One of the biggest dinosaur stories of 2009 was the discovery of a pint-sized tyrant called Raptorex. Described by a team of paleontologists led by Paul Sereno and dated to about 126 million years ago, the dinosaur showed that many definitive tyrannosaur characteristics—such a puny forearms—evolved when the predators were still small. But a story published in Nature's news section this week highlights some of the uncertainty about the specimen.
Despite becoming something of an instant dinosaurian celebrity, there have been two aspects of Raptorex that have caused paleontologists some degree of unease. The first is that it looks like a juvenile form of later, bigger tyrannosaurs, particularly the 70-million-or-so-year-old Tarbosaurus. In fact, this was how the fossil was unofficially diagnosed when it was purchased—more on that in a moment—although Sereno and co-authors cite the fusion of the sutures on the skull of the animal as an indication that it was a young adult animal. (Comparison with complete, juvenile Tarbosaurus skeletons could also help resolve this issue.) Likewise, it would be expected that juveniles of later tyrannosaurs would be similar in form to earlier species—such as Raptorex—with definitive, advanced tyrannosaurs traits only appearing much later during the growth of later species. If juvenile Tarbosaurus roughly looked like the adult stage of their ancestors, in other words, then it would be easy to confuse the two when viewed outside of their geologic context.
As with the debate over the suggestion that Torosaurus was the adult form of Triceratops, however, not all paleontologists agree that Raptorex is really the juvenile form of another dinosaur. Both cases are part of a larger effort to find out how dinosaurs changed as they grew and what this might mean for the identification of new species. As for Raptorex, though, anatomy alone can't solve the problem, especially since the most important issue yet to be resolved involves the dinosaur's geological age.
Rather than being found an excavated by scientists, the dinosaur is said to have been collected in the vicinity of Liaoning Province, China, by amateurs. After being dug up, it was later sold to a private collector who then contacted Sereno after having other scientists appraise the specimen. Frustratingly, whoever uncovered the fossil did not collect data about the place where the dinosaur was found, and most of what we know about geological context of the dinosaur comes from the rock which still clung to parts of its skeleton.
In addition to the type of rock it was found in, fossil shells and fish bones would appear to place Raptorex at about 126 million years ago in the Yixian Formation. Given that fish bones and shells of the kind found alongside Raptorex are seen in many fossil localities, however, more rigorous geological testing will be needed to determine where it came from and how old it was. Nailing down a date and locality for Raptorex is important. If Raptorex really is 126 million years old, then it could not be a juvenile of a known, giant tyrannosaur such as Tarbosaurus since it would have preceded it by about 50 million years. If Raptorex turns out to be the same geologic age as Tarbosaurus, however, then paleontologists will have to reexamine the skeleton in detail to determine whether it could be a juvenile form of a larger dinosaur.
These problems with Raptorex have been known to paleontologists since the time of its description, but the Nature News story brought it to the forefront. According to the report, Peter Larson and Jørn Hurum will be publishing a critical assessment of Raptorex which will identify the dinosaur as a juvenile Tarbosaurus. When and where that paper will be published is unknown, and there was no presentation of poster about the topic at the 70th annual SVP meeting.
Since this story broke during SVP, however, a few scientists did acknowledge the debate over Raptorex. In some of the tyrannosaur presentations given on Wednesday paleontologists pointed out that Raptorex was found to be distinct from Tarbosaurus in their independent analyses of tyrannosaur relationships, and a presentation about testing tyrannosaur growth by paleontologist Thomas Carr will likely provide a template for other scientists to test whether certain tyrannosaurs are juveniles of other forms.
In general, though, conference attendees I spoke to were frustrated by the Nature news coverage of the event—since no formal critique of Raptorex was published or presented, there was nothing new to talk about outside issues already known to exist. The ongoing discussion over Torosaurus and Triceratops seemed to be a more prominent topic at this year's conference, and the scientific debate over Raptorex awaits the publication of more data. Even when Hurum and Larson publish their paper, however, it will be unlikely to definitively close the case on Raptorex. Determining the true identity of this dinosaur will require multiple lines of evidence—from geology to bone histology—and this discussion will likely drag out through the literature for some time to come.
For more SVP coverage, see these posts:
Sereno, P., Tan, L., Brusatte, S., Kriegstein, H., Zhao, X., & Cloward, K. (2009). Tyrannosaurid Skeletal Design First Evolved at Small Body Size Science, 326 (5951), 418-422 DOI: 10.1126/science.1177428