The earliest known bird, whose fossil bones have been the subject of wild and woolly debate among paleontologists for more than a century, was not warm-blooded as most scientists believe, but was cold-blooded like a reptile and capable of only short flights before overheating, according to new research reported yesterday.

The new interpretation, based on a re-analysis of the muscle mass and skeletal features of the 145-million-year-old bird, is sure to enliven the heated debate over the origin of flight and the early evolution of birds, which paleontologists believe descended from dinosaurs.

If the first bird was cold-blooded, that could answer a list of questions over how the earliest birds flew and whether the first birds lived on the ground or in trees. The earliest birds had a skeleton and structure that is decidedly non-aerodynamic. Researchers have spent decades pondering how birds got airborne.

John Ruben thinks he has the answer.

"I believe that early birds were not warm-blooded, but cold-blooded. They were physiologically reptilian. They were still birds. They had feathers. They flew. But they were not warm-blooded," said Ruben, a zoologist at Oregon State University in Corvallis, who presented his new interpretation yesterday at the annual meeting of the Society of Vertebrate Paleontology in Lawrence, Kan.

Per square inch, the muscles of cold-blooded animals like lizards and snakes are twice as powerful as the muscles of warm-blooded animals like dogs and modern birds. According to Ruben, the earliest bird had a slight skeleton that could not have carried enough warm-blooded muscle to get it airborne. But if the creature was cold-blooded, its muscles would have been much more powerful.

"I don't think anyone has suggested {early} birds were cold-blooded. It seems like almost a requirement," Ruben said. "But believe me, this is going to be very controversial."

"It will undoubtedly be controversial," said John Ostrom of Yale University, who has published extensively on early bird fossils and evolution. "It is a whole new perspective."

Archeopteryx was about the size of a pigeon, but far more primitive than today's birds. Indeed, when the first Archeopteryx fossils were unearthed more than 100 years ago, most paleontologists considered the animal a small dinosaur, complete with pointy teeth, three fingers on each claw and a long bony tail.

However, a later fossil revealed a curious feature: Archeopteryx had feathers. Ever since, paleontologists have argued over how Archeopteryx, which lacks the aerodynamic skeleton of modern birds, could have flown, or whether it flew it all.

Some paleontologists believe Archeopteryx lived in trees, although there has also been debate over whether the bird was a powerful enough flier to get into trees to begin with. Some have suggested Archeopteryx may have clawed its way up into the branches and then launched itself into flight.

Others suspect Archeopteryx was terrestrial but able to take off by running to pick up speed, like a modern roadrunner. Yale's Ostrom doubts that Archeopteryx flew at all.

"The fact that it had feathers doesn't mean it flew," he said. Many ground birds -- such as emus, ostriches and kiwis -- do not fly. Ostrom believes the feathers could have served another role, providing camouflage, coloration, warmth or water replenishment.

"I'll show that Archeopteryx could do both. No problem. It could take off from a tree or from the ground. But it wasn't able to go long distance. Only short-distance hops," Ruben said. Reptiles, while good at sprints, tire quickly.

Eventually, Ruben said, birds evolved into warm-blooded animals capable of long-distance flight. Indeed, at the paleontological meeting another researcher gave his colleagues a first look at a fossilized bird that lived about 135 million years ago, some 10 million years later than Archeopteryx.

This new fossil, unearthed by a boy in China in 1987 but not revealed until yesterday, was a creature similar to a modern bird that probably lived in trees and flew adroitly.

"It was clearly a flying, perching bird," said Paul Sereno of the University of Chicago, who has prepared and studied the fossil with his Chinese colleague Cheng-gang Rao. "It definitely pushes back the date when the first modern, flying birds arose."