dinosaur species appear to be sibling taxa on a phylogenetic chart when they actually are separated by a number of intra subfamily species that created considerable gradistic space not readily accom- modated in one genus. In order to provide more clari- ty regarding these issues and tighten up the taxonomic specifics, diagnoses based on a substantial number of specimens of the pertinent taxa are used to determine the species on a gradistic basis. This comparative anat- omy approach is similar to work by Chure and Loewen (2022), Danison et al. (2024), and Paul (2025), who do not include statistical or phylogenetic analysis among fossils that are obviously closely related, but different enough to be divided at the genus and/or species levels. 206 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 An in-depth phylogenetic analysis of titanosaurids is far beyond the scope of this modest study. Nor would such likely to be productive because the differences between the Late Cretaceous North American specimens is of the modest degree expected within a subfamily that had been evolving with some degree of isolation from oth- er titanosaurs in South America and/or Asia, probably forming a mini-clade. The North American specimens are too few and incomplete to produce phylogenetic re- sults reliable enough to improve the taxonomy on the existing limited data base (as per the fossils examined in Danison et al., 2024). All specimens this researcher is aware of are listed and characterized in Systematics, with primary data and illustration sources noted. Overcoming evidence for taxonomic distinction at the species level between members of a subfamily, de- spite a large separation in time, requires that the fossils be very similar in form in all details of the crania and postcrania, as is a norm within species at a given onto- genetic stage (Paul, 2025). An exception is strong sexu- al dimorphism, but that has not been observed among dinosaurs (Mallon, 2017). Determining this norm for sauropod scapulae is aided by a sample of a number of such elements, all little distorted, for adult Camarasau- rus supremus, all found from the same quarry (Osborn and Mook, 1921; Figure 4); a badly distorted subadult scapula (Figure 76 in Osborn and Mook, 1921) is ex- cluded. Aside from some possinle slight alterations due to minor distortion, none of the scapulae are markedly divergent in configuration. They share the same basic distinctive profile. This includes the prominence and shape of the large acromion process. And a prominent triangular glenoid process. If divergence in the North American titanosaur scapulae is greater than in C. su- premus, especially from specimens from differing strati- graphic levels, then that is evidence they do not repre- sent a united taxon. Paul (1997, 2016, 2024, in press), Larramendi (2016), and Brassey (2017) detail the methods for restoring skeletons and body masses via volumetric models, with Paul (2019) focusing on the broad bellied titanosaurids, including specimens assigned to Alamosaurus. Lar- ramendi et al. (2021) modified the results with higher specific gravity values of 0.96, except for the sauropod pneumatic neck at 0.85. Because the fossil remains are so incomplete, an approximate composite restoration of the collective Maastrichtian North American titanosau- rids is all that can be prepared (Figure 5), which is then scaled to the most widely shared size class of large spec- imens, as well as a juvenile, with some modifications in each to represent their different growth stages based in part on age specific data from Lehman and Coulson (2002) and Tykoski and Fiorillo (2017). That the res- torations are composites based on incomplete fossils renders the resulting mass estimates more approximate than those based on more completely known taxa. Their composite natures also mean that there is not mass to dimensions allometric scaling between growth stages that may have existed, other than the head of the juve- nile is rendered a little larger in relation to the rest of the animal. LATE CRETACEOUS NORTH AMERICAN TITANOSAURID STRATIGRAPHY Although substantial uncertainties exist in some cases, there is broad consensus regarding the gener- al ages of the beds containing Late Cretaceous North American titanosaurid fossils (Table 1, Figures 2 and 3). A fossil from the upper Aguja Formation or the low- er Javenila Formation in the northeastern Chihuahua Province, Mexico, is late Campanian or early Maas- trichtian (Woodward and Lehman, 2005; Rivera-Sylva et al., 2006; Rivera-Sylva and Carpenter, 2014; H.E. Ri- vera-Sylva, National Autonomous University of Mexico, written communication, 2024). Their boundaries ap- parently being a major nonconformity (Fowler, 2017), there appears to be a significant time gap between the top of Aguja and the base of the Javelina Formations. If the specimen is from the Javelina Formation, it may be the earliest known of the North American titanosaurs, dating from near the end of the Campanian. If so, its southerly location may reflect the migration of titano- saurs from the south. Probably deposited over a short period of time, the thin Naashoibito section of the low- er Ojo Alamo Formation in northwestern New Mexico preserved the A. sanjuanensis types and numerous oth- er specimens. The absence of lambeosaurine hadrosaur 207 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 fossils precludes a late Maastrichtian age as explained by Jasinski et al. (2011). Fowler (2017) provisionally places the sediments at 70 Ma based on a disputed radiometric result that is discussed. Assertions that a 69-million-year age for Texas ti- tanosaurid fossils assigned to A. sanjuanensis (as per Lehman et al., 2006) show that the Ojo Alamo types are about that age (as per Jasinski et al., 2011) but cannot be verified because the very fragmentary easterly material is not taxonomically determinate. For example, Lehman et al. (2006) assign nondiagnostic Texas titanosaurid material to A. sanjuanensis covering a taxonomically improbably long time span of 3 million years. Also not temporally informative is a large tyrannosaurid scapu- la-coracoid (Jasinki et al. 2011) because gigantic mem- bers of the group do not belong to the latest Maastrich- tian species of Tyrannosaurus (Paul, 2025), which are known from the late Campanian (Stein and Triebold, 2013; Dalman et al., 2024). The apparently middle Maastrichtian middle sec- tion of the Hall Lake Formation of southern New Mex- ico (Amato et al., 2017; Dalman et al., 2022, 2024) con- tains fragmentary fossil remains (Lozinsky et al. 1984; Wolberg et al. 1986; also see the Systematic Paleontol- ogy section in this article). The lower North Horn For- mation of the Wasatch Plateau of central Utah produced USNM 15560 and a few other titanosaur elements. This part of the formation being shallower relative to the Cretaceous/Paleogene (K/Pg) boundary than it is else- where, the fossils are later late Maastrichtian, with the titanosaurid specimens not yet known from particular- ly close to the Cretaceous-Paleogene boundary (Figure 3 in Lawton et al., 1993; Cifelli et al., 1999; Difley and Ekdale, 1999; Sampson and Loewen, 2005; Difley, 2007; D’Emic et al., 2010; Jasinski et al., 2011; Curtice, 2016). The presence of a giant tyrannosaurid in the North Horn Formation in the same location at USNM 15560 is suggestive of such an age, but not entirely definitive as noted above. Also, the specimen of concern (Samp- son and Loewen, 2005) consists of only two temporal cranial elements that are not sufficiently diagnostic at the genus level to be of stratigraphic value regarding taxonomic identification (Paul, 2025). Missing from the specimen is the taxonomically critical lacrimal. That bone does not bear a hornlet that is found only in Ty- rannosaurus among American tyrannosaurids. Thus, lacking that element, the fossil cannot be shown to be a Tyrannosaurus. Being from significantly different lev- els of the Maastrichtian, specimens USNM 10846 and USNM 15560 are therefore separated by a few million years. The southwestern Texas Big Bend and nearby Chi- huahua region Javelina and lower Black Peaks Forma- tions feature fairly abundant titanosaurids that range Figure 4. Same scale comparison of undistorted adult Camarasaurus supremus AMNH scapulae from the upper Morrison Formation (Late Jurassic) Cope quarry at Garden Park, Colorado; bar equals 1 m. Shown as lefts (reversed when necessary). Based on Figures 74, 75, 77, 79, and 80 in Osborn and Mook (1921) and personal observation. (A) 5760/3, (B) 5761/4, (C) 5761/5, (D) 5761/1, and (E) 5761/4. 