Those of you at SVP will have already been aware of this new critter and I get to be smug and say that Steve Brusatte (formerly a guest poster on here with Shaochilong) showed me the photos months ago. However the paper is now out and Steve has been kind enough to write up another post for the Musings on his next groovy Asian theropod. Take it away please:
One of the great perks of studying at the American Museum of Natural History, and being a student of Mark Norell’s, is the material. Fossil material, and lots of it. Every year Mark and Mike Novacek trek to the Gobi Desert of Mongolia and scour the famous Late Cretaceous redbeds bursting with skeletons of Protoceratops, Velociraptor, Tarbosaurus, and many other famous dinosaurs. The AMNH team has been traveling to the Gobi for 20 years now, and the specimens keep on coming. Two decades of hard, back-breaking work in some of the world’s hottest climates have paid off. The Campanian and Maastrichtian dinosaur faunas of the Gobi are now among the best understood in the entire world.
When I began my PhD at the AMNH last fall Mark handed me an amazing specimen. The preservation, and the preparation job, was the best of any dinosaur fossil I had ever seen. Subtle details of the neurovascular foramina and antorbital fenestra and skeletal pneumaticity, and countless other wonderful things that are hardly ever preserved on dinosaur bones, were preserved in immaculate condition. And much of the skeleton was there, too. A nearly complete skull, missing just a few minor bones. A complete neck and large segments of the back, sacrum, and tail. A nearly complete pelvis and most of the hindlimbs. And, perhaps most spectacularly of all, the specimen was a tyrannosaur. Not just any dinosaur, but one of the best preserved fossils of one of the most familiar and celebrated groups of extinct vertebrates. Lucky me!
Over the past year I have meticulously studied this fossil, working with Mark Norell, Thomas Carr (one of the world’s foremost tyrannosaur experts), Gabe Bever (a postdoc at the AMNH), and Greg Erickson (Mr. Dinosaur Histology). Today, our first paper on the fossil, a short description, is published in Proceedings of the National Academy of Sciences. And it turns out that the fossil has a pretty cool story to tell.
The new skeleton is the first good fossil of the long mysterious tyrannosaur Alioramus. This puzzling genus is only known from one other fossil in a museum, the holotype of the species Alioramus remotus. This scrappy set of bones, which include various pieces of the skull and only a few fragments of the postcranium, was described by Kurzanov over 30 years ago. However, this specimen is fragmentary, has only been briefly described and poorly figured, and has largely been inaccessible to visiting researchers. As a result, Alioramus has remained a conundrum. It is clearly very similar to Tyrannosaurus, Tarbosaurus, Albertosaurus, and other tyrannosaurs. However, is it a tyrannosaurid proper (a member of the large-bodied, Late Cretaceous radiation of tyrannosaurs)? Is it a more basal member of the tyrannosauroid stem that survived until the Late Cretaceous? Or, perhaps, is it just a juvenile of Tarbosaurus? For 30 years we’ve known that only new specimens could solve this debate.
Thankfully, that new specimen has indeed arrived. The skeleton we described clearly represents Alioramus, as it shares several unique features with the type of A. remotus. However, we have identified a number of differences between the two specimens, and hypothesize that ours represents a new species, which we call A. altai, after the Altai Mountains of Mongolia. What’s most important, however, is that the new specimen shows that Alioramus is indeed a tyrannosaurid proper, but one with a unique body plan.
Our phylogenetic analysis finds Alioramus as deeply nested within the Tyrannosauridae, and as a member of the tyrannosaurine subclade that includes Tyrannosaurus, Tarbosaurus, and Daspletosaurus. In other words, it is more closely related to these taxa than it is to Albertosaurus and Gorgosaurus. However, among tyrannosaurids, Alioramus is very unusual. It has a very elongate, gracile skull, with numerous horn-like ornaments. Ornamentation and pneumaticity are extreme compared to other tyrannosaurids, but the longirostrine skull is very different from the deep and robust skulls of other tyrannosaurids. The massive, strong skull of Tyrannosaurus and other tyrannosaurids, as well as their peg-like teeth, strongly interlocking lower jaws, thick brows over the eyes, and deep jaw muscle attachment sites, together enable these animals to employ an unusual “puncture-pull” feeding style in which the teeth can literally crunch through bone. Obviously, Alioramus lacks all of these features and must have fed in a different way. And, Alioramus is small: it is only half the size of similarly-aged Tyrannosaurus and Tarbosaurus.
In short, the new fossil is the first evidence that some tyrannosaurids departed from the “classic” tyrannosaurid body plan. Alioramus was a small, gracile, long-snouted carnivore. Why might this be so? Tarbosaurus fossils are found at the same site as the new Alioramus fossils. These two large predators lived alongside each other during the Late Cretaceous in Mongolia. Perhaps the novel body plan of Alioramus allowed it to coexist with its larger, stronger cousin.
Our work on the fossil is just beginning. Stay tuned for two additional papers: a detailed description of the braincase (complete with lots of cool details discerned from CT scanning) and the full-on, gritty monograph of the entire skeleton. Until then, I would like to thank my co-authors, as well as the talented team of AMNH artists who brought the fossil to life: Amy Davidson for the exceptional preparation, Mick Ellison for photography, and Frank Ippolito for the skull and skeletal reconstructions.