Four-winged dinosaurs

[Mighty_Emperor]

Four-Winged Dinosaurs Found in China, Experts


Hillary Mayell
for National Geographic News
January 22, 2003


Paleontologists in China have discovered the fossil remains of a four-winged dinosaur with fully developed, modern feathers on both the forelimbs and hind limbs.

The new species, Microraptor gui, provides yet more evidence that birds evolved from dinosaurs, and could go a long way to answering a question scientists have puzzled over for close to 100 years: How did a group of ground-dwelling flightless dinosaurs evolve to a feathered animal capable of flying?

Xu Xing, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, and colleagues suggest in the January 23 issue of the journal Nature that the species is an early ancestor of birds that probably used its feathered limbs, along with a long, feather-fringed tail, to glide from tree to tree.

Microraptor gui is a new species of Microraptor, in the Dromaeosaur group—which are closely related to birds. An artist's impression illustrates feathered wings, which researchers believe the dinosaur used to glide from tree to tree, much like flying squirrels do today.


They argue that the animal represents an intermediate stage in the evolution of flight, from gliding much as flying squirrels do today to the active wing flapping of modern birds.

Xu's work has long been supported by the National Geographic Society's Committee for Research and Exploration.

The six specimens were excavated from the rich fossil beds of Liaoning Province in northeastern China. They are dated at between 128 to 124 million years old (Early Cretaceous).

"To have fully formed flight feathers on the hind legs is fascinating," said James Clark, Ronald Weintraub Associate Professor of Biology at George Washington University, Washington, D.C.

"There were some interesting speculations about 90 years ago that birds might have had four feathered limbs, but no one has suggested it in recent times, since all living birds use only their forelimbs," he said. "This find broadens the whole scope of thinking about the origins of flight."

The Bird-Dinosaur Connection

Much fossil evidence has been uncovered supporting the idea that birds evolved from a group of bipedal carnivorous dinosaurs called theropods. Within the theropod group, birds are most closely related to dromaeosaurids. Velociraptor, a star in the movie Jurassic Park, is probably the most famous of dromaeosaurs.

Earlier finds in Liaoning suggest that the earliest dromaeosaurs were small, feathered animals with forelimbs similar to those of Archaeopteryx, the oldest known bird at around 150 million years old, and feet with features comparable to modern tree-living birds.

"This species provides another link in the emerging transition from small, meat-eating dinosaurs to birds," said Hans-Dieter Sues, curator of vertebrate paleontology and associate director for science and collections at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. "These fossils fill in a blank in the fossil record."

Although the M. gui fossils are about 25 million years younger than Archaeopteryx, the four-winged dinosaur is a more primitive form derived from a very early evolutionary branch of dromaeosaurs. The Chinese scientists suggest that the four-winged dinosaur is the most recent known common relative shared by both birds and dinosaurs.

From the Ground or the Trees

M. gui, which is about three feet (1 meter) long, provides evidence that the evolutionary transition to birds included an intermediate phase when there were flight feathers on the hind limbs. The existence of such a stage in the evolution of birds was long ago postulated by naturalist William Beebe in 1915, and by the Danish ornithologist Gerhard Heilmann in 1927. Whether that means they could glide, or even that they were tree-dwellers, remains open to debate.

There are two competing hypotheses that attempt to explain the origin of flight: "ground up" and "tree down."

"Ground up" proponents argue that ground-dwelling dinosaurs achieved flight by developing legs designed for speed and rudimentary wings that when vigorously flapped could either increase velocity or give enough lift to enable the dinosaurs to launch themselves into the air.

The "tree down" hypothesis suggests that birds' most recent ancestors were tree-dwelling creatures that took advantage of gravity and learned to glide before flapping their wings for fully powered flight.

"The argument over whether birds evolved the ability to fly from trees or from the ground is oversimplified and there's no way you can test it," said Clark.

It's just as likely that there was an evolutionary intermediate phase before the gliding phase that got the dinosaurs off the ground and into the trees, he said.

Xu and colleagues support the tree-down hypothesis, arguing that the four-winged dinosaur was built for gliding and the long feathers on M. gui's feet would be a hindrance to running.

"This time the evidence is overwhelming," Xu said. "It's hard even to imagine how these little animals could have moved around bipedally."

Other paleontologists urge caution in interpreting the fossil evidence.

"Xu and colleagues argue that before you could get to flight you had to go through a gliding phase," said Clark. "And intuitively it makes sense that it would be simpler for flight to evolve for a gliding creature than it would be for a ground-dwelling animal.

"That's certainly a possibility, and very plausible, but interpreting function from a fossil is highly speculative," he said. "There's just a lot we aren't going to know."

Mark Norell, chairman of the division of paleontology at the American Museum of Natural History, concurs. "I think you have to be really cautious about inferring biomechanical properties from fossils. However, what these fossils do tell us is that there are some really strange creatures out there, unlike anything around today.

"This shows that people really have to change their conceptions of what dinosaurs are all about," he said. "They're incredibly varied, incredibly diverse, and include everything from small feathered creatures to the large dinosaurs people are most familiar with."

http://news.nationalgeographic.com/news/2003/01/0121_030122_dromaeosaur.html

There is apicture gallery too.

Emps