T. rex Body Plan Debuted in Raptorex at 100th the Size
|At only nine feet in length, Raptorex already had the powerful jaws, puny arms and quick legs of its much larger and more famous descendants. Courtesy of Todd Marshall
|Weighing as little as 1/100th that 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.
A nine-foot dinosaur from northeastern China had evolved all the hallmark anatomical features of its famous descendant, Tyrannosaurus rex,at least 125 million years ago. Raptorex displays a large head compared to its torso, tiny arms and lanky feet well-suited for running. Its brain cast also displayed enlarged olfactory bulbs — as in T. rex — indicating a highly developed sense of smell.
Raptorex shows that tyrannosaur design evolved at “punk size,” said University of Chicago paleontologist Paul Sereno, a National Geographic Explorer-in-Residence, “basically our bodyweight. And that’s pretty staggering, because there’s no other example that I can think of where an animal has been so finely designed at about 100th the size that it would eventually become.”
“It’s really stolen from tyrannosaurids all the fire of the group,” Sereno said. All that Raptorex left for its descendants is “a suite of detailed features largely related to getting bigger.”
Sereno marvels at the scalability of the tyrannosaur body type, which when sized up 90 million years ago completely dominated the predatory eco-niche in both Asia and North America until the great extinction 65 million years ago at the end of the Cretaceous Period.
|Sculptors add skin, scales and rudimentary feathers to a cast of the nearly complete skull of the new tyrannosaur Raptorex. Courtesy of Mike Hettwer
“On other continents like Africa, you have as many as three large predators living in the same areas that split among them the job of eating meat,” he said. But in Africa, the allosaurs never went extinct, as they did in North America, possibly presenting an evolutionary opportunity for Raptorex. “We have no evidence that it was a competitive takeover,” said Sereno, “because we have never found large tyrannosaurs and allosaurs together.”
Henry Kriegstein, a private fossil collector, brought the nearly complete Raptorex skeleton to Sereno’s attention after buying it from a vendor. After Sereno and colleagues finish a more detailed study of the fossil, it will be returned to a museum in Inner Mongolia, the place where the fossil was illicitly excavated.
Sereno and five co-authors describe the newly discovered dinosaur in the early online edition of Science, September 17, 2009. The research is funded by The Whitten-Newman Foundation and the National Geographic Society.