208 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 from perhaps the later early to late Maastrichtian, with some in the Black Peaks very close to the K/Pg bound- ary (Lehman and Coulson, 2002; Woodward, 2005; Lehman et al., 2006; Rivera-Sylva and Carpenter, 2014; Fowler, 2017). Part of the top of the Javelina is also very late Maastrichtian, although it does not necessarily reach the K/Pg boundary (Caitlin et al., 2018; Lehman et al., 2022). Late Cretaceous titanosaurids were present in North America for 5 to perhaps 6 million years (Figure 3). That is abundant time for taxa to experience sub- stantial evolution, especially at the species level. The possibility that one species lasted even half that long is hardly tenable—a million years for a species would be a bioevolutionary stretch. A genus being present for 5 million years is plausible but not necessarily probable. Because the upper Javelina and lower Black Peaks For- mations that contain titanosaurids are late Maastrich- tian (Lehman and Coulson, 2002; Tykoski and Fiorillo, 2017), they are good potential candidates for being the same taxon as specimen USNM 15560, but that still re- quires comparative osteological confirmation. Impre- cise dating of the Ojo Alamo fossils especially and also of the North Horn Formation in Utah and some Texas and Mexican material (as per Wick and Lehman, 2013), leaves most of the information on the stratigraphic chart (Figure 3) approximate. However, exacting strati- graphic placement is not necessary regarding the issues at hand because the only two named taxa are based on type materials with 3 to 4 million years between them. RESULTS AND DISCUSSION That the New Mexican A. sanjuanensis holotype and paratypes USNM 10846 and USNM 10847 were found about 300 m from one another (Gilmore, 1922) disfavors Figure 5. Same scale comparison of mature (scaled to USNM 15560) and juvenile (scaled to TMM 43621) skeletal resto- rations of generalized composite North American Maastrichtian titanosaurids; scale bar equals 2 m. 209 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 their being from the same individual. That the paratype ischium appears to be too small to belong to the same animal as the large scapula (Figures 1A and 1B) increas- es the probability that the two specimens, USNM 10846 and USNM 10847, are from two individuals. Because the scapula is missing some parts from the holotype (Figure 1A), the species is dangerously close to being a nomen dubium (doubtful name) (Lucas and Sullivan, 2000). The standard titanosaurid form paratype ischium (Figure 1B), that may or may not be the same species, does little to improve the taxonomic understanding of lower Ojo Alamo Formation’s titanosaurids. The same applies to additional fragmentary and usually badly damaged titanosaurid fossil remains from the same level of the Ojo Alamo Formation—the aft end of a cervical (Figure 1C), an anterior caudal (Figure 1D), a distal fe- mur, a fairly complete pes, and numerous other elements (D’Emic et al., 2011; Fowler and Sullivan, 2011; Jasinski et al., 2011). It therefore is not known how many taxa are represented by these nonoverlapping fossils, contrary to Jasinski et al. (2011) who insist on a presumption of one taxon while not noting that multiple titanosaurid and Formation Location Age Specimen Lower Black Peaks West Texas latest Maastrichtian BIBE 45854, 45958; TMM 43621-1 Lower North Horn Central Utah late Maastrichtian USNM 15560 (Utetitan holotype), BYU 9087, 11392, 11393 Upper Javelina West Texas middle to late Maastrichtian AMNH 21531; TMM 41541-1 Middle Javelina West Texas middle to late Maastrichtian TMM 46052-1 Lower Javelina West Texas, northeastern Chihuahua Province middle to late Maastrichtian TMM 40699, 41060, 41061, 41398-1- 2, 41450-1-2; TTU 542, 546; UTEP P-25, uncataloged Chihuahua fossil remains Javelina (stratigraphic position was not avail- able) West Texas middle to late Maastrichtian TMM 40597-5, 41063, 41396-1, 41541, 42495-7, 43090-2, 43598-1-6, 43599-1-3, 45601-1, 45602-1, 45854- 1-8, 45855-1-4, 45856-1, 45857-1, 45859-1, 45861-1, 45862-1, 45863-1, 45864-1, 45865-1, 45888-1, 45890- 1&2, 45891-1-17, 45864-1 Middle Hall Lake southern New Mexico middle Maastrichtian TKM 007, 009 Lower Ojo Alamo northwestern Chihuahua early Maastrichtian USNM 10846 (Alamosaurus ho- lotype), 10847, 15658; NMMNH 22544, 25072, 25077, 25077, 27291, 28741, 29031, 29722, 29723, 29724, 29725, 29726, 29727, 29728, 49967; PMU 24305, 24893; SMP VP 1097, 1136, 1138, 1139, 1271, 1494, 1539, 1541, 1581, 1625, 1626, 1641, 1715, 1718, 1850, 1864, 1866, 1876, 2043, 2065, 2097, 2104, 2175, 2230, 2233, 2507, 2696, 3323, SSM 5428 Upper Aguja or lower Javelina northeastern Chihuahua late Campanian or early Maastrichtian Uncataloged tibia Table 1. North American titanosaur-bearing formations and their usually approximate stage ages and fossil contents; see Sys- tematics for additional information. 210 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 other sauropod taxa often inhabit the same habitats. Found on the eastern rim of Joe’s Valley in Utah’s Wasatch Plateau, USNM 15560 is the most complete North American latest Cretaceous titanosaurid spec- imen (Gilmore, 1946). Because its scapula is a little smaller than that of specimen USNM 10846 (Figure 1A), USNM 15560 was probably somewhat lesser sized in life. Specimen USNM 15560 ischium is about the same size as that of specimen USNM 10847, which in- dicates that those two dinosaurs were similar in overall dimensions (Figure 1B). Thus, it is not clear whether the Utah dinosaur was somewhat smaller in life than the New Mexico dinosaur or about the same size. Impor- tantly, the USNM 15560 scapula is well preserved. Its acromion process point is markedly more prominent than that of A. sanjuanensis USNM 10846 (Figure 1A). D’Emic et al. (2011) seem to attribute the differences of the two specimens to distortion of USNM 15560, but the exact basis of this assertion is not documented. I have examined the specimens and did not observe any distortion after directly comparing the two bones. The latter has a markedly deeper indentation in the profile of the posterior edge immediately dorsal to the glenoid (Gilmore, 1946), resulting in a prominent triangular glenoid process similar to that of some C. supremus (Figure 2). These divergences on their own are not in good accord with taxonomic synonymy. The paired tu- bercles just above the posterior indentation cited as evi- dence for conspecifity by D’Emic et al. (2011) are