The first functional biplane took off more than 100 million years before the Wright brothers puttered over Kittyhawk, North Carolina. That's the conclusion of a new study that suggests that a small, tree dwelling dinosaur called Microraptor gui held its two pairs of wings one over the other, much like the Wright brothers' design. The findings support the idea that the ancestors of present-day birds began to fly by gliding between trees.
Microraptors are among the closest relatives of modern birds and lived during the Cretaceous Period nearly 125 million years ago. Unlike today's birds, these chicken-sized animals had long primary feathers, or flight feathers, not only on their forelimbs, but also on the lower part of their hind legs. The animals sported sharp, curved claws that appear suited for perching, and their long leg feathers would have hindered walking or running on the ground. Combining these lines of evidence, paleontologists Xing Xu, Zhonghe Zhou, and colleagues at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, proposed in 2003 that microraptors were tree-dwellers that spread out their wings one behind the other, much like a dragonfly (ScienceNOW, 22 January 2003).
Paleontologist Sankar Chatterjee of Texas Tech University in Lubbock was skeptical. When he and aeronautical engineer Jack Templin reanalyzed the skeletons, they realized that the hind leg posture proposed by Xu's team is impossible. To extend their hind limbs horizontally, the animals would have to dislocate their hips. Instead, the dinosaurs must have held their back legs under their bodies, much like eagles and falcons do when they are homing in on prey. Thus the hind wings would be in a staggered position below and slightly behind the larger forewings. By simulating the dinosaurs' aerodynamics, the duo concluded that this arrangement would have also caused the primary feathers on the hind legs to face the wind, reducing the amount of resistance and making the animal an excellent glider. The authors report their findings in this week's Proceedings of the National Academies of Sciences.
"I think the biplane idea is a better idea," says Zhou. Microraptors' glidiing ability makes the tree-down hypothesis for bird evolution more credible, says Chatterjee. Vertebrate paleontologist Larry Witmer of Ohio University in Athens regards the dichotomy between arboreal and ground-dwelling hypotheses as false and argues that the evolution of flight probably involved a combination of the two. Yet, he says, the "evocative and provocative piece of work" does offer a solution to the problem of how this dinosaur "with wings on its feet, like Mercury the God" flew. The idea will need further testing with experiments using microraptor models, Witmer says.