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Scientists have unveiled one of the earliest dinosaurs yet found, an agile meat-eater from the late Triassic period, some 230 million years ago.

Researchers writing in the journal say Eodromaeus or "dawn runner" was a small, two-legged creature of not much more than 1.2m in length and 4-6kg in weight.

They reconstructed the dinosaur, a probable ancestor of Tyrannosaurus rex, from an almost complete set of bones found in the Valley of The Moon, in northwestern Argentina.

Researchers from Argentina and the US first chanced upon fossilised bones belonging to Eodromaeus in 1996, but it has taken them this long to reconstruct the fragments.

Many of the fossils were covered in iron encrustations and required painstaking work under the microscope before casts could be made and a complete skeleton recreated.

The finished creature claim the researchers, is one of the earliest dinosaurs yet discovered, and could represent the beginnings of the theropod line which eventually leads to T.rex.

"It's very close to the root of the dinosaurs," said lead author Professor Paul Sereno of University of Chicago. "This is a two legged animal, pint-sized if you will. It was agile - we know that from the limb proportions, it had grasping hands with really powerful claws.

"It was a predator, we know that from the grasping hands but also especially from the long curved teeth. It was a meat-eater, a specialist and it was in many ways very close to the original meat-eater, the first of the theropod line that would eventually evolve into huge forms like T.rex."

Ancestral dinosaur

Eodromaeus has also cast light on an earlier discovery by the same team, another two-legged creature called Eoraptor, for which additional bones have now been collected.

The latter creature has been reclassified by the researchers as an early herbivore rather than a predator, and though similar in appearance to Eodromaeus, they now think Eoraptor could be an early example of the sauropods which eventually spawned such giants as Diplodocus.

"These two dinosaurs actually represent the routes of two incredibly different radiations," said Professor Sereno. "We're looking at just a few years away from that first 'eve' dinosaur and looking at two of the great lineages, which looked very similar at this point."

Even though their descendents may have gone on to great things, neither of the creatures were dominant in their time, and the researchers believe their eventual rise may be down to blind chance, and perhaps some unknown environmental catastrophe.

"Eodromaeus and Eoraptor are punks on the block," said Professor Sereno. "There were other large mammal-like reptiles around that would have been the biggest herbivores and a squat four-legged predator related to alligators that would have swallowed Eodromaeus in one bite."

"Eodromaeus needed some opportunity, some change of events to get the upper hand, so it was extinctions of other animals that were already there which ultimately gave dinosaurs their foot in the door about 200 million years ago."

Dr Paul Barrett, research scientist with the Natural History Museum, said the research was "very interesting" and could bring us closer to the first dinosaur, which he thinks must have lived very shortly before Eodromaeus.

"We have ideas of what dinosaurs looked like from later members of the groups but they're likely to look more similar as they get closer to that ancestral dinosaur," he said.

These two dinosaurs, he said, could represent two of three great dinosaur lineages - the theropods and the sauropods - the other being the ornithischia group to which creatures such as Stegosaurus and Triceratops belonged.