Your Friends The Titanosaurs: Utetitan zellaguymondeweyae

Back almost four years ago when we first waded into Alamosaurus, I noted that "I was waiting on it because I was concerned that it might suffer a taxonomic detonation at any time, so I thought I'd hold off as long as possible." Titanosaurs work on their own schedule. Although people have danced around the issue for decades by suggesting that A. sanjuanensis is dubious or refraining from including some specimens in the species, nobody had taken the step of formally proposing another name for any of the material until Gregory S. Paul in October 2025 with Utetitan zellaguymondeweyae. This genus and species are based on the North Horn specimen that has long been the "practical purposes" type* of A. sanjuanensis.

*Seems like there ought to be a name for non-type specimens that everyone uses instead of the type. "Apatotype", for "deceptive type"? Or "pseudotype" for "false type"? Think like the "Mantel-piece" of Iguanodon mantelli. Brachiosaurus had a whole apatotype species until it was split off for Giraffatitan.

Genus and Species: Utetitan zellaguymondeweyae. The genus name honors the Ute people, formerly of the central Utah area where the type specimen was found. The species name honors Zella Guymon Dewey, Paul's maternal grandmother and also at one time a resident of the area that produced U. zellaguymondeweyae (Paul 2025).

Citation: 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: https://doi.org/10.31711/giw.v12.pp201-220.

Geography and Stratigraphy: The lower North Horn Formation on the southwest toe of North Horn Mountain in Manti-La Sal National Forest, Emery County, Utah (Gilmore 1946; Paul 2025).

Holotype: USNM 15560 (National Museum of Natural History), a partial skeleton consisting of 30 articulated caudal vertebrae beginning with the first, three partial ribs, 25 chevrons, both ischia with attached fragments of the other pelvic bones, the left scapula and coracoid, both sternal plates, articulated right arm with metacarpals, and an osteoderm (Gilmore 1946; Carrano and D'Emic 2016; Paul 2025). The radius is now missing per the USNM online database. Badly weathered dorsal vertebrae and the sacrum were left in the field (Gilmore 1946) and later disintegrated (Curtice 2017).

We've already lavished attention on USNM 15560, so there's no need to go through all that again. Paul's taxonomic argument focuses on the biostratigraphy and anatomy of the New Mexican holotype versus USNM 15560. If you've read his field guides, you'll know that he argues for geologically short "lifespans" for dinosaur species, and this comes into play here: he regards the Naashoibito Member of the Ojo Alamo Formation (type A. sanjuanensis) as a few million years older than the lower North Horn Formation (USNM 15560), and therefore it is likely that the Ojo Alamo and North Horn material represent distinct forms. I'm not ready to commit either to firm dates on the two formations involved or to the firm application of the underlying principle of short-lived species (I'm not confident we know enough about the absolute dating of dinosaur species ranges to make rules about it). [Update, 2025/10/24: Hardly any sooner do I write this than Flynn et al. (2025) put out a paper dating the Naashoibito Member to approximately 66.4–66.0 million years ago, in other words the very end of the Cretaceous, which doesn't help the stratigraphic argument.]  I'd much rather rely on anatomical differences. Do those pan out?

I'm a stickler type, so I'm going to look at only the scapulae, since that's the only bone in the A. sanjuanensis holotype. (Sorry, paratype ischium.) There indeed are a variety of differences in the profiles of the A. sanjuanensis type and the USNM 15560 scapula. Paul's interpretation is that the differences are significant enough for two (related) genera. I could be persuaded to go for two species of one genus instead. The catch there, of course, is the inconvenient fact of A. sanjuanensis being based on a specimen of one (1) bone, so maybe it's best to cut loose from the anchor entirely, at least until such time as a similar scapula to A. sanjuanensis turns up with a more complete skeleton, preferably from the Naashoibito Member in New Mexico.

A crop from Figure 1 of Paul (2025), showing three scapulae drawn to the same scale (left: Alamosaurus sanjuanensis holotype USNM 10846; above center: BIBE 45958; right USNM 15560), with landmarks highlighted by arrows. CC-BY-3.0.

