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Miniature T-Rex Discovered

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 the ingredients of the king of the carnivores - adorned a newly discovered dinosaur, Raptorex kriegsteini, reported by the journal Science. But Raptorex was only about 1/90 the size of Tyrannosaurus, and flourished about 40 million years before T.Rex appears in the fossil record.

"The most important and interesting thing is this was completely unexpected," says the study team's Stephen Brusatte of the American Museum of Natural History in New York. "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, a 5- or 6-year-old near adult based on its mature skeleton, looks like a T. Rex in miniature, right down to its undersized front claws.

"A cute pocket Tyrannosaurus," says paleontologist Thomas Holtz of the University of Maryland in College Park, who was not part of the study. "He's not completely unexpected, but 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."

A few smaller predecessors to T. Rex turned up in earlier fossil searches, but they had long clawed arms and narrower heads, Holtz adds, "much more primitive looking."

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 Serenoto 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.

Some evidence suggests that as plant-eating dinosaurs grew larger - the ancestor of Stegosaurus, for example, grew from the size of a hog some 130 million years ago to the size of an elephant later on - Tyrannosaurid dinosaurs grew larger, Holtz says, in an evolutionary arms race for size.

Because few carnivore fossils are available from the era about 90 million years ago, scientists cannot tell whether smaller Tyrannosaurs out-competed the large meat-eaters of their time, or if they replaced those creatures after they were wiped out in an extinction event. "Juvenile Tyrannosaurs likely filled the ecological niche their smaller predecessors had occupied," he says, which may have added to their success.

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