Part of what so fascinates us about dinosaurs is that they came in such a wide array of forms.
Unfortunately the fossil record does not contain a 100 percent complete record of ancient life. Only a very few creatures ever became fossilized, and of those even fewer have been found by scientists. Early dinosaurs, especially, are very rare, but in last week's issue of Science a team of paleontologists announced the discovery of a dinosaur that helps explain that origins of one of the great branches of the dinosaur evolutionary tree. Named Tawa hallae, it provides some crucial clues to how predatory dinosaurs evolved.
Since the end of the 19th century it has been known that there are two major groups of dinosaurs. There were the ornithischians (the hadrosaurs, horned dinosaurs, ankylosaurs, and a few others) and the saurischians (the theropods and sauropodomorphs). Tawa was close to the origin of theropod dinosaurs, and by comparing it to other early dinosaurs paleontologists were able to refine their ideas about early saurischian evolution.
The bones of Tawa were found in approximately 215-million-year-old rocks of the Chinle Formation in New Mexico. This was a time when early dinosaurs were diversifying, but were not yet like the giants of the Jurassic and Cretaceous. Instead Tawa was a relatively small dinosaur that shared many traits in common with other early theropods such as Coelophysis, thus placing it as part of the early radiation of predatory dinosaurs.
What is even more interesting, however, is that the nearly complete remains of Tawa allowed the scientists to confirm other previously ambiguous dinosaurs as theropods. Since the time of their discovery, the evolutionary positions of Eoraptor and Herrerasaurus have been controversial, but their shared resemblances with Tawa confirm that they were among the earliest theropod dinosaurs. This not only allows scientists to better understand the origins of predatory dinosaurs, but to refine hypotheses of what to look for in the common ancestor of theropods and the sauropodomorphs. In fact, Tawa appears to be part of a radiation of early dinosaurs that migrated from what is now South America into what we presently call North America, confirming that the deposits of South America is probably still some of the best places to look for the earliest dinosaurs.
There is little doubt that the origin of theropods, and dinosaurs in general, will be debated for some time to come, but Tawa has helped to put some early forms in their place. For more on this new discovery, see Bill Parker's post at Chinleana.