This also leaves the question of what to do with the rest of the stuff. If there's one thing that's numbingly apparent after considering something like 150 titanosaurs and titanosaur-adjacent species, it is that one should never assume the presence of just one titanosaur species in a given formation. Strictly speaking, again we're best off limiting comparisons to specimens with scapulae. Paul identified one immature specimen from the Black Peaks Formation of Texas (BIBE 45958 [Big Bend National Park]) as having several of the features of USNM 15560 and assigned it to U. zellaguymondeweyae. That's one down, a bunch to go. There is a minor challenge at work here: North American titanosaurs have been resolutely against providing paleontologists with specimens of more completeness than a single bone. It's almost comical; out of all the titanosaur specimens, the only examples approaching a skeleton are USNM 15560 and three or so from Texas (TMM 41541-1 [Texas Memorial Museum], TMM 43621-1, and TMM 45891-##), and none of them are exactly the Camarasaurus lentus from Dinosaur National Monument. Conspicuously, none of these are from New Mexico. Furthermore, none of the three Texas specimens happen to have come down to us from the mists of time with a scapula. There are, however, some overlapping bones with USNM 15560. The ischia of two of the partial skeletons from Texas, TMM 41541-1 and 43621-1, are more similar to each other and to the A. sanjuanensis paratype ischium than they are to that of USNM 15560 (Paul 2025). With TMM 43621-1 being from the Black Peaks Formation, this would put two titanosaur taxa in this unit, which, as noted, would be typical for titanosaur diversity. This would not be the first time somebody reported differences between TMM 41541-1 and USNM 15560: Fronimos and Lamanna (2018), in a presentation abstract for a Society of Vertebrate Paleontology meeting, recognized differences in the caudals that they suggested meant TMM 41541-1 wasn't A. sanjuanensis on the assumption that USNM 15560 was.

With Alamosaurus, there are other considerations as well. Generations of paleontologists and dinosaur fans have grown up with Alamosaurus, the one and only North American titanosaur, so whatever deficiencies it has in its holotype, it has more than made up for them in the court of public opinion. It has managed to trundle along for decades under the open secret that it is based on very limited material, absorbing specimens from across southwestern North America via geographic/stratigraphic arguments and a short chain of more complete specimens. (Reminds me of Euskelosaurus, except Euskelosaurus was more convenient than popular.) Remove one key specimen like USNM 15560 from the chain and the whole thing collapses. That's really for the best: assigning specimens to names should be based on anatomy, not geography or stratigraphy. We can surely get by with "undetermined Titanosauria" and "indeterminate Titanosauria" until more complete material comes along, rather than constructing a sauropod that never was.

References

Carrano, M. T., and M. D. D'Emic. 2015. Osteoderms of the titanosaur sauropod dinosaur Alamosaurus sanjuanensis Gilmore, 1922. Journal of Vertebrate Paleontology 35(1):e901334. doi:10.1080/02724634.2014.901334.

Curtice, B. 2017. Remembering the Alamosaurus: Jensen relocates Gilmore's Alamosaurus quarry, USNM 15560, North Horn Formation, Emery County, Utah, and discovers a second individual Alamosaurus. Journal of the Arizona-Nevada Academy of Science 47(1):1–5. doi:10.2181/036.047.0101. 

[Flynn, A. G., S. L. Brusatte, A. A. Chiarenza, J. García-Girón, A. J. Davis, C. W. Fenley IV, C. E. Leslie, R. Secord, S. Shelley, A. Weil, M. T. Heizler, T. E. Williamson, and D. J. Peppe. 2025. Late-surviving New Mexican dinosaurs illuminate high end-Cretaceous diversity and provinciality. Science 390(6771): 400–404. doi: https://doi.org/10.1126/science.adw3282.]

Fronimos, J. A., and M. C. Lamanna. 2018. An articulated titanosaurian (Dinosauria, Sauropoda) postcranial skeleton from the Late Cretaceous of Texas, with implications for the taxonomic status of Alamosaurus sanjuanensis. Journal of Vertebrate Paleontology, Program and Abstracts, 2018:131. 

Gilmore, C. W. 1946. Reptilian fauna of the North Horn Formation of central Utah. U.S. Geological Survey, Washington, D.C. Professional Paper 210-C. (line illustrations are better in the USGS pdf, but photos are better in this Google Books scan.)

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: https://doi.org/10.31711/giw.v12.pp201-220.