The fossil of a never-before seen species of dinosaur that could have been among the first to roam the planet 230 million years ago has been unearthed in Argentina.
A fossil of the petite, two-legged Eodromaeus, which means 'dawn runner', was found in Ischigualasto, a well-explored rock formation in northwestern Argentina that has yielded hundreds of precious fossil discoveries.
Scientists say the find of two near-complete dino fossils right next to each other has helped shed light on the development of predatory dinosaurs known as theropods (which means beast-footed), including the famous T. Rex.
"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," says palaeontologist Paul Sereno of University of Chicago .
"Who could foretell what evolution had in store for the descendants of this pint-sized, fleet-footed predator?"
Fossils of small theropods are rare, and this one measures about two meters long. Scientists believe it weighed 4.5 to 7 kilograms.
The Eodromaeus had a long neck and tail, sharp claws and biting canine teeth.
Early dinosaurs common and diverse
By examining its limbs, scientists believe they have found differences between the Eodromaeus and its contemporary, the Eoraptor, which they now believe belonged to a different family lineage, the huge, long-necked and four-legged sauropods.
Both species were about the same size and ran on two legs, suggesting that the three main types of dinosaur (ornithischians, sauropodomorphs, theropods) that lived during the late Triassic period shared similar body types.
But the newly discovered Eodromaeus had a skull that resembled other theropods, while the plant-eating Eoraptor "had more sauropod-like features, including enlarged nostrils and an inset first lower tooth," says the study's authors.
An examination of the overall fossil record from the area shows that early dinosaurs "were more common and diverse than previously thought," says the study which appears in the journal Science.
By logging thousands of fossils in the area, from lizard-like creatures to mammal-like reptiles that outnumbered the dinosaurs, scientists believe that dinosaurs likely came to dominate the landscape slowly, over several million years.
"Dinosaurs took their sweet time to dominate the scene," says lead author of the study Ricardo Martinez of Argentina's National University of San Juan .
"The story from this valley suggests that there was no single advantage or lucky break for dinosaurs but rather a long period of evolutionary experimentation in the shadow of other groups," says Sereno.