"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."
More National Geographic News Stories on Dinosaurs:
Dinosaur Cannibal?Mystery in New Mexico
Utah Dinos May Have Been Killed By Drought
Cuban Dinosaur: First Confirmed Remains Discovered
Tetrapod Fossil FoundFirst Ever in Asia
New Picture of Dinosaurs Emerging
Fossil Implies Our Early Kin Lived in Trees, Study Says
Weird Buck-Toothed Dinosaur Found
Dinosaur Tracks Preserved on Scottish Island
Comets May Have Led to Birth and Death of Dinosaur Era
Dinosaur Tracks Shed Light on Sauropod Evolution
Fossil of Dog-Size Horned Dinosaur Unearthed in China
Tyrannosaurus rex Was a Slowpoke
Researchers Rethink Dinosaur Die Off Scenario
Researchers Melt Polar Dinosaur Mysteries
Scientist's Finds Spur New Thinking on Dino Evolution
Dino-Era Vomit Fossil Found in England
Study Paints New Picture of Dinosaur's Nose
Skeleton of New Dinosaur "Titan" Found in Madagascar
"Tidal Giant" Roamed Coastal Swamps of Ancient Africa
"Feathered" Fossil Bolsters Changing Image of Dinosaurs
Oddly Angled Teeth Make Masiakasaurus Stick Out
New Find: Pterosaur Had Strange Crest, Fishing Style
Dinosaur Beak Probably Used to Strain Food, Not Kill Prey
Additional Dinosaur Resources from National Geographic:
Paul Sereno: National Geographic Explorer-in-Residence and Dinosaur Hunter
Destinations: Dinosaur National Monument
Use this National Geographic News article in your classroom with these Xpeditions lesson plans and student activity:
K-2: Dinosaur Bodies
3-5: How Do Scientists Find Dinosaur Fossils?
6-8: The Science of Digging Up Dinosaurs
9-12: The Evolution of Dinosaurs Over Geologic Time
K-2: Those Fussy Dinosaurs!
9-12: Physical Characteristics of Places: The Fossil Record
Activity: A Dinosaur's Neighborhood
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