Observatory - Bones Show Early Divergence of Dinosaur Lineage - NYTimes.com 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, support the idea that the lineages diverged early on 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 University of Texas researcher and lead author of a paper in Science describing the find. Dr. Nesbitt worked on the fossils while at the American Museum of Natural History and Columbia University. Like later theropods, Tawa walked on two legs and had sharp teeth for tearing apart its food: other animals. The most complete specimen, a juvenile, was about six feet long. Tawa shares some features with an early South American dinosaur, Herrerasaurus, that had been a source of confusion for paleontologists. Tawa, in effect, shows that Herrerasaurus was a theropod. Because Herrerasaurus was found near some early sauropods and ornithischians, the new finding strongly suggests that all three main lineages diverged early on. The New Mexico fossil beds included several other theropods that the scientists found were more closely related to different groups of South American theropods than to each other. That suggests theropods diverged and radiated from South America. And if theropods had that dispersal pattern, the findings suggest, the other lineages probably did, too.