A relatively tiny new ancestor of the Tyrannosaurus rex has been unearthed in China, researchers said Thursday.
The three-meter-long (10-foot) dinosaur dubbed the Raptorex only weighed about 60 kilograms (150 pounds) and was nearly 100 times smaller than the king of the dinosaurs.
But it was nearly identical in structure -- even down to the scrawny arms -- and had all of the traits which made T. Rex such a successful predator, said lead author Paul Sereno, a paleontologist with the University of Chicago.
"It was jaws on legs," Sereno said.
This unexpected new link in the evolution of the mighty predator which once dominated the northern half of the globe has provided researchers with an entirely new picture of how T. Rex evolved.
"Raptorex, the new species, really throws a wrench into the observed pattern," said co-author Stephen Brusatte of the American Museum of Natural History.
"Here we have an animal that's 1/100th of the size of T. Rex -- about my size -- but with all of the signature features -- big head, strong muscles, tiny little arms -- that were thought to be necessary adaptations for a large-body predator."
The Raptorex fossil shows that the skinny arms evolved not in order to help it offset a heavier overall bodyweight, but instead as a tradeoff for agility and speed.
The powerful muscles of the back legs would have helped the T. Rex chase down its prey while the smaller front legs allowed it to remain upright and attack with its deadly jaws.
The Raptorex fossil -- which was estimated to be a juvenile of five to six years old when it died -- is about 125 million years old.
The tyrannosaurus genus did not reach its full size until about 85 million years ago and was wiped out about 65 million years ago in the great extinction which ended the Cretaceous Period.
"What that means is that for most of their evolutionary history, about 80 percent of the time that they were on earth, tyrannosauruses were small animals that lived in the shadow of other types of very large dinosaur predators," Brusatte said in a conference call with reporters.
It's likely that T. Rex was able to grow to its colossal size because other competing predators became extinct, Sereno said.
"We cannot say that this incredibly successful, scalable blueprint for a predator was responsible for their total domination... because we never saw them cohabiting in environments with these other, earlier types of predators," he said.
But once tyrannosauruses were able to expand in body size, "there was no turning back until the asteroid hit because they really had it down pat."
The incredibly well-preserved and nearly complete fossil was almost lost to science after it was unearthed illegally and spirited out of China for sale on the private market.
An American eye surgeon and dinosaur enthusiast purchased the still-embedded fossil and recognized its potential value to science.
Henry Kriegstein contacted Sereno, who agreed to analyze the fossil so long as Kriegstein was willing to return it to China once the work is complete.
"We rapidly achieved that agreement and Raptorex sees the light of day," Sereno said.
"I hope this is a pathway (which can be used again so) that other important specimens that do find their way out of the ground in the dark of night do not get lost to science."
The full name of the new species is Raptorex kriegsteini, in honor Kriegstein and the dinosaur's link to raptors and the T. Rex.
The study was published in the journal Science.