The nearly complete fossil was found 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. The fossil, which was illicitly excavated, will be returned to a museum in China.

Dr. Sereno said the fossil was that of a young adult, about 5 or 6 years old and near the end of its growth period. Besides the oversized head, jaws and legs, it had 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, in an animal that is basically the body weight of a human,” Dr. Sereno said.

Raptorex, like T. rex, would have killed animals with its teeth and jaws. The forelimbs would not have been the primary means for attacking prey. In fact, Dr. Sereno said, the forelimbs would have gotten smaller as the head got larger. “This is an agile, fast-running animal,” he said. “By adding a lot of weight at the top, something has to give way. What gave way was the forelimb.”

Thomas R. Holtz Jr., a paleontologist at the University of Maryland who was not involved in the work, said the discovery helped “clear up the origin of the characteristic features of the Tyrannosaurs.”