The titanosaur Alamosaurus has long been the only titanosaur to be described from the latest Cretaceous of North America, and numerous titanosaur remains from the Maastrichtian of Laramidia have long been referred to the genus, including postcranial skeleton USNM 15560 from the North Horn Formation of Utah and a postcranial remains from Big Bend National Park. However, a handful of sauropod workers have questioned the referral of some of these specimens on grounds of anatomical overlap or a lack of shared diagnostic features, even going as far as to suggest that more than one species is present among referred Alamosaurus specimens. Now, Paul (2025) has come out with a new paper revising the taxonomic attribution of specimens referred to Alamosaurus, erecting Utetitan zellaguymondeweyae for USNM 15560 and restricting Alamosaurus sanjuanensis to remains found in New Mexico.
 |
| Same scale comparison of select postcranial elements of Maastrichtian titanosaurs from North America (after Paul 2025). Arrows point to morphological features listed by Paul as diagnostic for Alamosaurus and Utetitan. |
The morphological criteria for distinguishing Utetitan from Alamosaurus given by Paul (2025) deserve some attention. The diagnoses of these two genera given in the paper are primarily based on scapular and ischial characters, and while D'Emic et al. (2011) diagnose Alamosaurus sanjuanensis on the basis of scapular and caudal characters, ischial features which Paul (2025) cites to distinguish Alamosaurus from Utetitan would suggest that USNM 10487 could be distinct from USNM 15560. However, since USNM 10486 and USNM 10487 were found far apart and belong to distinct individuals, it is unclear if Alamosaurus possessed the ischial characters cited as diagnostic for Utetitan. Paul lists a robust femur as another diagnostic trait for Utetitan, but he does not compare the proportions of the femur belonging to BYU 9087 with those of TKM 009 from the Hall Lake Formation of southern New Mexico. Lastly, Paul's erection of the subfamily Utetitaninae to accommodate Alamosaurus and Utetitan is problematic because most of the diagnostic characters cited by Paul for Utetitaninae are confined to very limited anatomical regions of the skeleton and cladistic analyses (e.g. Navarro et al. 2022) consistently place Alamosaurus as a saltasaurid, either an opisthocoelicaudiine or a saltasaurine.
In his discussion of the stratigraphy of specimens referred to Alamosaurus, Paul (2025) suggests that a 5-6 million year timespan for Maastrichtian titanosaurs from North America is improbable because it is abundant time for taxa to experience substantial evolution, especially at the species level. Although not cited by Paul, an abstract presented at the 2018 SVP conference hints at the specimen TMM 41541-1 being a distinct taxon from USNM 15560. Also, the specimen BIBE 45854 (described by Tykoski and Fiorillo 2017) from the lower part of the Black Peaks Formation in southern Texas not only hails from the late Maastrichtian like Utetitan and juvenile specimen TMM 43621-1 but also has been recovered as a member of Lognkosauria by Navarro et al. (2022) in contrast to Alamosaurus, indicating that the giant lognkosaurians and smaller saltasaurids co-existed in southern Laramidia during the late Maastrichtian. Paul's provisional referral of all titanosaur material from the Ojo Alamo Formation of New Mexico on grounds of biochronology should be taken with a grain of salt because Paul (2025) does not compare the humerus TKM007 and femur TKM009 (both from the Hall Lake Formation) to the Utetitan specimen BYU 9087 or any isolated titanosaur humeri and femora from the Javelina Formation. Given the yet-as-unpublished data indicating the distinctness of TMM 41541-1 from Utetitan and the cladistic results of Navarro et al. (2022) finding Alamosaurus and BIBE 45854 in different positions within Titanosauria, it seems probable that more than two titanosaur taxa existed in New Mexico during the early to middle Maastrichtian.
The paper by Paul (2025) is by no means the last word on the topic of titanosaur diversity in Laramidia during the Maastrichtian, but it helps to highlight the pitfalls of referring all Maastrichtian titanosaur remains to Alamosaurus absent shared diagnostic features or overlap with the A. sanjuanensis holotype specimen.
References:
D’Emic, M.D., Wilson, J.A., and Williamson, T.E., 2011. A sauropod dinosaur pes from the latest Cretaceous of North America and the validity of Alamosaurus sanjuanensis. Journal of Vertebrate Paleontology 31: 1072–1079.
Navarro, B.A., Ghilardi, A.M., Aureliano, T., Díaz, V.D., Bandeira, K.L.N., Cattaruzzi, A.G.S., Iori, F.V., Martine, A.M., Carvalho, A.B., Anelli, L.E., Fernandes, M.A., and Zaher, H., 2022. A new nanoid titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of Brazil Ameghiniana 59(5): 317–354. doi:10.5710/AMGH.25.08.2022.3477
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 12: 201–220. doi:10.31711/giw.v12.pp201-220
Tykoski, R.S., and Fiorillo, A.R., 2017. An articulated cervical series of Alamosaurus sanjuanensis Gilmore, 1922 from Texas—new perspective on the relationships of North America’s last giant sauropod. Journal of Systematic Paleontology 15: 339–364.
