Unearthed in 230-million-year-old rocks, scientists say they creature would have been as tall as a 7-year-old human but no heavier than a house cat, ScienceNews.org reported Thursday.
Dubbed Eodromaeus, or "dawn runner," it joins its kin Eoraptor, a dinosaur known to have lived in the same time and place and very similar to Eodromaeus with one crucial difference: One ate plants while the other ate meat.
"From 20 feet away you'd do a double take -- will the animal run from you or take your leg off?" team member Paul Sereno, a paleontologist at the University of Chicago, says of the similar-looking creatures.
Yet each eventually led to a separate branch of dinosaur evolution, he says.
"There's no way to look at them and realize that the ultimate descendants of one results in a tyrannosaur, and the other something like [the plant-eating] Diplodocus," Sereno says.
In addition to Eodromaeus, researchers discovered a wealth of other Argentinean fossils that give clues to how the earliest dinosaurs evolved and what their environment was like.
All came from the evocatively named Valley of the Moon in northeastern Argentina and are buried in a fossil mother lode known as the Ischigualasto.
"What we have is this unbelievable graveyard of the earlier dinosaurs," says Sereno. "We don't have a lot of places like this."