A new dinosaur, named Raptorex kriegsteini, is dwarfed by a Tyrannnosaurus rex in this illustration released by the journal Science. (Todd Marshall/Science via The New York Times)

(© 2009 New York Times News Service) Paleontologists said Thursday that they had discovered what amounted to a miniature prototype of Tyrannosaurus rex, complete with the oversize head, sharp teeth, long legs – and, as every schoolchild knows, puny arms – that were hallmarks of the king of the dinosaurs.

But this scaled-down version, which was about nine feet long and weighed only 150 pounds, lived 125 million years ago, about 35 million years before T. rex roamed the earth. So the discovery calls into question theories about the evolution of T. rex, which was about five times longer and almost 100 times heavier.

"The thought was these signature Tyrannosaurus features evolved as a consequence of large body size," Stephen L. Brusatte of the American Museum of National History, an author of a paper describing the dinosaur published online by the journal Science, said at a news conference announcing the discovery. "They needed to modify their entire skeleton so they could function as a predator at such colossal size."

The new dinosaur, which has been named Raptorex kriegsteini, "really throws a wrench into this observed pattern," Brusatte said.

The nearly complete fossil was found in ancient lake deposits in northeastern China and bought by a collector, Henry J. Kriegstein, who alerted Paul C. Sereno, a paleontologist at the University of Chicago and lead author of the paper. Under the terms of the work, the fossil, which was illicitly excavated, will be returned to a museum in China.

Sereno said that the fossil was that of a young adult, about 5 or 6 years old, and that several characteristics, including the fused nature of many of the bones, suggested it was near the end of its growth period.

In addition to the sharp incisors and oversized head and legs, Raptorex had enlarged jaw muscles, long shinbones and long, compressed feet that helped it run fast after smaller dinosaurs and other prey. "We see this all to our great surprise," Sereno said, "in an animal that is basically the body weight of a human."

Raptorex, like T. rex, would have killed animals with its teeth and jaws. The forelimbs, which Brusatte described as "pathetically tiny," would not have been the primary means for attacking prey.

In fact, Sereno said, the forelimbs would have gotten smaller as the dinosaur’s head got larger. "A skeleton is a tradeoff," he said. "This is an agile, fast-running animal. By adding a lot of weight at the top, something has to give way. What gave way was the forelimb."

Sereno said Raptorex’s features were "a body blueprint for a predator – jaws on legs, as it were – that is one of the most successful of the Mesozoic," the period that ended with the demise of the dinosaurs in the mass extinction 65 million years ago.

That blueprint, he said, proved to be scalable as the Tyrannosaurs grew in body size. "When they did there was no turning back until the asteroid hit," he said. (NYT)