The small, two-legged dinosaur lived 230 million years ago and was an early ancestor of T. Rex.
A new dinosaur named "Dawn Runner" was one of the world's first known meat-eating dinosaurs.
It lived 230 million years ago at a site in Argentina where other early dinosaurs have been found.
The earliest three main dinosaur groups all shared the same basic body shape.
Paleontologists have just unearthed "Dawn Runner," the world's second oldest known carnivorous dinosaur that eventually gave rise to Tyrannosaurus, Allosaurus and other famous meat-eating dinosaurs.
Dawn Runner (Eodromaeus murphi) lived during the dawn of the Dinosaur Era 230 million years ago in what is now the Ischigualasto formation in northeastern Argentina, according to a paper in the latest issue of Science.
Since other very early dinosaurs -- including the world's oldest known carnivorous dino, Herrerasaurus -- have also been found at this site, it may have been home to Earth's first ever dinosaur.
"We don't know for sure yet if this site, also called The Valley of the Moon, was the birthing ground of dinosaurs, but it has yielded the earliest and best dated sequence for these animals," co-author Paul Sereno told Discovery News.
Sereno, a paleontologist at the University of Chicago, and his colleagues dug up Dawn Runner, along with numerous other prehistoric species, at the South American site.
"Dawn Runner was a 10 to 15-pound, scrappy two-legged predator that carried the blueprint for all other predatory dinosaurs that were to come," Sereno said.
He explained that this blueprint included features such as a grasping hand, air pockets in the neck, a balancing tail that's stiff at the end, a pubic bone used for squatting and sitting, a strap-shaped shoulder girdle, and other traits shared by T. rex and all other later carnivorous dinosaurs.
The researchers also determined that another dinosaur from the site, Eoraptor, was previously misidentified as a theropod (a carnivorous predatory dino). It was, in fact, an early ancestor of the sauropod lineage of dinosaurs, which included the enormous, long-necked plant eaters. Unlike Dawn Runner, Eoraptor possessed more sauropod-like features, such as enlarged nostrils and an inset first lower tooth.
Despite these differences, both dinosaurs were less than 6.5 feet long and ran on two legs. Since the world's earliest bird-like dinosaur also shared these characteristics, this means that the three principal groups of dinosaurs (the ornithischians, the sauropodomorphs and the theropods) all shared the same basic body plan in the late Triassic.
"It was approximately 30 million years later that dinosaurs rose to prominence," Sereno said. "They did not claw their way to dominance. It was instead more of an opportunistic replacement."
Thomas Holtz, a University of Maryland paleontologist, told Discovery News that the conclusions of Sereno and his colleagues "seem quite reasonable."
Holtz said "dinosaurs spent most of the Triassic in the shadows" of other animal groups, such as crurotarsans (relatives of modern crocodiles) and therapsids (protomammals). Dinosaurs only diversified "into more familiar types with the great extinction at the end of the Triassic."
Holtz believes the new findings help "to reinforce that the oldest dinosaurs were little, bipedal animals with grasping hands and most likely an omnivorous diet: sort of reptilian raccoons."
He wasn't surprised that the three early representatives for the three major groups of dinosaurs all initially looked similar because "after all, all of these are branches very, very close to the base of the dinosaur family tree, so there really hadn't been much time for these groups to diverge from each other."
Non-avian dinosaurs enjoyed a good run on the planet, living for at least 165 million years before they went extinct. Mammals then became some of the most successful animals after the big K-T extinction event about 65 million years ago.
"Mammals survived, in part, because they were small, furry and warm," Sereno said. "The smallest dinosaurs were still much larger than the smallest mammals."