wide- ly separated and quite prominent on the Utah scapula despite its not being a larger animal. Their presence on USNM 10846 is at most marginal if they are true mor- phological features. They are not widely spaced, and such structures are present on some other titanosau- rids from other regions and geologic ages (D’Emic et al., 2011). These structures are consequently not nec- essarily species specific diagnostic characters, contrary to D’Emic et al. (2011) and Tykoski and Fiorillo (2017). Instead, they are different enough to help define at least two species, if not genera. The same comments apply to the both similar but also different dorsal flaring of the scapula blades cited as taxonomically significant by D’Emic et al. (2011) and Tykoski and Fiorillo (2017), that of specimen USNM 15560 being greater than that of specimen USNM 10846. The cumulative differenc- es between the two USNM scapulae are readily seen to markedly exceed that observed between the five ca- marasaur scapulae from one quarry (compare Figures 1A and 1B to Figures 4A through 4E). So much so that USNM 15560 looks somewhat like a camarasaur scap- ula; USNM 10846 is not close to having such a shape. Far from sharing the sufficient anatomical uniformity to surmount the stratigraphic differential, the mor- phological divergences are sufficient to indicate two different taxa even if the bones were found at the same level of the same formation. A badly damaged imma- ture Ojo Alamo scapula (Jasinski et al., 2011) does not have a prominent acromion process, paired tubercles, or ventroposterior indentation in accordance with the A. sanjuanensis holotype. Of great interest is the juve- nile ventral scapula BIBE 45958, which is a near perfect match in the relative strong prominence and form of the acromion process, the strong indentation just above the glenoid resulting in a substantial triangular gle- noid process, and very similar development and place- ment of the two tubercles compared to the much larger USNM 15560. The two elements are so alike that spec- imen BIBE 45958 looks like the juvenile of the same specimen as USNM 15560 despite the geographic dis- tance between them. Both are markedly different from specimen USNM 10846 found in New Mexico. Such a close similarity between these two late Maastrichtian specimens reinforces the probability that shared char- acteristics are genetically determined, and therefore of taxonomic significance. Thus, specimen BIBE 45958 can be confidently assigned to the same taxon as con- temporary specimen USNM 15560, but not to the geo- logically earlier specimen USNM 10846. The claims by D’Emic et al. (2011) and Tykoski and Fiorillo (2017) that four ischia are similar in form are not correct, they are all divergent from one another (Figure 1B). Specimens USNM 10847 and TMM 41541-1 have distinctly longer lateral processes relative to midline length than does specimen USNM 15560, and the first two have far deeper and more subcircular concave arcs to their postero-lateral edge profiles. Specimen TMM 41541-1 is more curved than specimen USNM 15560. There is, therefore, no reason to conclude these pelvic 211 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 bones are indicative of just one or even just two taxa. The same applies to the North American titanosaur ilia and pubes. The elements of a large individual from the late Maastrichtian Black Peaks Formation are very different from those of a similar sized, much earlier fossil from the lower Javelina Formation (Figure 7 in Fronimos and Lehman, 2014). The younger example has a pubic peduncle about half the length of, yet more massive than, that of the other, and the shapes of the anterior and aft ilial plates are quite distinctive. The latter Maastrichtian pubis has a longer and more gracile ventral process than its older counterpart. These two specimens could easily represent different genera, with the earlier possibly being Alamosaurus, although not necessarily A. sanjuanensis. Complicating matters is another Black Peaks For- mation ilium (Figure 8 in Lehman and Coulson, 2002), which is different in all respects from the other two tita- nosaurids. Its anterior plate is shallower and the poste- rior plate deeper. Meanwhile the pubic peduncle is very long like the earlier potential Alamosaurus, and very divergent from the short process of the contemporary ilium. That means two terminal Maastrichtian regional titanosaurid taxa is possible. That the three ilia appear to share an indentation on the dorsal rim may indicate an alliance within a common titanosaurid subclade. The relative proportions and shape of the Texas juve- nile TMM 43621-1 ischium is more similar to that of the A. sanjuanensis paratype and specimen TMM 41541-1 than to specimen USNM 15560, but the juvenile status of the first obscures the systematic significance of this observation (Figure 1B). The anterior caudals of spec- imen USNM 15560 from Utah are not in close accord with those of either earlier New Mexico specimen SMP VP-2104, or similar Texas specimen TMM 41541-1. The latter two having a broader centrum relative to their own heights (Figure 1E). It is possible that the differ- ences are due to ontogenetic changes, SMP VP-204 and TMM 41541-1 being larger than USNM 15560, but that is speculative. Proximal ends of a humerus and femur from the North Horn Formation are close to USNM 15560, but do not belong to that specimen (Curtice, 2016). A tita- nosaurid femur and humerus are present in the correl- ative McRae Formation of southern New Mexico (Loz- insky et al., 1985; Wolberg et al., 1986), and a number of humeri and femora, many complete or close to it, from juvenile to adult are known from Texas (Lehman and Coulson, 2002; Fronimos and Lehman, 2014; Wick and Lehman, 2014). As those researchers have noted, these fossil elements are sufficiently similar enough to repre- sent a single taxon, but they were not definitive at ad- dressing whether the fossils are or not. That may be due to the fossil assemblage greatly differing in the sizes of the specimens due to ontogeny and, in some cases, the bones being incomplete and/or distorted. Early Maastrichtian Ojo Alamo Formation femora are too incomplete (Lucas and Sullivan, 2000; Fowler and Sullivan, 2011) to compare to the late Maastrich- tian Utah and Texas specimens, and a humerus from the first locale is not yet known. Wick and Lehman (2014) observe that two femora from the upper Javeli- na Formation are more robust, even though immature, than the three specimens found lower in the formation. That may be of taxonomic significance, although sexual and individual variation cannot be ruled out. The sim- ilar size of the Ojo Alamo and North Horn specimens cited by D’Emic et al. (2011) as suggestive of one species is not an important taxonomic item in many species being similar in size, which is all the more so because some of the Ojo Alamo material is markedly larger than anything from the North Horn to date (Figure 1D; Fowler and Sullivan, 2011). Sexual and individual vari- ations may or may not justify taxonomic synonymy if the specimens were from the same stratigraphic level of the same formation, but in view of their being separated by millions of years the possibility of a monospecies is small. There is not sufficient reason to maintain the Ojo Alamo and North Horn Formations’ titanosaurid fossils in the same species, or even genus, with the stratigraphic and morphological data at hand. Therefore, taxonomic division is strongly favored. Because they lived at the same time or close to it (Figure 3), the late Maastrichtian Utah and Texas tita- nosaurid fossils are candidates for being the same taxon at least at the genus level, but such should not be auto- matically presumed to avoid the risk of oversimplifying the systematic situation at the end of the Cretaceous 212 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 based on the very limited fossil data set. Whereas the similarities between the two Black Peaks ilia favor that they the same taxon perhaps at the species level, their differences with the North Horn Formation ilium in- dicate the former and the latter are not the same taxon, probably at the genus level. In contrast, the Utah and BIBE 45958 scapulae are so extremely alike that they are entirely compatible with the latter being a juvenile of the same species as USNM 15560. That the last two are so alike while being so different from earlier USNM 10846 reinforces the systematic separation of early from end Maastrichtian titanosaurids. Concerning ontogeny, the small juvenile status of TMM 43621-1 (Lehman and Coulson, 2002) severely limits its taxonomic utility as a potential type specimen. Meanwhile, the great differences in cervical morpholo- gy between large (Tykoski and Fiorillo, 2017) and small (Lehman and Coulson 2002) may represent differential ontogeny, or systematics, or both. The dorsals of a large specimen (Fronimos and Lehman, 2018) and a juve- nile (Lehman and Coulson 2002) appear fairly similar, but the size differences, and the disarticulation of the juvenile’s neural spines from the centra, hinder needed anatomical comparisons. The lack of sufficient overlap between the large cervicals described by Tykoski and Fiorillo (2017) with other latest Cretaceous regional titanosaurids means their taxonomic value is currently nil. TMM 43621-1 and BIBE 45854 are therefore inde- terminate specimens. Because the TMM 41541-1 ischium and caudal are so distinct from specimen USNM 15560, they are not readily referable to the same taxon and may be a distinct form. Perhaps closer to A. sanjuanensis than USNM 15560, specimen TMM 41541-1 should be considered indeterminate titanosaurid fossils until more informa- tion becomes available. Two late Maastrichtian osteo- derms found with Utah USNM 15560 (Carrano and D’Emic, 2015) and from Texas (Fronimos, 2021) are compatible with their being one taxon, but they do not establish such. Nor do they offer comparison to earlier Alamosaurus for which no armor is yet known. The sug- gestions by Fronimos and Lehman (2018) and Lehman et al. (2018) that latest Maastrichtian lower Blacks Peak Formation and earlier Javelina Formation titanosaurids are distinct taxa is sound, but their assignment of the former to the much older A. sanjuanensis is not well founded. Even if future information indicates that some of the Texas titanosaurids constitute a distinct taxon, none of the known specimens is of holotype quality. The few vertebral, pectoral, and pelvic characters shared by the collective end Cretaceous specimens, their geographic isolation from non-North American titano- saurids, and considering genetic drift, suggest North American titanosaurids formed a small clade distinct from other titanosaurids that are diagnosed as a sub- family—a higher level titanosaur taxonomy is currently too unsettled to assign this little group to a larger family. But there is no strong evidence which are considered a subfamily . But there is no strong evidence that they all belong to one species in one genus, such would be probable only if all the southwestern titanosaurids were structurally very similar and came from one narrow stratigraphic level. However, that is far from the actual situation because there is too much time stratigraph- ic separation (Figure 3) and morphological variation (Figure 2) between the North American titanosaurids to place them in one species that lasted for millions of years during the Maastrichtian or even earlier up to the K/Pg crisis in natural selection stasis. Therefore, the dinosaurs that dwelled together experienced rapid and extensive Darwinian evolution, including the ap- parent loss of whole clades of North American alber- tosaurines, lambeosaurines, centrosaurines, and classic chasmosaurines in favor of over-sized tyrannosaurines, edmontosaurines and triceratopsines (Paul, 2016, 2024, 2025, in press). No other dinosaur species is currently projected to have lasted so long based on extensive fos- sil remains. My interpretation is the Utah material can be read- ily differentiated from the New Mexico fossils, and at least two taxa need to be recognized, with the under- standing that more, perhaps many more, taxa may have been present over that time on the continent. Diagnos- ing the relatively complete USNM 15560 as a holotype is more readily done than for any other North Ameri- can titanosaurid. The failure to identify USNM 15560 as the basis of a taxon, and using it to help identify the far less adequately characterized A. sanjuanensis, is not 213 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 well advised. The more difficult question is whether the taxo- nomic separation is just specific or is also generic. Con- tinuing to place the North Horn Formation specimen in Alamosaurus is problematic because the genus is founded on such poor material. The differences are sub- stantial, and—very importantly—ongoing congeneric status, risks encouraging the continued use of the Ojo Alamo and North Horn material to define one dubious genus and tossing assorted specimens in a probable ge- nus level taxonomic wastebasket (as per D’Emic et al., 2011; Tykoski and Fiorillo, 2017). So USNM 15560 is designated the holotype of a new genus and species. The generic name recognizes the original peoples of the re- gion, and the specific title honors a centurion resident of the Wasatch Plateau area in which it was found. A few other incomplete titanosaurid fossil remains from the lower North Horn are referred on a tentative basis, but the characters they exhibit should not be used to diagnose the taxon until further comparisons are made. Despite living at a similar time, most of the Texas specimens are not placed in the new taxon because, in part, they are not sufficiently identifiable at the genus and/or species level, and avoids potentially contami- nating the taxon and its identification with dinosaurs from a different horizontal location. This also precludes future taxonomic inertia from leading researchers to presume synonymy as has been the past tendency, rath- er than more aggressively investigating the taxonomic situation. The new genus is not intended to be another wastebasket taxon for incomplete specimens across the late Maastrichtian of the North American southwest. Because it is quite possible that more than one tita- nosaurid lived in the same place at the same time, Alam- osaurus sanjuanensis is authoritatively identified on the one element that definitely belongs to the taxon, USNM 10846. The characteristics of the paratype ischium are highly tentative, the reference of the specimen to the spe- cies being suspect. The other lower Ojo Alamo Forma- tion specimens are referred to A. sanjuanensis on a very provisional basis and need to be considered as potentially distinct taxa as they do not exhibit the diagnostic features and the age is inconclusive. The tibia from Chihuahua is a taxonomic floater. The differential identifications are intended to distinguish the North American titanosaur taxa from one another, not from foreign examples. SYSTEMATIC PALEONTOLOGY Dinosauria Owen 1842 