

Flashy feathers get all the attention in the evolution of dinosaur flight, but a more complex internal adaptation provided the aerodynamics necessary for lizards to go airborne.

Fossil analysis suggests that pterosaurs had sacs of air in their bodies, starting in their lungs and spreading beyond, even hollowing out their bones. They were, in short, not nearly as heavy as their size suggested.

"We offer a reconstruction of the breathing system in pterosaurs, one that proposes the existence of a mechanism with the same essential structure to that of modern birds — except 70 million years earlier," study co-author Leon Claessens, a biologist at College of the Holy Cross, said in a press release.

The findings, published Thursday in Public Library of Science ONE, answer a lingering mystery: pterosaur skeletons have features suggestive of wing flapping, but the energy necessary to keep such heavy bodies in flight would have been huge.

Claessens' team couldn't look at fossilization-unfriendly soft tissue, but instead compared the skeletons of large modern birds and pterosaurs. Using x-ray movies and CT scans, they charted the skeletal mechanics of wing flapping in birds, then found similar bone structures in the dinosaurs. They also observed nearly identical relationships between body size and bone density.

Without internal balloons and hollow bones, dinosaurs couldn't have flown, much less reached sizes comparable to small airplanes. Fortunately for humans, however, birds have followed an evolutionary path less prone to gigantism.

After all, who'd want to deal with Cessna-sized pigeons?

Citation: "Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism." By Leon P. A. M. Claessens, Patrick M. O'Connor, David M. Unwin. Public Library of Science ONE, Feb. 18, 2009.

Images: PLoS ONE

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