CHICAGO – Dinosaur hunters working in Inner Mongolia Wednesday announced they have discovered the fossil of an enormous birdlike dinosaur 300 times heavier than its closest relatives.

Dubbed Gigantoraptor erlianensis, the species lived 75 million to 95 million years ago, late in the dinosaur era. The relatively complete fossil specimen was about 25 feet long, 11.5 feet high at the shoulder and would have weighed more than 3,000 pounds, yet exhibited birdlike characteristics including a beak, hollow bones and, very likely, ornamental feathers.

The report in this week’s issue of Nature adds complexity to the story of bird evolution, a gradual shrinking process by which the huge thunder lizards were reduced to small, light and feathered creatures. The discovery of such a large hybrid reveals the twists and turns that evolution can take.

“This species is the largest of its kind, and a lot bigger than anyone ever expect these animals to get,” said Peter Makovicky, assistant curator of dinosaurs at the Field Museum in Chicago. “When animals get big they tend to look less birdlike; for instance, we have no birds that reach a ton or more in body size. But here, some of the birdlike traits are actually retained or developed even though the animal is very large.”

Dr. Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing and colleagues found the fossil in the Erlian Basin in northeastern China. Xu said the region has attracted attention since the 1920s but that several interesting discoveries have emerged in the area recently, including several other feathered dinosaurs.

By analyzing the state of growth in one of the fossil bones, researchers estimate the dinosaur was relatively young, about 11 years old at the time of death. That suggests a full-size Gigantoraptor may be even larger than this particular fossil indicates.

While the fossil does not prove the dinosaur had feathers, researchers agree that it likely did.

“One would assume Gigantoraptor is feathered since its ancestry lies with a group of animals that we know also had feathers,” Makovicky said.

However, its feathers would definitely not be used for flight.

“Being 3,000 pounds heavy, no one would expect this species to fly,” said Xu.

Experts say feathers likely first evolved as a form of insulation, similar to fur in mammals. But because retaining body heat is less of an issue for larger animals, researchers believe Gigantoraptor’s feathers would have served a different purpose: helping the animal communicate.

“Dinosaurs were clearly animals that liked to display to each other; that’s why they had crests, horns, and bumps,” said Philip J. Currie, professor of dinosaur paleobiology at the University of Alberta. “Feathers almost certainly developed first as insulation in small species, but other dinosaurs surely started using them as display. A species might have crown feathers at the top of its head or feathers at the end of its tail.”

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AP-NY-06-13-07 1650EDT