Sauropoda Marsh 1878 Titanosauria Bonaparte and Coria 1993 Utetitaninae n. sf. Provisional diagnosis: circumferential depression limited to ventral half of the anterior condyle of bicon- vex first caudal centrum, multiple large foramina pierce lateral surface of first caudal, paired tubercles dorsal to scapula glenoid, base of blade cross section asymmet- rically concave with thinner anterior and thicker pos- terior margin, antero-dorsal blade edge nearly straight whereas postero-dorsal edge flares posteriorly. Small indentation on dorsal rim of ilium. Alamosaurus Gilmore 1922 Type species A. sanjuanensis Diagnosis: as for type species Alamosaurus sanjuanensis Gilmore 1922 Holotype: USNM 10846 (nearly complete left scapula- coracoid, Gilmore, 1922; Figure 1A). Paratype: USNM 10847 (nearly complete right ischi- um, Gilmore, 1922; Figure 1B). Provisional referred specimens: NMMNH P-22544 (proximal and distal right tibia), 25072 (nearly com- plete left scapula), 25077 (fragmentary right femur) (Mateer, 1976), NMMNH 25077 (fragmentary right femur), 27291 (anterior caudal, NMMNH 28741 (par- tial mid caudal), 29031 (partial caudal), 29722 (partial mid caudal), NMMNH 29723 (partial anterior caudal), NMMNH 29724 (seven tooth fragments), NMMNH 29725 (tooth fragment), NMMNH 29726 (four tooth fragments), NMMNH 29727 (tooth fragment), NMMNH 29728 (tooth fragment), NMMNH 49967 (nearly complete right pes), (Lucas and Sullivan, 2000; D’Jasinski et al., 2011); PMU 24305 (fragmentary right ilium), PMU 24893 (last sacral and first caudal); SMP VP 1097 (partial tooth), SMP VP 1136 (posterior left ilium), SMP VP 1138 (partial left femur), SMP VP 1139 214 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 (partial right? pubis), SMP VP 1271 (tooth root), SMP VP 1494 (2 partial left caudals), SMP VP 1539 (partial scapula), SMP VP 1541 (skull fragments), SMP VP 1581 (distal caudal), SMP VP 1625 (distal left femur), SMP VP 1626 (nearly complete right fibula), SMP VP 1641 (fragmentary dorsal), SMP VP 1715 (coracoid and pos- sible associated fragments), SMP VP 1718 (fragmentary femur), SMP VP 1850 (posterior cervical, Figure 1C), SMP VP 1864 (partial dorsal), SMP VP 1866 (partial dorsal), SMP VP 1876 (nearly complete dorsal), SMP VP 2043 (proximal tibia), SMP VP 2065 (partial tibia), 2097 (partial left ischium, right femur), SMP VP 2104 (partial anterior caudal, fragments, Figure 1D), SMP VP 2175 (distal right radius), SMP VP 2230 (nearly com- plete caudal), SMP VP 2232 (partial ribs), SMP VP 2233 (anterior right ilium), SMP VP 2507 (tooth), SMP VP 2696 (chevron, fragments), SMP VP 3323 (partial left pubis) (Lucas and Sullivan, 2000; Fowler and Sullivan, 2011; Jasinski et al. 2011); SSM 5428; USNM 15658 (dis- tal caudal, Kues et al., 1980), TKM 007 (nearly complete right humerus), 009 (femur), (Lozinsky et al., 1984). Location, horizon, age: northwestern New Mexico, low- er (Naashoibito Member) Ojo Alamo Formation; south- ern New Mexico? lower Hall Lake? early Maastrichtian. Diagnosis: acromion process not strongly prominent (1), posterior profile of scapula gently sinuous so a tri- angular glenoid process is absent (2), paired tubercles dorsal to glenoid very subtle and closely spaced (3), flare of postero-dorsal edge of blade subtle (4), later- al process of ischium long? (5), postero-lateral profile of ischium has a strong concave subcircular arc? (6). Utetitan n.g Etymology: after the original Ute peoples of central Utah upon whose ancient lands the fossil was found. Type species U. zellaguymondeweyae n.s. Diagnosis: as for type species. Utetitan zellaguymondeweyae n.s. Etymology: after the author’s maternal grandmother Zella Guymon Dewey (1901–2002), born and raised in Huntington, Utah, 30 km northeast of the USNM 15560 quarry, who called the Wasatch Plateau “the hills of home.” Shortly after the specimen was excavated her family moved from Salt Lake Valley to the wartime Washington, D.C. suburbs a few kilometers from the fos- sil’s new Smithsonian location where I visited with her on occasion. She is buried in Arlington National Ceme- tery with her husband William E. Dewey (1899–1962). Gilmore, 1922 Alamosaurus sanjuanensis Holotype: USNM 15560 (partial dorsal ribs, caudals 1-30, 25 chevrons, both sternals, right scapula cora- coid, humerus, radius, ulna, metacarpus, both ischia, osteoderm, Gilmore, 1946). Referred specimens: BIBE 45958 (partial left scapula (Tykoski and Fiorillo, 2017); very probably BYU 9087 (proximal left humerus and femur, Curtice, 2016), BYU 11392 (anterior caudal), BYU 11393 (anterior caudal), (D’Emic et al., 2011). Location, horizon, age: central Utah, lower North Horn Formation; West Texas, upper Javelina? and lower Black Peaks Formations; late Maastrichtian. Diagnosis: acromion process strongly prominent (1), posterior profile of scapula strongly sinuous so a prominent triangular glenoid process is present (2), paired tubercles dorsal to glenoid fairly prominent and widely separated (3), flare of postero-dorsal edge of blade more prominent (4), lateral process of ischi- um modest in size (5), postero-lateral profile of is- chium moderately concave arced (6), femur robust? Middle/late Maastrichtian Utetitan or titanosaurid incertae sedis: AMNH 21531 (right femur, Wick and Lehman, 2014); BIBE 45854 (cervicals 6-14; Tykos- ki and Fiorillo 2017), TMM 40597-5 (pubis), TMM 40699 (mid caudal), 41060 (right ilium), TMM 41061 (right rib), TMM 41063 (left pubis), TMM 41396-1, TMM 41398-1-2 (cervical centrum, proximal left hu- merus), TMM 41450-1 and TMM 41450-2 (1st caudal, right ulna), TMM 41541 (partial posterior cervical, right ischium), TMM 41541-1 (anterior dorsal ribs, 1st caudal), TMM 42495-7 (pubis), TMM 43090-2 (par- tial femur), TMM 43598-1-6 (partial cervical, nearly complete anterior dorsal, partial dorsal centrum, partial anterior caudals, partial left femur, right metatarsal II), TMM 43599-1-3 (partial rib, anterior caudal), TMM 215 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 43600-2 (right humerus), TMM 43621-1 (7-8 partial cervicals, 6-7 partial dorsals, partial sacral and sacral rib, left coracoid, left humerus, left ulna, left ilium, left fibula, distal right tibia), TMM 45601-1 (right fibula), TMM 45602-1 (right tibia), TMM 45854-1-8 (rib ma- terial), TMM 45855-1-4 (left dorsal rib/s, proximal left scapula, proximal right metacarpal II?), TMM 45856-1 (left ulna), TMM 45857-1 (partial left humerus), TMM 45859-1 (right femur), TMM 45861-1 (nearly complete left fibula), TMM 45862—1 (partial left ischium), TMM 45863-1 (right humerus), TMM 45864-1 (partial mid caudal), TMM 45865-1 (distal caudal), TMM 45888- 1 (osteoderm, Fronimos, 2021), TMM 45890-1 and TMM 45890-2 (nearly complete right tibia, nearly com- plete right ungual), TMM 45891-1-17 (5 complete and partial dorsals, sacral fragments, partial anterior cau- dal, transverse process, left and right nearly complete humeri, nearly complete left ilium, fragment of right ilium, nearly complete left pubis, nearly complete left ischium,), TMM 45864-1 (anterior mid-caudal), TMM 46052-1 (nearly complete left femur) (Lawson, 1972; Woodward and Lehman, 2009; Fronimos and Lehman, 2014; Wick and Lehman, 2014); TTU 542 (nearly com- plete right femur, Wick and Lehman, 2014), TTU 546 (proximal caudal, Lehman and Coulson, 2002), UTEP P-25 (nearly complete left femur, Wick and Lehman, 2014). Late Campanian and/or early Maastrichtian Ala- mosaurus or titanosaurid incertae sedis: uncatalogued Chihuahua fossils (right tibia, minority of skeleton, Ri- vera-Sylva, 2006; Rivera-Sylva and Carpenter, 