Study finds T. rex had 8-foot mini precursor

— -- Tyrannosaurus rex's likely ancestors started small — but perfectly armed — some 125 million years ago, dinosaur researchers reported Thursday.

Puny arms, massive jaws, swift legs — all characteristics of the king of the carnivores — adorned a newly discovered dinosaur, Raptorex kriegsteini, reported by the journal Science. But Raptorex was only about 1/5 the length of tyrannosaur and flourished about 40 million years before T. rex appears in the fossil record.

"This was completely unexpected," says Stephen Brusatte of the American Museum of Natural History in New York City, a co-author of the Science report. "Until now, we thought that T. rex's features had evolved as a consequence of its large body size." Instead, the 8-foot-long raptorex looks like a T. rex in miniature.

"A cute pocket tyrannosaurus," says paleontologist Thomas Holtz of the University of Maryland-College Park, who was not part of the report study. "He helps clarify some of the mystery of what happened before tyrannosaurids got so large."

Raptorex likely dined on pig-sized plant-eating dinosaurs and turtles of its era, says study team chief Paul Sereno of the University of Chicago. "This was an animal that ran down its prey and used its huge jaw muscles to catch them."

Raptorex kriegsteini takes its name from fossil collector Henry Kriegstein, who had bought the fossil, still encased in a stone block, from a fossil dealer. He then asked Sereno to look it over to verify its scientific value. "He has donated it to science and it will be returned to China" from where it was illegally excavated, Sereno says.

Tyrannosaurus rex enjoyed top predator status only for the last 20 million years of the Age of Dinosaurs, which ended with a mass extinction event around 65 million years ago. Smaller predecessors to T. rex likely "lived in the shadow of larger carnivores" prior to that time, Sereno says.