Weighing as little 1% of its descendant T.rex, 125-million year old Raptorex shows off the distinctive body plan of this most dominant line of predatory dinosaurs. Based on a fossil skeleton discovered in Inner Mongolia, China.
Credit: Todd Marshall
CHICAGO: At just 1% of the size of its later relative, a tiny new ancestor of Tyrannosaurus rex has been unearthed in China.
The three-metre-long dinosaur dubbed Raptorex only weighed an estimated 60 kg, 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.
"It was jaws on legs," said lead author Paul Sereno, lead author a study which details the find and a palaeontologist with the University of Chicago in Illinois, USA.
New picture of evolution
This unexpected new link in the evolution of the mighty predator which once dominated the northern half of the globe is offering a completely 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 in New York City.
"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," he said.
Raptorex 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 species to chase down its prey while the smaller front legs allowed it to remain upright and attack with its deadly jaws.
The fossil - which was estimated to be a juvenile of five to six years old when it died - is about 125 million years old.
Nearly lost to science
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," Brusatte said.
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."
