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WINGS OF CHANGE: The small feathered dinosaur Microraptor possessed asymmetric wings on parts of its arms and legs—signs of aerodynamic streamlining. Click for full-size image Image: COURTESY OF XING XU
The invention of the biplane marked the beginning of manned flight, and now some researchers believe that the wing-on-top-of-wing design was present in an early flying animal as well. A new reconstruction of a controversial feathered dinosaur called Microraptor gui has the small beast gliding between treetops on a stacked pair of wings. If the finding withstands scrutiny, then the biplane design is 125 million years older than the Wright brothers' 1903 flight would suggest.
Discovered four years ago in northeastern China, Microraptor is one of more than half a dozen small feathered dinosaurs found in the region that may represent some of evolution's early—and possibly dead-end—experiments with feathered flight.
On parts of the creature's arms and legs, the discoverers observed a series of long asymmetric feathers, with one side of the feather narrower than the other. In modern birds such asymmetry is considered a hallmark of aerodynamic function—like the asymmetry of an airplane wing. Based on the orientation of the feathered remains, the group imagined the dinosaur flying spread-eagled, with wings trailing from each limb.
Paleontologist Sankar Chatterjee of Texas Tech University and his colleague, retired aeronautical engineer R. Jack Templin of Ottawa, Ontario, take issue with the reconstruction in a paper published online by the Proceedings of the National Academy of Sciences USA. "We realized there's something wrong in the anatomy because no dinosaurs could splay their legs sideways," Chatterjee says. "No birds can do that."
Chatterjee and Templin instead envisioned Microraptor's legs tucked under its body, with its rear feathers parallel to and beneath the feathers on its outspread arms. "It's a very common posture in living raptors," such as when holding prey, Chatterjee says. That's also a more realistic position, they say, because it puts the leading, narrower edges of the leg feathers forward against the direction of airflow, like the arm feathers. The spread-eagled wing plan had the feathers in clashing orientations, they note.
"It is pure conjecture," asserts evolutionary biologist Kenneth Dial of the University of Montana in Missoula. Dial describes the report as "an armwaving evolutionary interpretation" and says, "this is what has plagued the 'origin of avian flight' literature for 150 years." [See note below.]
Others see it differently. The biplane model is "creative and interesting," says evolutionary ornithologist Richard Prum of Yale University. "We have such radically new creatures, like this guy [Microraptor]. Everybody's scrambling to understand it," he says. "It's important that we explore lots of ideas."
In 2005 Dial and another researcher, writing in Nature, in which Microraptor's discovery was also published, issued a strongly worded comment arguing there was no evidence that such small feathered dinos flew (as opposed to ran), or that their wings could even support the loads required for flight.
"It's patently obvious that this creature had some kind of aerial behavior because of the nature of the wings," Prum insists. He says it's up in the air whether modern birds evolved from a four-winged design or Microraptor's lineage developed this trait on its own.
Prum says to better understand how the dinosaur might have flown, researchers will have to build realistic models that show how its feathers would likely have been positioned as the limbs moved around. "There are legitimate questions as to how this thing functioned," he says.
Note: parts of this article were modified after publication.