Fossils suggest birds may have four-winged ancestor

Fossils with leg feathers discovered in northeast China suggest birds may have a four-winged ancestor that glided from ancient trees above its dinosaur cousins long before two wings took over to power flight.

The small animal called "Microraptor gui" -- in honor of Chinese paleontologist Gu Zhiwei -- was about 2 1/2 feet long and had feathers covering its legs that were similar to the feathers in its wings. The fossils were dated to 128 million years ago.

Other researchers hailed the discovery, but said the fossils don't necessarily mean all winged creatures share Microraptor gui as a common ancestor.

"It's a phenomenal find," said Luis Chiappe, a paleontologist at the Natural History Museum of Los Angeles County. Chiappe did not participate in the dig, but he has visited the fossil site in the Chinese province of Liaoning, northeast of Beijing. Liaoning has yielded several important specimens in recent years.

"We don't have anything that resembles this in the whole dinosaur and bird spectrum," Chiappe said.

Debate revived

Scientists say the fossils revive a debate between two theories of how dinosaurs might have evolved into birds. One theory holds that some of these apparent bird ancestors learned to flap their wings to power flight while they were gliding.

The other theory suggests they learned to fly by increasing their running speed with their wings and taking off from the ground.

Details of the fossils appear in the current issue of the journal Nature.

Paleontologist Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences described six fossils with leg feathers arranged in a pattern similar to wing feathers in modern birds.

"They are long and some have asymmetrical veins like flight feathers," Xu said.

The feathered legs amount to rear wings, Xu said. He speculated they could have been an intermediate stage before the arrival of the birdlike Archaeopteryx, whose flight was powered by two real wings.

Or, the feathered legs could've been an evolutionary dead-end, researchers said.

Legs didn't flap

The legs probably didn't flap like wings. They were oriented like the legs of typical dinosaurs and modern birds, meaning they fit vertically into hip sockets. It would have been extremely difficult to rotate the legs 90 degrees to gain lift for the creature and may have been aerodynamically unstable unless it was just used for gliding, Chiappe said.

"And if that were case it would be a total oddity - the weirdest creature in the world of dinosaurs and birds," Chiappe said.

Other scientists said the fossils add diversity to the story of flight, even if they don't immediately provide answers.

Ken Dial, head of a biological flight laboratory at the University of Montana, said there is room for both gliding and flapping dinosaurs in evolutionary history.

"Gliding represents a splendid example of convergent evolution," Dial said. "We should not be surprised to unearth gliding dinosaurs as we have numerous living-day examples of gliders in nearly all the vertebrate groups -- reptiles, mammals, birds and even parachuting amphibians."

Last week, Dial reported in the journal Science the way young birds such as turkeys and quail use their wings suggests their ancestors eventually learned to fly by running and flapping.

Paul Sereno, a University of Chicago paleontologist, said the best way to determine whether Microraptor gui was an intermediate stage in bird evolution or a dead end is to find other dinosaur fossils with feathered legs.

Sereno called the Xu study a "landmark paper" but added: "Whether this represents an intermediate form that all birds passed through is a question that's going to be hotly debated."