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. [Illustration: Todd Marshall]
When you think of Tyrannosaurus rex, a small set of striking physical traits comes to mind: an oversized skull with powerful jaws, tiny forearms, and the muscular hind legs of a runner. But, researchers have just unearthed a much smaller tyrannosauroid in China, no more than three meters long, that displays all the same features—and it predates the T. rex by tens of millions of years.
This finding means that such specialized physical features did not develop as the prehistoric predators evolved into larger sizes. Instead, the massive jaws, tiny forearms, and tremendous hind legs were present all along in tyrannosauroid dinosaurs—and at all sizes—to help them catch and eat prey throughout their reign in the Cretaceous Period.
Paul Sereno from the University of Chicago and National Geographic explorer-in-residence, along with colleagues, studied the new, small-bodied fossil, naming it Raptorex kriegsteini, and estimated that it was a young adult when it died. They examined the skull, teeth, nose, spine, shoulders, forearms, pelvis, and hind legs of the new fossil, comparing the features to larger evolutionary versions of tyrannosauroid dinosaurs.
"First, we used the best mechanical preparation of the specimen possible, which entails the finest needles and air abrasives under a microscope," Sereno said in an email interview. "Then we made molds and casts of the cranial bones, assembled a cast skull, and sent that skull through a CT scanner at the University of Chicago hospital to get the snout cross-section... We used silicone on the skull roof to cast the forebrain of R. kriegsteini... Finally, I made a thin-section from one femur, or thigh bone, for microscopic examination, and determined that the individual had lived to be five or six years old."
The researchers conclude that the "predatory skeletal design" of R. kriegsteini was simply scaled up with little modification in its carnivorous descendants, whose body masses eventually grew 90 times greater. This was a stepwise process, over the course of many millions of years. Sometime after Raptorex, perhaps as little as 10 million years later, says Sereno, "tyrannosaurs emerged as the dominant large-bodied predators in Asia and North America." But nearly 50 million years would pass before the very largest of descendants, like T. rex, evolved.
Sereno and his colleagues also use this new fossil to propose and describe three major morphological stages in the evolutionary history of tyrannosauroid dinosaurs.
The first stage is comprised of small- to medium-sized tyrannosauroids of the Middle Jurassic to Early Cretaceous Period that display some of these initial feeding specializations. The second stage, heralded byR. kriegsteini, involves all of the remaining tyrannosaur functional specializations, such as stronger jaws and lanky, fleet-footed hindlimbs. The third and final stage is characterized by a marked increase in body size.