Eodromaeus was only four feet long from nose to tail-tip and weighed a mere 10 to 15 pounds.
In a rare find, fossil bones of two of the creatures were unearthed side-by-side at a desert site known as the Valley of the Moon in northern Argentina.
Researchers pieced together a near-complete skeleton of the new species, which they have described in the journal Science.
Study leader Dr Paul Sereno, from the University of Chicago, US, said: “It really is the earliest look we have at the long line of meat eaters that would ultimately culminate in Tyrannosaurus rex near the end of the dinosaur era.”
The site where Eodromaeus was found, in the foothills of the Andes, has already yielded clues about the early origins of the dinosaurs.
In 1991 Dr Sereno found another species in the same valley called Eoraptor. About the same size as Eodromaeus, it was a plant-eater whose descendants would include the giant long-necked sauropods such as Diplodocus. ( ITN )