The early evolution of dinosaurs, in the late Triassic period, is fuzzy, to say the least.

Paleontologists know that the first dinosaurs appeared about 230 million years ago, but fossil evidence is so spotty that it is unclear where and when the major lineages — theropods, sauropods and ornithischians — began to diverge.
Some excellent 215-million-year-old fossils unearthed in Ghost Ranch, in northern New Mexico, are helping to clarify things. The bones, of a theropod that the discoverers have named Tawa hallae, provide strong support for the idea that the major lineages diverged early in dinosaur evolution in the part of the supercontinent Pangea that is now South America.
“What Tawa does is it helps signify the relationships at the base of dinosauria,” said Sterling J. Nesbitt, a postdoctoral researcher at the University of Texas and lead author of a paper in Science describing the find.
Dr. Nesbitt did most of the work on the fossils while at the American Museum of Natural History and Columbia University.
Like T. rex and other later theropods, Tawa walked on two feet and had sharp serrated teeth, good for tearing apart its food — other animals. Its body was probably covered in something like feathers.
The most complete specimen found, a juvenile, was a little over two feet high and about six feet long.
What makes Tawa significant is that it shares some features with an early dinosaur, Herrerasaurus, that had been a source of confusion for paleontologists. Tawa, in effect, shows that Herrerasaurus was a theropod.
Because Herrerasaurus was found in what is now South America near some early sauropods and ornithischians, this strongly suggested that all three main lineages diverged early on.
The New Mexico fossil beds included several other theropods. The researchers found that these dinosaurs were only distantly related to each other and Tawa, and were more closely related to different groups of South American theropods.
That suggests theropods diverged and radiated from South America. And if the theropods had that pattern of dispersal, the findings suggest, the other lineages probably did, too.
(Source: The NYT)