Diminutive version of T. rex found

Raptorex illustration courtesy EurekAlert

Smaller version predates T. rex by 50 million years; shows co-existence of carnivores likely

Fossils from a 9-foot-long dinosaur believed to have lived 125 million years ago have all the anatomical features of the full-size Tyrannosaurus rex but one. Dubbed Raptorex, the mini T. rex is about 100th the size of the full size Tyrannosaurus that scientists have been studying since the late 1800s. The Raptorex weighed about 150 pounds (60 kg) compared to the 6-ton T. rex.

The discovery has some researchers puzzled because there are no known examples of a specimen that “has been so finely designed at about 100th the size that it would eventually become," according Paul Sereno, a paleontologist at the University of Chicago and National Geographic Explorer-in-Residence.

The fossil is “not a missing link,” said Sereno, nor is it a young T. rex.  Examination of the skeleton found it to be an adult, six or seven years old.

The nearly complete skeleton of the Raptorex was found in northeastern China, a region rich with well-preserved fossils, and illegally smuggled out of the country, according Sereno. The paleontologist acquired the skeleton from Henry Kriegstein, a private fossil collector. He said after studies are complete, the fossil will be returned to a museum in Inner Mongolia, near where it was excavated.

Sereno is the lead author on a paper on Raptorex published in the journal Science.

Raptorex has all the features of the T. rex, from its large head compared to its torso, tiny arms and lanky feet well-suited for running. A brain cast of the Raptorex brain showed that like T. rex, it had enlarged olfactory bulbs that indicated a keen sense of smell.

Lacking T. rex’s size, it’s doubtful that Raptorex enjoyed being top of the predatory food chain like its descendent, the T. rex, or “tyrant lizard” in Latin.

What are the implications of the discovery?

“Raptorex complicates our knowledge of how tyrannosaur dinosaurs evolved,” Mark Witton, a paleontologist at the University of Portsmouth, UK, told Flesh and Stone.  

“Raptorex shows that there is more diversity of form in small tyrannosaurs than we previously thought,” said Witton. “As well as relatively slender animals, there are some small bruisers in this group as well. Secondly, it appears that the hypercarnivory of tyrannosaurs began development before large size. Ecologically speaking, Raptorex can be viewed as a mini-tyrannosaur that was functioning in a Tyrannosaurus-like fashion, but on a much smaller scale.

“Later tyrannosaurs are weird because, so far as we can tell, they are the only large carnivores in their respective environments. Other large, non-tyrannosaurid carnivorous dinosaurs are surrounded by similarly-sized contemporaries, but large tyrannosaurs really dominate their food pyramids. Raptorex shows, though, that this wasn't always the case and that some dinosaur landscapes would've been similarly diverse, with flesh-eating and bone-crushing foraging strategies allowing co-existence of numerous carnivores.

“What isn't so clear, though, is the rise of the tyrannosaurs to their dominant place in dinosaur ecosystems. The fossil record during this portion of their evolutionary history isn't that great, so quite how they went from background bone-crushers like Raptorex to dominant, apex predators like Tyrannosaurus still requires more investigation.”

Raptorex will be featured on a National Geographic program, “Bizzare Dinos” on Oct. 11.

Citation:

Paul C. Sereno, Lin Tan, Stephen L. Brusatte, Henry J. Kriegstein, Xijin Zhao and Karen Cloward, Tyrannosaurid Skeletal Design First Evolved at Small Body Size, early online edition of Science, Sept. 17, 2009.