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Tail vertebrae from a previously known Alamosaurus specimen (A), compared with a newly-discovered Alamosaurus tail vertebra (B) and a tail vertebra from the large titanosaur Futalognkosaurus (C). From Fowler and Sullivan, 2011.

Alamosaurus was an unusual sauropod. What makes it so remarkable is not so much its appearance—the dinosaur seems to be a fairly typical member of a group called titanosaurs—but when and where it lived. Even though North America once hosted multiple, coexisting genera of sauropods during the Late Jurassic, that diversity was eventually lost until, about 100 million years ago, there were none left on the continent. By this time the horned dinosaurs and hadrosaurs were the primary herbivores on the landscape. Then, after a 30 million year absence, sauropods returned to what is now the southwestern United States in the form of Alamosaurus. A new study suggests this dinosaur may have been one of the biggest ever.

Among the various dinosaur superlatives, the title of “biggest dinosaur” is always the most hotly contested. It’s also among the most difficult to assign. Despite their size, many of the contenders for biggest dinosaur, whether they’re going for the all-time title or just in a particular slice of prehistory, are known from only partial remains. Some degree of estimation is often required to envision the whole animal. Then there’s the problem of what characteristics make a dinosaur the largest of its kind—is it just length? Or do weight and height also get figured in? Many of the contenders for largest sauropod ever are estimated to have been about 90 to to 110 feet long, and so far, no dinosaurs have exceedingly outstripped that range. (I am not counting the supposed giant Amphicoelias since the original material was lost long ago and no additional remains have been confirmed.) This range may represent some kind of upper limit for sauropod body size due to constraints of structure or biology.

According to the new paper by paleontologists Denver Fowler from the Museum of the Rockies and Robert Sullivan of the State Museum of Pennsylvania, newly-discovered fossils from the latest Cretaceous of New Mexico may indicate that Alamosaurus reached the top tier of titanosaur size. The material in question consists of two partial vertebrae and a partial femur found at different field sites, and the assignment of these bones to Alamosaurus is based upon the fact that this sauropod is the only one presently known from the Late Cretaceous deposits where the remains were found. (Though it should be noted that the fragmentary nature of this specimen and others from around the same time makes comparisons between dinosaurs and estimates of actual diversity difficult.) The fact that the bones were found at different field sites means that the three bones came from different individuals of different sizes and ages, but comparison of these bones with those of other sauropods can provide a rough idea of how large Alamosaurus grew to be.

The two vertebrae described by Fowler and Sullivan appear to indicate that Alamosaurus could reach roughly the same size as the titanosaurs Futalognkosaurus and Puertasaurus. Both of these dinosaurs from South America are estimated to have been within the roughly 90 to 110 foot size range that many other big sauropods fall into, although the fact that paleontologists have not yet found complete skeletons of either dinosaur means we can’t be really sure just how big they got. This makes estimating the actually upper size range of Alamosaurus problematic. The vertebrae Fowler and Sullivan describe are certainly larger than those previously assigned to Alamosaurus, but accurately estimating the size of the dinosaur requires reliance on previous estimates of other partially known specimens. Alamosaurus appears to have been in the upper sauropod size class—and was probably among the biggest ever to have lived in North America—but additional fossil material from this dinosaur and other giants will be needed to figure out exactly how enormous they became.

There was one other wrinkle to this story that caught my attention. A Montana State University press release about the study stated that a tyrannosaur tooth was found near another Alamosaurus vertebrae that was being excavated by the same team. Whether this is an indication of predation or scavenging by the tyrannosaur remains to be seen—teeth are resilient and easy to transport—but the association is a further confirmation that Alamosaurus shared its habitat with Tyrannosaurus rex. The two dinosaurs have been found in the same deposits before, such as Utah’s North Horn Formation, and the occurrence of the two dinosaurs in New Mexico makes me wonder exactly how a large tyrannosaur would go about hunting an enormous sauropod. Clashes of titanic dinosaurs were not restricted to the Late Jurassic of North America or the Cretaceous of South America. At the close of the Cretaceous, prehistoric New Mexico may have been the setting for confrontations between the largest herbivore and carnivore ever to live in North America.

References:

Fowler, D., & Sullivan, R. (2011). The first giant titanosaurian sauropod from the Upper Cretaceous of North America Acta Palaeontologica Polonica DOI: 10.4202/app.2010.0105