| Mini T. rex ancestor found in China|
|Scientists say a miniature Tyrannosaurus rex ancestor unearthed in China sheds new light on the evolution of the giant meat-eater.|
The three-metre-long Raptorex, which was found in China, weighed only about 60 kilograms and was nearly 100 times smaller than T. rex.
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, says lead author Dr Paul Sereno, a palaeontologist with the University of Chicago.
"It was jaws on legs," says Sereno.
The study is published today in the journal Science.
This unexpected 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," says co-author Stephen Brusatte, of the American Museum of Natural History.
"Here we have an animal ... about my size, but with all of the signature features ... 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 trade off 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% 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," says Brusatte.
It is likely T. rex was able to grow to its colossal size because other competing predators became extinct, says Sereno.
"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 recognised its potential value to science.
Dr Henry Kriegstein contacted Sereno, who agreed to analyse the fossil so long as Kriegstein was willing to return it to China once the work is complete.
The full name of the new species is Raptorex kriegsteini, in honour of Kriegstein and the dinosaur's link to raptors and the T. rex.