2014). SKELETAL RESTORATIONS AND BODY MASS The large (scaled to USNM 15560, close to USNM 10846, USNM 10847, and TMM 41541) and juvenile (scaled to TMM 43621-1) skeletals (Figure 5) are broad- ly similar to those produced by Lehman and Coulson (2002), with an improvement that the mature version is able to incorporate the adult cervicals subsequently described by Tykoski and Fiorillo (2017), and a juve- nile partial scapula from the same reference. Reliably estimating the relative size of the neck in adults is cur- rently not possible, the large cervical series not being accompanied by more posterior postcranial elements (Tykoski and Fiorillo, 2017) necessary for cross scaling. The length of the cervical series in the large skeletal is scaled to the same ratio relative to the dorso-sacrals of the juvenile, and that results in a large mature neck. But the quite large BIBE 45854 cervicals may favor the neck being a fifth or more longer relative to the body, or they may have been attached to a larger bodied individual. The volume greater than mass results for juvenile TMM 43621-1 is approximately 1.3 mt, the somewhat larger BIBE 45958 is approximately 3 mt. The similar sized TMM 46052-1 (femur 1510 mm), USNM 10487, and USNM 15560 are approximately 16 to 17 mt. The larger USNM 10846 is in the range of 20 mt, and still heftier UTEP P-25 (femur 1675 mm) and TMM 41541 (femur 1730 mm) is 22 to 24 mt. The TMM 41541 mass value is about three quarters the estimate for the specimen based on bone strength factors (Lehman and Coulson, 2002), which is in fair agreement; although long bone circum- ferences have a much greater mass estimate error range than do scientifically proportioned volumetric models (Paul, 1997, 2019; Larramendi, 2016; Brassey, 2017; Larramendi et al., 2021). A middle cervical, anterior caudal, and femur from the Ojo Alamo Formation have been offered as evidence of much larger putative Ala- mosaurus individuals (Fowler and Sullivan, 2011). The incomplete nature of all these bones prevents rigorous quantitative comparisons. The cervical is quite large, but the true dimensions of the very fragmentary verte- bra (Figure 1D) are correspondingly very unclear—the short distance from the posterior rim of the centrum to the postero-lateral end of the wing of the parapoph- ysis suggests the total length was much less than that of the mid cervicals of Puertasaurus (Figure 1A2 ver- sus 1B2 in Fowler and Sullivan [2011] sans speculative outlines), which reached 60 to 70 mt as do some other South American titanosaurids (Paul, 2019; Larramendi et al., 2021; Paul and Larramendi, 2023). The cervical appears to be similar in size to the more ordinary sized Black Peaks Formation cervical series (Figure 1D) in which that big individual may have been in the range of 30 or somewhat more tonnes. The caudal centrum is markedly larger than those of USNM 15560 (Figure 1E) 216 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 and is similar in size to that of Futalognkosaurus (Fig- ures 2B and 2C in Fowler and Sullivan, 2011), which was approximately 30 mt (Paul 2019; Larramendi et al., 2021). The large femur also suggests a 30-mt indi- vidual, as does the large pes described by D’Emic et al. (2011), and the tibia from Chihuahua (Rivera-Sylva et al., 2006). The largest known Ojo Alamo titanosaurids were 30 mt or little more, which is in general agreement with Paul (2019) and Larramendi et al. (2021), although higher estimates cannot be ruled out on the limited data on hand. At this time, it appears that neither Alamosau- rus nor Utetitan matched the enormous bulk of South American or Indian super titanosaurids and other sau- ropods of 50 to possibly well over 100 mt (Paul, 2019; Larramendi et al., 2021; Paul and Larramendi, 2023). Nor can these end Mesozoic sauropods said to be the largest known North American sauropods, those be- ing nontitanosaurids known from the Late Jurassic and Early Cretaceous (Paul, 2019, 2024, in press). But the small North American fossil titanosaurid sample allows for much larger examples having existed. With all the elements used to produce the PMNS composite display mount originating from the late Maastrichtian the fossil is not Alamosaurus. Because part of fossil is modeled after USNM 15560 it can be tagged Utetitan, although there is a possibility the fossil is at least a species and perhaps generic chimera. It is possible that Utetitan did not have as wide a hindlimb gauge as most titanosau- rids (Wick and Lehman, 2014). CONCLUSION The possibility that all the titanosaurids of the late Campanian and Maastrichtian stages of the North American southwest over the 5 or more million years of the return of sauropods to the continent were just one taxon, especially species—the universal Alamosau- rus sanjuanensis thesis—is not a systematically viable hypothesis. A closer look at the specimens from the region shows sufficient anatomical diversity to demon- strate that more than one taxon, probably including two known genera, were present over the stratigraphic stage as is to be expected among dinosaurs that are prone to exhibit rapid evolution and diversification starting in the Campanian. Sauropods in the latter stage were probably one or more distinct indeterminate taxa whose detailed char- acteristics are not yet known. Alamosaurus sanjuanensis was very probably restricted to the early Maastrichtian lower Ojo Alamo Formation of New Mexico and any lateral regional formations. Even then, multiple taxa may have been extant at that time and preserved among the known fossils. Additional fossil remains discovered over time may resolve this question. Later stage sauro- pods probably went extinct later in the Maastrichtian, possibly after having spawned related taxa via cladogen- esis and/or anagenesis. The late Maastrichtian lower North Horn Forma- tion in Utah was home to the distinct Utetitan zellaguy- mondeweyae that probably went extinct at the end of the Mesozoic. Titanosaurids from contemporary lateral formations such as the Black Peaks Formation in Texas may be the same species, but more than one titanosau- rid taxon was present in the North Horn and/or Black Peaks habitats at that time is at least plausible, if not probable. Again, there is a need for additional fossil re- mains. I suggest that incomplete North American tita- nosaurid elements not be assigned to a specific taxon unless there are strong anatomical and stratigraphic reasons to do so, if not they should be tagged as enig- matic taxa. With the titanosaurid fossils in Alamosaurus spanning millions of years, their biostratigraphic utili- ty is limited to showing that the sediments containing them were latter Campanian and Maastrichtian. Under the new scheme fossil remains that can be assigned to A. sanjuanensis are indicative of early Maastrichtian age, those to U. zellaguyondemeya to late in the stage. Neither Alamosaurus nor Utetitan or any close relatives appear to have been much over 20 to 30 mt, which is far short of observed titanosaurid maximums, although higher masses cannot be eliminated. Being classic titanosaurids, Alamosaurus and Uteti- tan could not have directly descended from much ear- lier North American sauropods (D’Emic et al., 2010). This analysis does not provide any compelling evidence regarding the Asian versus South American origin of North American Maastrichtian titanosaurids; howev- 217 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 er, Lucas and Sullivan (2000) and Tykoski and Fiorillo (2017) suggest that an improved understanding of the low level taxonomy