"We believe Gigantoraptor kept those feathers from its ancestors," said Xu, who likened the dinosaur's appearance to that of a mammoth-size ostrich.
Previously the biggest known feathered animal was an 1,100-pound (500-kilogram) prehistoric flightless bird that lived in Australia six to eight million years ago, he added.
The researchers theorize that Gigantoraptor may have used its feathers for display or for incubating its eggs. Past studies suggest oviraptorosaurs may have had long feathers on their arms and bodies for covering their eggs.
"This is one of the hypotheses to explain how long feathers evolved on arms," Xu said.
Gigantoraptor may also account for huge clutches of fossilized theropod eggs measuring ten feet (three meters) in diameter previously found in China, said Luis M. Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County.
"It is perfectly possible that animals like Gigantoraptor were responsible for laying these gigantic nests," Chiappe said.
The species shows "gigantism needs not to be correlated with loss of birdness" in theropod dinosaurs, he added.
The fossil also revealed various unexpected features, the team said, including more birdlike arms and legs than those seen in related dinosaurs.
"It looks like it has a number of features which are present in birds which it didn't inherit from its own relatives," said dinosaur researcher Paul Barrett of the Natural History Museum in London, England.
"Those birdlike features may have appeared once in this animal and once in birds," he added.
The massive oviraptorosaur also "throws up all sorts of questions regarding the biology of these animals," Barrett said. "It makes it even more difficult to work out whether these things would have been herbivores or carnivores."
The study team notes that the toothless dinosaur shares other physical traits with herbivores such as a small head and a long neck. (Related: "'Bizarre' New Dinosaur Shows Evolution to Plant Eating, Study Says" [May 4, 2005].)
But it was also armed with fearsome claws almost 20 centimeters (8 inches) long. "The claws are so long they are more like those of a meat-eating dinosaur," Xu, the lead researcher, said.
"Maybe it ate small animals, or clams, or plant seeds, or even eggs," he added.
His team says the dinosaur seems to have had an accelerated growth rate, faster than that of other large theropods.
"That would probably be the mechanism by which it got to its large body size," Barrett, of the Natural History Museum, said. "Instead of growing for longer to get larger, it probably grew faster."
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