It just seems small compared to the giant T. rex that evolved millions of years later and was as much as 100 times more massive.
"It really is the blueprint for the later (T. rex) dinosaurs," Sereno said, "it was a blueprint that was scalable."
Described for the first time in Thursday's ScienceExpress, the online edition of the journal Science, the new dinosaur has been named Raptorex kriegsteini.
Sereno reports that Raptorex has all the hallmarks of T. rex, including a large head, tiny arms and lanky feet -- just in a smaller size.
"What we're looking at is a blueprint for a fast-running set of jaws," Sereno said at a briefing arranged by the American Association for the Advancement of Science.
The giant T. rex dominated much of the planet from about 90 million years ago until the great extinction 65 million years ago.
Raptorex would have weighed only about 150 pounds, said Sereno, of the University of Chicago and also a National Geographic explorer in residence.
The newly described remains were found by fossil hunters in northern China, smuggled out of that country and offered for sale to collector Henry Kriegstein of Higham, Mass., Sereno said. Kriegstein, for whom the animal is now named, donated the materials to science and they will be returned to China.
The fossil was encased in a single block of stone, Sereno said. That stone allowed the researchers to trace the find to its original location.
The way the bones were fused indicates the animal died at the age of five or six, which is nearly adult. It would have matured at eight or 10 and been old by 20, added co-author Stephen Brusatte of the American Museum of Natural History.
The find also shows that features such as the animal's tiny arms did not evolve as T. rex grew larger, but were present in the much earlier forms, Brusatte said.
"Much of what we thought we knew about T. rex turns out to be simplistic or out-and-out wrong," Brusatte said.
Sereno said Raptorex was a predator. Some scientists debate whether T. rex was a predator or scavenger.
Dinosaur expert John R. Horner of the Museum of the Rockies at Montana State University was cautious about the find.
"It's hard to evaluate their conclusions," he said, calling the report interesting but adding that the drawing in the paper shows some differences from a T. rex in addition to being smaller.
However, he added, he didn't see anything that would disprove their theory that Raptorex was an ancestor of T. rex.
The research was funded by the Whitten-Newman Foundation and the National Geographic Society.
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