of the beasts may eventually be pro- ductive. The absence of fossil sauropod fossils in Alas- kan dinosaur-bearing beds (Chiarenza et al. 2022)—al- though there are substantial stratigraphic gaps in the Bering Land Bridge dinosaur fossil record—may be due to the severe climate restricting the food base for the land giants (Paul, 2024). Additionally, the very large, attenuated necks and tails of sauropods could have pre- cluded the heat retention needed in polar winters. If so, then a South American origin is favored. That once back in North America titanosaurids were just one static tax- on for millions of years is not evolutionarily logical. The combination of shared vertebral and pectoral characters of A. sanjuanensis, U. zellaguymondeweyae, and other Maastrichtian North American titanosaurids on one hand, along with significant differences between them on the other, indicate that utetitans formed their own small local clade, presumably because of their re- stricted geographic isolation and experienced Darwin- ian selection that caused them to become anatomically distinct from their foreign ancestors. Individuals inves- tigating titanosaurid phylogeny should use the North American fossils with caution, the collective partial fossils represent multiple genera and species that risk complicating cladistic results if they are assumed to represent just one taxon. Alamosaurus sanjuanensis was not the last known North American titanosaurid, that was Utetitan zellaguymondeweyae, and possibly another taxon or more. ACKNOWLEDGMENTS Thanks go to Richard Gilreath and Matthew Carra- no (Smithsonian Institute), and Thomas Lehman (Texas Tech University) for providing information on images and specimens. I also thank the anonymous reviewers for their comments and edits to the manuscript. REFERENCES Amato, J.M., Mack, G.H., Jonell, T.N., Seager, W.R., and Up- church, G.R., 2017, Onset of the Laramide orogeny and associated magmatism in southern New Mexico based on U-Pb geochronology: Geological Society of America Bulletin, v. 129, B31629.1. Brassey, C.A., 2017, Body-mass estimation in paleontolo- gy—a review of volumetric techniques: The Paleonto- logical Society Papers, v. 22, 133–156. Carrano, M.T., and D’Emic, M.D., 2015, Osteoderms of the titanosaur sauropod dinosaur Alamosaurus sanjuanensis Gilmore, 1922: Journal of Vertebrate Paleontology, v. 35, e901334. Caitlin E.L., Peppe D.J., Williamson, T.E., Heizler, M., Jack- son, M., Atchley, S.C., Nordt, L., and Standhardt, B., 2018, Revised age constraints for Late Cretaceous to ear- ly Paleocene terrestrial strata from the Dawson Creek section, Big Bend National Park, West Texas: Geological Society of America Bulletin, v. 130, p. 1143–1163. Chiarenza, A.A., Mannion, P.D., Farnsworth, A., Carrano, M.T., and Varela, S., 2020, Climatic constraints on the biogeographic history of Mesozoic dinosaurs: Current Biology, v. 32, p. 570–585. Chure, D.J., and Loewen, M.A., 2020, Cranial anatomy of Al- losaurus jimmadseni, a new species from the lower Mor- rison Formation of Western North America: PeerJ, v. 8, article e7803. Cifelli, R.L., Nydam, R.L., Eaton, J.G., Gardner, J.D., and Kirk- land, J.I., 1999, Vertebrate faunas of the North Horn For- mation, Emery and Sanpete Counties, Utah, in Gillette, D.D., editor, Vertebrate paleontology in Utah: Utah Geo- logical Survey Miscellaneous Publication 99-1, p. 377–388. Curtice, B., 2016, Remembering the Alamosaurus—Jensen relocates Gilmore’s Alamosaurus quarry, USNM 15560, North Horn Formation, Emery County, Utah, and dis- covers a second individual Alamosaurus: Journal of the Arizona-Nevada Academy of Science, v. 47, p. 1–5. Dalman, S.G., Lucas, S.G., Jasinski, S.E., and Longrich, N.R., 2022, Sierraceratops turneri, a new chasmosaurine cer- atopsid from the Hall Lake Formation of south-central New Mexico: Cretaceous Research, v. 130, article 105034. Dalman, S.G., Loewen, M.A., Pyron, R.A., Jasinski, S.E., Malinzak, D.E., Lucas, S.G., Fiorillo, A.R., Currie, P.J., and Longrich N.R., 2024, A giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the evolution of tyrannosaurid gigantism: Scientific Reports, v. 13, article 22124. Danison, A., Wedel, M., Barta, D., Woodward, H., Holley, 218 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 E., Lee, A., and Snively, E., 2024, Chimerism of speci- mens referred to Saurophaganax maximus reveals a new species of Allosaurus: Vertebrate Anatomy Morphology Palaeontology, v. 12, p. 81–114. D’Emic, M.D., Wilson, J.A., and Thompson, R., 2010, The end of the sauropod hiatus in North America: Palaeogeogra- phy, Palaeoclimatology, Palaeoecology, v. 297, p. 486–490. D’Emic, M.D., Wilson, J.A., and Williamson, T.E., 2011, A sau- ropod dinosaur pes from the latest Cretaceous of North America and the validity of Alamosaurus sanjuanensis: Journal of Vertebrate Paleontology, v. 31, p. 1072–1079. Difley, R., 2007, Biostratigraphy of the North Horn Forma- tion at North Horn Mountain, Emery County, Utah, in Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., Central Utah—diverse geology of a dynamic landscape: Utah Geological Association Publication 36, p. 439–454. Difley, R., and Ekdale, A.A., 1999, Stratigraphic aspects of the Cretaceous-Tertiary (K-T) boundary interval at North Horn Mountain, Emery County, Utah, in Gillette D.D., editor, Vertebrate paleontology in Utah: Utah Geological Survey Miscellaneous Publication 99-1, p. 389–398. Fowler, D.W., 2017, Revised geochronology, correlation, and dinosaur stratigraphic ranges of the Santonian-Maas- trichtian (Late Cretaceous) formations of the Western Interior of North America: PLoS ONE, v. 12, article e0188426. Fowler, D.W., and Sullivan, R.M., 2011, The first giant titano- saurian sauropod from the Upper Cretaceous of North America: Acta Palaeontologica Polonica, v. 56, p. 685–690. Fronimos, J.A., 2021, Morphology and neurovascular anat- omy of a titanosaur osteoderm from the Upper Creta- ceous of Big Bend National Park, Texas: Cretaceous Re- search, v. 120, article 104670. Fronimos, J.A., and Lehman, T.M., 2014, New specimens of a titanosaur sauropod from the Maastrichtian of Big Bend National Park: Journal of Vertebrate Paleontology, v. 34, p. 883–899. Gates, T.A., Prieto-Márquez, A., and Zanno, L.E., 2012, Mountain building triggered Late Cretaceous North American megaherbivore dinosaur radiation: PLoS ONE, v. 7, article e42135. Gilmore, C.W., 1922, A new sauropod dinosaur from the Ojo Alamo Formation of New Mexico: Smithsonian Miscel- laneous Collections, v. 72, p. 1–9. Gilmore, C.W., 1946, Reptilian fauna from the North Horn Formation of central Utah: U.S. Geological Survey Pro- fessional Paper 210, p. 29–52. Jasinski, S.E., Sullivan, R.M., and Lucas, S.G., 2011, Taxonom- ic composition of the Alamo Wash local fauna from the Upper Cretaceous Ojo Alamo Formation, San Juan Ba- sin, New Mexico, in Sullivan, R.M., Lucas, S.G., and Spiel- mann, editors, Fossil record 3: New Mexico Museum of Natural History and Science Bulletin 53, p. 216–270. Larramendi, A., 2016, Shoulder height, body mass, and shape of proboscideans: Acta Palaeontologica Polonica, v. 61, p. 537–574. Larramendi, A., Paul, G.S., and Hsu, S., 2021, Review and reappraisal of the specific gravities of present and past multicellular organisms, with an emphasis on verte- brates, particularly pterosaurs and dinosaurs: Anatom- ical Record, v. 304, p. 1833–1888. Lawson, D.A., 1972, Paleoecology of the Tornillo Formation, Big Bend National Park, Brewster County, Texas: Austin, University of Texas, M.A. thesis, 182 