Fossil hunters announced yesterday that they have found the oldest known tyrannosaur - an ancestor of the fearsome Tyrannosaurus rex that had a bizarre combination of features, including a large, fragile crest on its head that would have attracted mates but made it vulnerable in a fight.
The diminutive dinosaur, which lived 160 million years ago, stood 3.6 feet tall and measured 9.8 feet long. That was a far cry from T. rex, which came along 90 million years later and stood about 15 feet high and 40 feet long, weighed roughly 6 tons and had a large mouth that bristled with 6-inch-long, sharp, serrated teeth.
The discovery of the fossil, in China, sheds light on the early evolution and geographical distribution of coelurosaurs, a group of small, meat-eating dinosaurs that includes the closest relatives of birds.
Similar to ornamental features seen in birds like cassowaries and hornbills, the complex head crest may have been brightly colored and used for display, the study team suggested. They speculated that the structure may even have been a cumbersome sexual ornament similar to the peacock's tail or the antlers of an elk.
Described as about as thick as a tortilla and 2.5 inches high, the crest certainly wouldn't have been much use as a weapon and might have gotten in the animal's way when hunting, said James M. Clark, biology professor at George Washington University in Washington and co-author of the study that appeared in the British journal Nature.
"It seems paradoxical that a presumably predatory dinosaur like Guanlong would possess such a delicate cranial crest," he said. "Based on similar ornaments found in extinct vertebrates, we hypothesize that the primary function of the crest was to make the animal more noticeable or attractive to other members of its species."
Researchers named the new creature Guanlong wucaii. The first part means "crowned dragon" in Chinese, after the dinosaur's head crest. The second refers to the rich colors of rocks that produced the specimens at the locality of Wucaiwan.
"The crest is a very bizarre-looking structure. I actually had a chance to see the fossil last summer in China while they were working on it," said Peter Makovicky, curator of dinosaurs at the Field Museum in Chicago. "It's remarkably well preserved. And it has this strangely open triangular structure that flares a little bit at the back. Very delicate."
The specimens were found in the Junggar Basin of Xinjiang, China, on the western reaches of the Gobi desert near the old Silk Road.
The team found two specimens of the fossil preserved with three specimens of other carnivorous dinosaurs, said Mark Norell of the American Museum of Natural History, a member of the research team.
One specimen was determined from growth rings in the bone to have died in the 12th year of life; most of the elements of the skeleton were preserved. The other specimen, which was 6 when it died, is nearly complete.
The fossil was identified as a tyrannosaur based on the shape of its teeth, the shape of openings in its skull and features of its pelvis, the researchers said.
"On the one hand, the new fossil looks like just what paleontologists have been expecting for a primitive dinosaur," said Xu Xing of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, a research fellow at the American Museum of Natural History.
"On the other hand, no one expected that a tyrannosaur would bear a crest like this - large and delicate. Even after so many great discoveries, we have to say there is a lot we don't know about dinosaurs. They are really a diverse group of animals."
Tyrannosaurs are best known from the end of the Cretaceous Period, about 65 million to 70 million years ago. Fragmentary specimens from the middle part of the earlier Upper Jurassic had been suggested as tyrannosaurs but were too incompletely known to be sure.
The new fossils, which date to the beginning of the Late Jurassic, about 160 million years ago, offer a peek at what tyrannosaurs looked like shortly after they branched from other coelurosaurs.
"People care about tyrannosaurs," Norell said. "People are interested in them - not just scientists - and this fossil goes a long way to understanding their early history."
Peter Gorner writes for the Chicago Tribune.