A tiny tyrannosaur

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, published online by the journal Science at the Science Express website on September 17, means that such specialized physical features did not evolve as the prehistoric predators grew in size. Instead, they were present for feeding efficiency at all sizes of the dinosaurs during 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.

Sereno and his colleagues also use this new fossil to propose and describe three major morphological stages in the evolutionary history of tyrannosauroid dinosaurs.

Dr. Sereno's coauthors are Lin Tan of the Long Hao Institute of Geology and Paleontology in Hohhot, PRC; Stephen Brusatte of the American Museum of Natural History in New York, NY; Henry Kriegstein of Higham, MA; Xijin Zhao of the Chinese Academy of Sciences in Beijing, PRC; and Karen Cloward of Western Paleontological Laboratories, Inc. in Lehi, UT. The paper is entitled, “Tyrannosaurid Skeletal Design First Evolved at Small Body Size.”

This research was funded by the David and Lucile Packard Foundation and the National Geographic Society.

The American Association for the Advancement of Science (AAAS) is the world's largest general scientific society, and publisher of the journal Science (www.sciencemag.org). AAAS was founded in 1848, and serves 262 affiliated societies and academies of science, reaching 10 million individuals. Science has the largest paid circulation of any peer-reviewed general science journal in the world, with an estimated total readership of 1 million. The nonprofit AAAS (www.aaas.org) is open to all and fulfills its mission to “advance science and serve society” through initiatives in science policy; international programs; science education; and more. For the latest research news, log onto EurekAlert!, www.eurekalert.org, the premier science-news Web site, a service of AAAS.