Last March a team of paleontologists led by Roger Benson described what appeared to be a partial hip of a tyrannosauroid dinosaur from Australia—the first-ever trace of this group of dinosaurs on the southern continent. Now, in a comment and reply printed in last week's Science, Matthew Herne, Jay Nair and Steven Salisbury argue that the case for a tyrant from down under isn't as strong as Benson proposed.
The argument hinges upon parts of the front, downward-oriented part of the hip known as the pubis. Tyrannosaurs, particularly some of the last ones to have evolved, are well-known for having distinctive, robust pubic bones, and the specimens recovered from the vicinity of Victoria, Australia certainly look like they could belong to a tyrannosaur. Yet, according to Herne and colleagues, the anatomical details initially thought to diagnose the bones as belonging to a tyrannosaur are seen among other theropods, too. Precisely what kind of theropod the bones represent is difficult to determine, but Herne and his co-authors propose that it came from one of the theropod dinosaur varieties already known from Australia (such as other types of coelurosaur and the carcharodontosaurians).
As would be expected, Benson and the other authors of the original paper disagree. In a reply published along with the new commentary, the scientists maintain that a peculiar feature of the hip—known as the pubic tubercle—is most similar to the same feature in tyrannosaurs to the exclusion of similar theropods. Even though this feature is broken, the authors behind the original description argue that the orientation of the missing portion can still be determined, and if it were complete it would show a condition similar to that of the tyrannosauroid dinosaurs. Furthermore, discoveries made during the past several decades have shown that dinosaur diversity cannot be simply divided into northern (Laurasian) and southern (Gondwanan) groups. The closest relative of the Australian theropod Australoventaor, for example, is Fukuiraptor from Japan, indicating that some groups of dinosaurs crossed what were once thought to be geographic barriers.
Two groups of researchers looked at the same fossils and came to very different conclusions. One group interpreted the hip bones as those of the first tyrannosaur known from the southern hemisphere, while the other argues that there is nothing which explicitly identifies it as such. For the moment, though, the identification of the bones could go either way. More fossils will be needed to know for sure, and I anxiously await the announcement of further remains from this contested Australian dinosaur.
For more on this discovery, see this in-depth summary at The Bite Stuff.
Herne, M., Nair, J., & Salisbury, S. (2010). Comment on "A Southern Tyrant Reptile" Science, 329 (5995), 1013-1013 DOI: 10.1126/science.1190100
Benson, R., Barrett, P., Rich, T., Vickers-Rich, P., Pickering, D., & Holland, T. (2010). Response to Comment on "A Southern Tyrant Reptile" Science, 329 (5995), 1013-1013 DOI: 10.1126/science.1190195