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.