225,000,000-year-old dinosaur found.

A team of American and Argentinian paleontologists has discovered the world's most primitive dinosaur, an approximately 225,000,000-year-old carnivore that walked on two legs. Eoraptor, or "dawn stealer," lacks nearly all the specialized features found in later dinosaurs. "We are just a couple of steps away from the ancestor of all dinosaurs," indicates Paul Sereno, an assistant professor in the Department of Organismal Biology & Anatomy, University of Chicago, who analyzed the new fossil. "We can project back to what the most primitive dinosaurs should have looked like, but the only way you can test that picture is to go out and find something. This fossil confirms our suspicions that dinosaurs began as small, carnivorous, bipedal animals."

Sereno named the new genus "dawn stealer" because it appeared at the dawn of the dinosaurs and, though clearly a meat-eater, was not large enough to hunt the herbivorous reptiles common at the time. "It would have been a crafty hunter, probably eating small animals and snatching the young of larger species."

In October, 1991, Sereno and Alfredo Monetta of the National University of San Juan, Argentina, led a dinosaur-hunting trip to Argentina's Ischigualasto Valley. Student Ricardo Martinez found the fossilized Eoraptor less than a mile from the place where, in 1988, Sereno discovered a nearly complete skeleton of the oldest known dinosaur, Herrerasaurus. "As the students were walking away from a less interesting specimen, Ricardo happened to pick up the skull, which was lying on the ground, covered in rubble," Sereno explains. "There was just the glint of a couple of teeth sticking out to give Ricardo the clue that he was holding the jaw of an ancient reptile in his hand. We soon realized that it was a skull, probably from a small dinosaur."

Catherine Forster, a post-doctoral...