An international team of scientists believes two Chinese fossils of feathered dinosaurs -- animals with down-covered bodies, strong legs and stubby arms -- are the strongest evidence yet that birds evolved from dinosaurs.
"This shows that dinosaurs are not extinct, but are well-represented by 10,000 species of birds," paleontologist Philip Currie said Tuesday.
Although the fossilized dinosaurs are thought to have been capable of running swiftly, flapping feathered wings and fanning out impressive tail feathers, they were unable to actually fly, said Currie, who is curator of the Royal Tyrrell Museum of Paleontology in Canada.
The fossils, which were unearthed in the Liaoning province in China, date back more than 120 million years and offer, according to Currie, conclusive proof that birds evolved from dinosaurs, a theory that has been hotly contested for more than 20 years.
A team of scientists identified the fossils as two separate species, and their findings were published in National Geographic magazine and the journal Nature.
Resembling the earliest known bird
The new fossils closely resemble the earliest known bird, called Archaeopteryx, which dates back 140 million to 150 million years.
Although the new fossils closely resemble Archaeopteryx in some ways, they lack the precise form of true birds -- in particular the length of wing and design of individual feathers. For this reason, the researchers believe the fossils were true dinosaurs that are the immediate ancestors of the first birds.
"They represent a missing link between dinosaurs and birds which we had expected to find," said Ji Quiang, director of the National Geological Museum in Beijing, who worked on the fossils.
Both fossils were removed by the Beijing team from a dry lake bed formation in northeast China. The area has earned fame in recent years for containing rich deposits of dinosaur remains.
Fast runners, unable to fly
The two species, called Caudipteryx zoui, or "tail feather," and Protoarchaeopteryx robusta, both were fast runners and were probably unable to fly, judging from the short arms and long legs. Their feathers may have been for insulation or display, Currie said.
Protoarchaeopteryx, which was about the size of a modern-day turkey, is the more primitive and earlier of the two fossils, said Mark Norell, who also worked on the fossils. Norell is chairman and associate curator in the department of vertebrate paleontology at the American Museum of Natural History in New York,
Feathers covered its body but there is no preserved evidence of wing feathers.
Caudipteryx had more plumage, including a generous tail fan. It stood about three feet tall.
Both animals closely resemble meat-eating dinosaurs called theropods, Currie said.
"These fossils are things we predicted would be there but seriously in my lifetime I never thought we were going to find them," Norell said.
One scientist casts doubt
Alan Feduccia, an evolutionary biologist at the University of North Carolina, Chapel Hill, said the discoveries are "very interesting," but he said they do not provide immediate and final proof that birds evolved from dinosaurs.
He said the new fossils are dated after those of the first bird, suggesting that the fossils could be either feathered dinosaurs or primitive birds that happened to resemble dinosaurs.
"The age dates for these things are still unresolved," said Feduccia. "We need to back up and take a closer look at these things before drawing any final conclusions."
Currie said there also may be a division between theories on the evolution of feathers and the evolution of flight. It was once thought that they evolved together.
He said the feathers on the dinosaurs probably evolved for warmth, suggesting that some of the animals may have been warm-blooded.
Flight may have been a later development.
Currie said there are two theories for the origin of flight: that fast-running, birdlike animals developed wings that enabled them to lift away from the ground, or that animals living in trees evolved first into gliders, such as the flying squirrel, and then later into true fliers.
The two fossils appear to support the ground-to-air theory instead of the tree-to-ground theory.
Correspondent Kyoko Altman, The Associated Press and Reuters contributed
to this report.