p. Lawton, T.F., Talling, T.F., Hobbs, R.S., Trexler, J.H., Jr., Weiss, M.P., and Burbank, D.W., 1993, Structure and stratigraphy of Upper Cretaceous and Paleogene strata (North Horn Formation), eastern San Pitch Mountains, Utah—sedimentation at the front of the Sevier orogenic belt: U.S. Geological Survey Bulletin 1787-II, p. 1–33, 2 plates, scale 1:12,000. Lehman, T.M., and Coulson, A.B., 2002, A juvenile specimen of the sauropod Alamosaurus sanjuanensis from the Up- per Cretaceous of Big Bend National Park, Texas: Journal of Paleontology, v. 76, p. 56–72. Lehman, T.M., McDowell, F.W., and Connelly, J.N., 2006, First isotope (U-PB) age for the late Cretaceous Alam- osaurus vertebrate fauna of West Texas, and its signifi- cance as a link between two faunal provinces: Journal of Vertebrate Paleontology, v. 26, p. 922–928. Lehman, T.M., Wick, S.L., Beatty, H., Straight, W.H., and Wagner, J., 2018, Stratigraphy and depositional history of the Tornillo Group of West Texas: Geosphere, v. 14, p. 2206–2244. Lehman, T.M., Cobb, J., Sylvestor, P., and Souders, A.K., 2022, The Cretaceous-Paleogene contact in the Tornil- lo Group of Big Bend National Park, West Texas, USA: 219 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 Geosphere, v. 18, p. 1851–1884. Lozinsky, R.P., Hunt, A.P., Wolberg, D.L., and Lucas, S.G., 1984, Late Cretaceous dinosaurs from the McRae For- mation, Sierra County, New Mexico: New Mexico Geol- ogy, v. 6, p. 72–77. Lucas, S.G., and Hunt, A.P., 1989, Alamosaurus and the sau- ropod hiatus in the Cretaceous of the North American Western Interior: Geology Society of America Special Paper 238, p. 75–85. Lucas, S.G., and Sullivan, R.M., 2000, The sauropod dinosaur Alamosaurus from the Upper Cretaceous of the San Juan Basin, New Mexico, in Lucas, S.G., and Heckert, A.B., editors, Dinosaurs of New Mexico: New Mexico Muse- um of Natural History Bulletin 17, p. 147–156. Mallon, J.C., 2017, Recognizing sexual dimorphism in the fossil record—lessons from nonavian dinosaurs: Paleo- biology, v. 43, p. 495–507. Mallon, J.C., 2019, Competition stuctured a Late Cretaceous megaherbivorous dinosaur Assemblage: Science Re- ports, v. 9, article 15447. Mateer, N., 1976, New topotypes of Alamosaurus sanjuanen- sis Gilmore: Bulletin of the Geological Institutions of the University of Uppsala, New Series, v. 6, p. 93–95. O’Connor, R.E., Romanov, M.N., Kiazim, L.G., Barrett, P.M., Farré, M., Damas, J., Ferguson-Smith, M., Valenzuela, N., Larkin, D.M., and Griffin, D.K., 2018, Reconstruction of the diapsid ancestral genome permits chromosome evo- lution tracing in avian and non-avian dinosaurs: Nature Communications, v. 9, p. article 1883. Osborn, H.F., and Mook, C.C., 1921. Camarasaurus, Am- phicoelias, and other sauropods of Cope: Memoirs of the American Museum of Natural History, New Series, v. 3 part III, 247–387. Paul, G.S., 1997, Dinosaur models—the good, the bad, and using them to estimate the mass of dinosaurs, in Wol- berg, D.L., Stump, E., and Rosenberg, G.D., editors, Di- nofest International Symposium Proceedings, Academy of Natural Sciences, Philadelphia: Philadelphia, Acade- my Sciences of Natural Sciences, p. 129–154. Paul, G.S., 2006, A revised taxonomy of the iguanodont dinosaur genera and species: Cretaceous Research, v. 29, p. 92–216. Paul, G.S., 2016, The Princeton field guide to dinosaurs (2nd edition): New Jersey, Princeton Univeristy Press, 360 p. Paul, G.S., 2019, Determining the largest known land ani- mal—a critical comparison of differing methods for re- storing the volume and mass of extince animals: Annals of the Carnegie Museum, v. 85, p. 335–358. Paul, G.S., 2024, The Princeton field guide to dinosaurs (3rd edition): New Jersey, Princeton Univeristy Press, 384 p. Paul, G.S., 2025, A presentation of the current data on the exceptionally diverse nontyrannosaurid eutyrannosaur and tyrannosaurini genera and species of Western North America during the End Cretaceous North American Interchange: Mesozoic, v. 2, p. 85–138. Paul, G.S., in press, The Princeton field guide to sauropods and prosauropods: New Jersey, Princeton University Press. Paul, G.S., and Larramendi, A., 2023, Body mass estimate of Bruhathkayosaurus and other fragmentary sauropod re- mains suggest that the largest land animals were about as big as the greatest whales: Lethaia, v. 56, p. 1–11. Rivera-Sylva, H.E., and Carpenter, K., 2014, Mexican sau- rischian dinosaurs, in Rivera-Sylva, H.E., Carpenter K., and Frey, E., editors, Dinosaurs and other reptiles from the Mesozoic of Mexico: Bloomington, Indiana Univer- sity Press, p. 143–155. Rivera-Sylva, H.E., Guzman-Gutierrez, J.R., and Palomi- no-Sanchez, F.P., 2006, Preliminary report on a verte- brate fossil assemblage form the Late Cretaceous of Chi- huahua, Mexico: Hantkeniana, v. 5, p. 66–68. Ryan, M.J., and Evans, D.C., 2005, Ornithischian Dinosaurs, in Currie P.J., and Koppelhus, E.B, editors, Dinosaur Pro- vincial Park—a spectacular ancient ecosystem revealed: Bloomington, Indiana University Press, p. 312–348 Sampson, S.D., and Loewen, M.A., 2005, Tyrannosaurus rex from the Upper Cretaceous of North Horn Formation of Utah; biogeogrpahic and paleoecological implications: Journal of Vertebrate Paleontology, v. 25, p. 469–472. Scannella, J.B., Fowler, D.W., Goodwin, M.B., and Horn- er, J.R., 2014, Evolutionary trends in Triceratops from the Hell Creek Formation, Montana: PNAS, v. 111, p. 10245–10250. Stein, W.W., and Triebold, M., 2013, Preliminary analysis of a sub-adult tyrannosaurid skeleton from the Judith Riv- er Formation of Petroleum County, Montana, in Parrish, J.M., Molnar, R.E., Currie, P. J., and Koppelhus, E.B., ed- itors, Tyrannosaurid paleobiology: Bloomington, Uni- 220 Stratigraphic and Anatomical Evidence for Multiple Titanosaurid Dinosaur Taxa in the Late Cretaceous (Campanian-Maastrichtian) of Southwestern North America Paul, G.S. Geology of the Intermountain West 2025 Volume 12 versity of Indiana Press, p. 55–77. Tschopp, E., Mateus, O.V., and Benson, R.B., 2015, A speci- men-level phylogenetic analysis and taxonomic revision of Diplodocidae: PeerJ, v. 3, article e857. Tykoski, R.S., and Fiorillo, A.R., 2017, An articulated cer- vical series of Alamosaurus sanjuanensis Gilmore, 1922 from Texas—new perspective on the relationships of North America’s last giant sauropod: Journal of System- atic Paleontology, v. 15, p. 339–364. Weishampel, D.B., Dodson, P., and Osmolska, H., 2004, Di- nosaur distribution (Late Cretaceous, South America), in Weishampel, D.B., Dodson, P., and Osmólska, H., ed- itors, The Dinosauria (2nd edition): Berkeley, University of California Press, p. 600–604. Wick, S.L., and Lehman, T.M., 2014, A complete titanosaur femur from West Texas with comments regarding hind- limb posture: Cretaceous Research v. 49, p. 39–44. Williamson, T.E., and Wiel, A., 2008, Stratigraphic distri- bution of sauropods in the Upper Cretaceous of the San Juan Basin, New Mexico, with comments on North America’s Cretaceous ‘sauropod hiatus’: Journal of Ver- tebrate Paleontology, v. 28, p. 1218–1223. Wolberg, D.L., Lozinsky, R.P., and Hunt, A.P., 1986, Late Cretaceous vertebrate paleontology of the McRae For- mation, Elephant Butte area, Sierra County, New Mexi- co, in Clemons, R.E., King, W.E., Mack, G.H., and Zidek, J., editors, Truth or Consequences region: New Mexico Geological Society 37th Field Conference Guidebook, p. 227–234. Woodward, H.N., and Lehman, T.M., 2005, Bone histolo- gy of the sauropod dinosaur Alamosaurus sanjuanensis from the Javelina Formation, Big Bend National Park, Texas: Microscopy and Microanalysis, v. 13, p. 508–509.