Pet-sized ancestor of mighty Tyrannosaurus rex discovered by scientists

Tyrannosaurus rex had a pint-sized skinny ancestor that lived when dinosaurs were the underdogs 230 million years ago, scientists have discovered.

Eodromaeus was only four feet long from nose to tail-tip and weighed a mere 10 to 15 pounds.

But the two-legged dinosaur gave rise to the biggest and most ferocious land predators that ever lived, including T rex.

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 described today 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.

"Who could foretell what evolution had in store for the descendants of this pint-sized, fleet-footed predator?"

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.

Although Eodromaeus - name means "dawn runner" - was armed with sharp canine teeth and claws, it would have been up against strong competition.

At that time other creatures out-numbered and dominated the dinosaurs, including the lizard-like rhynchosaurs and mammal-like reptiles.

"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," said Dr Sereno.