Eodromaeus: Nasty, Brutish, and ShortDeadly and dog-size, the dinosaur Eodromaeus (shown in reconstruction) lived in Argentina 230 million years ago, a new study says. The new species is providing fresh insight into the era before dinosaurs overtook other reptiles and ruled the world, a new fossil study says. (Watch video.)
"This is the most complete picture we have of what a predatory dinosaur lineage – what it looked like at the very beginning," said study co-author Paul Sereno. "It was small but nasty—this animal was fast."One of the earliest known dinosaurs, Eodromaeus was only about 4 feet (1.3 meters) long and would have barely reached the knees of an adult human. But this unassuming little dinosaur gave rise to the theropods, including Tyrannosaurus rex and the "terrible claw," Deinonychus, the new study suggests.Like those fearsome descendants, Eodromaeus had a long rigid tail, a unique pelvis shape, and air sacs in its neck bones that may have been related to breathing—and which add to evidence that theropod dinosaurs eventually evolved into today's birds.(Take a dinosaur quiz.)—Ker Than

Photograph courtesy Mike Hettwer