Tyrannosaurus rex's tiny ancestors: fossil suggests peculiar features weren't limited to dino king.

A miniature version of Tyrannosaurus rex is throwing a bone to paleontologists interested in how the king of dinosaurs evolved.

The newly discovered species, called Raptorex kriegsteini, lived tens of millions of years before T. rex and had a large head, puny forelimbs and strong legs and jaws, researchers report online September 17 in Science. These features suggest that Raptorex was a direct ancestor of T. rex, the scientists say, and that smaller animals could have possessed and benefited from traits once thought particular to large predators.

"It was the common perception that the arms got smaller as the animals grew bigger" says study coauthor Paul Sereno of the University of Chicago. "No one had any idea there was something like Raptorex lurking around."

The findings are based on a nearly complete 125-million-year-old fossil unearthed in China. The size of the fossil suggests that an adult T rex would have weighed 90 times as much as a fullgrown Raptorex, Sereno says. But, despite its size, Raptorex had a body plan similar to that of Tyrannosaurus dinosaurs, which were the dominant predators from 90 million to 65 million years ago, during the Late Cretaceous period. In addition to a large head and tiny arms, Raptorex had long legs and specialized running feet, as well as large head cavities linked to a keen sense of smell, the researchers report.

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"We see this all to our great surprise in an animal about the size of a human," Sereno says.

Other dinosaurs thought to be T. rex's earliest ancestors were lankier, with long arms, small heads and a simpler foot.

Raptorex's features suggest that Tyrannosaurus' ancestors adapted earlier than thought to their role as runners and hunters. With strong legs, big heads and short arms, these dinosaurs became a "running set of jaws," Sereno says.

Paleontologist Thomas R. Holtz Jr. of the University of Maryland in College Park says the discovery of the dinosaur is strong proof that Tyrannosaurus relatives had monstrous features long before reaching six tons. However, he says, "There's still a gap of a few tens of millions of years before we pick up the classic tyrannosaurids," and it is not clear how the body plan changed during that time.