Thursday, February 09, 2006

Before The Tyrannosaurus, Guanlong Roamed China

AUTHOR: John Noble Wilford

SOURCE: New York Times

Chinese and American scientists have discovered what appears to have been the granddaddy of all tyrannosaurs, a primitive crested dinosaur that lived 160 million years ago in northwestern China.

The scientists announced yesterday that an analysis of two fossil specimens suggested that they were either remains of the most primitive tyrannosaur known or the first branch on the family tree leading to Tyrannosaurus rex, the symbol of tooth and claw predation in the age of reptiles.

James M. Clark, a paleontologist at George Washington University, said the discovery "shows us how ancestors of tyrannosaurus took the first step that led to the giant T. rex almost 100 million years later."

The research team, led by Dr. Clark and Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, named the new species Guanlong wucaii. The first, or generic, name is derived from the Mandarin word for "crowned dragon," a reference to its large, fragile crest. The second, or species, name refers to the rich colors of the Junggar Basin, the remote discovery site north of the Tian Shan range.

The discovery, made in 2002, is described in detail in today's issue of the journal Nature. Dr. Clark and other team members discussed the ancestral tyrannosaur yesterday at a news conference in Washington.

Two specimens of the new species were uncovered near one another. The most revealing one, the scientists said, was a nine-foot-long, 12year-old adult with the crested head believed to be typical of the species. The other was a smaller, 7-year-old juvenile. Almost immediately, Dr. Clark said, "we knew we had something fairly rare."

The clearest evidence of an ancestral link to tyrannosaurs were the teeth and pelvic structure of the two skeletons. Closer examination, Dr. Clark said, dispelled any lingering skepticism and showed a definite relationship with later tyrannosaurs.

Mark A. Norell, a paleontologist at the American Museum of Natural History in New York and a team member, said, "The discovery of this basal tyrannosaur is giving us a much broader picture of the diversity of this group and its ancestors."

Dr. Norell noted several primitive traces in the skeletons, including the presence of long forearms and three-fingered hands. The well-known T. rex, which lived about 70 million years ago, toward the end of the Cretaceous period of geologic time, evolved short forearms that were virtually nonfunctional two-fingered hands, and a mammoth body two or three times the length of these early ancestors.

The differences suggest that the newfound animals were an intermediate step in evolution between primitive coelurosaurs, a group of birdlike dinosaurs, and tyrannosaurs.

The skeletons were found in sediments from the late Jurassic period, when the site in the desert basin was a warm land of lakes and marshes. The region of the discovery, near China's borders with Mongolia and Kazakhstan, was previously explored by a Chinese-Canadian fossil hunting expedition in the 1980's. Other paleontologists said they were not surprised that the region had yielded more discoveries from earlier epochs in the time of dinosaurs.

Only a few scraps of dinosaur fossils were previously uncovered in the Jurassic deposits, but Dr. Clark said the age of the Guanlong specimen was "about where we would expect the oldest tyrannosaurs to be."

The earliest previously known tyrannosaur was a 130-million-year-old feathered specimen, Dilong paradoxus, which American and Chinese scientists reported two years ago. No signs of feathers were found on the two Guanlong specimens.

The presence of a crest on the Guanlong adult's head was a complete surprise, Dr. Clark said, showing that there was "clearly still much more to be learned about early tyrannosaurs."

The research team said the crest was about as thin as a tortilla and only two and a half inches high. It appeared to be filled with air sacs and reminded the paleontologist of the ornamental features found on some living birds, like cassowaries and hornbills.

Dr. Norell said the crest was too thin to have provided much protection, or to have been used in butting heads in combat. More likely, he said, the crest of these "crowned dragons" had something to do with attracting mates or identifying fellow species members.


Perhaps the most interesting thing about this ancestral Tyrannosaurid is the possibility that it may have been feathered, as shown in the figure. Although no signs of feathers were found on the two specimens of Guanlong, other fossil Coelurosaurs (including Dilong paradoxus) show evidence of feathers. Just as hair became less luxuriant among larger mammals (who retain heat well without insulation), it may be that feathers became less important among the larger Tyrannosaurids. Alternatively, we may not have yet found any feathered Guanlong simply because it takes very special conditions to preserve something as fragile and evanescent as feathers.


Location Online:
New York Times

Original posting/publication date timestamp:
February 9, 2006


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