It has been a good month for tyrannosaur research. We have been introduced to the comparatively tiny tyrannosauroid Raptorex, have learned that
"But wait," I hear you say. "I have been on a Walt Disney World ride that has an Alioramus in it. It's not really new." While it is true that the genus Alioramus, which lived about 70 million years ago in what is now Mongolia, was first described in 1976 by Russian paleontologist Sergei Kurzanov, it was only known from an incomplete skull. The new specimen described by Brusatte and colleagues in the journal PNAS is much more complete and represents a new species of the same genus, which they have named Alioramus altai. It provides a much better look at what this relative of larger tyrannosaurs like Tarbosaurus was like.
What is most immediately recognizable about Alioramus altai is its skull. Its close tyrannosaurid relatives all had heavy, deep skulls that allowed them to exert crushing bite forces that could ram their teeth through bone. Alioramus altai, on the other hand, had a shallower skull and long-snouted appearance. I have no doubt it skull could deliver a deadly bite, but not the damage the jaws of something like Tyrannosaurus could inflict.
In fact, this long-snouted appearance is similar to what paleontologists expect juvenile Tyrannosaurus and Tarbosaurus to have looked like. Could Alioramus altai have just been a juvenile Tarbosaurus (another tyrannosaurid from Mongolia that lived about the same time)? The paleontologists looked at the growth pattern in the bones of the dinosaur to find the answer.
While the specimen of Alioramus altai the team described was not yet a full-grown adult, it was distinctly different from some juvenile Tarbosaurus specimens that have been found. In addition to its more slender jaws, it had at least eight small horns covering its face, including a row on the top of its nose and two below the eyes. (The fact that this specimen was a juvenile suggests that adults of this species would have even more impressive knobs and protuberances on the skull.) Other tyrannosaurids have a few similar ornaments on their skulls, but Alioramus altai was far more decorated. It also appears that Alioramus altai would have been somewhat smaller than some of its giant relatives, although an adult specimen will have to be found to determine just how big it got.
Alioramus altai also has some important implications for our understanding of tyrannosaur evolution. It was one of the last tyrannosaurus, living close in time to when Tyrannosaurus lived in prehistoric North America, but it was a very different kind of predator. This means that it was not an evolutionary stage leading up to a dinosaur like Tyrannosaurus but represented a distinct kind of tyrannosaur that probably fed on smaller prey. Its discovery has increased the diversity of the known types of tyrant dinosaurs, and its discovery suggests that other unique tyrannosaurus may yet be found.
For more on this discovery, see a guest post by lead author of the new study, Stephen Brusatte, at the Archosaur Musings blog.