Modern birds sustain their flight with an efficient ventilation system that keeps air flowing to their muscles. But no one knew how pterosaurs, the first flying vertebrates, powered their wings. A new study in PLoS One concludes that ancient pterosaurs, flying reptiles that lived 220 million to 65 million years ago, did much the same, with a mobile rib cage and a system of air sacs distributed throughout the bones to help move air around.
Pterosaurs dominated the sky in the Mesozoic era. The reptiles ranged from sparrow-sized peewees to giants with the wingspan of a small airplane. Although pterosaurs and birds share very distant reptilian ancestors, pterosaurs were not birds. As a result of studying the reptiles' wing anatomy, scientists have long been confident that they flew by actively flapping--those wings weren't meant for gliding. But how did pterosaurs get enough energy to power those muscles and stay in the air? "When we think of reptiles, we think of animals that literally creep along the ground and are relatively sluggish," says paleontologist David Unwin of the University of Leicester in the United Kingdom. "If you take a crocodile or lizard and put wings on them, they would very quickly run out of energy to flap," because flapping takes enormous effort.
In 2003, paleontologist Leon Claessens of the College of the Holy Cross in Worcester, Massachusetts, visited Unwin, who was then at the Natural History Museum in Berlin. Unwin showed him a Rhamphorhynchus, a pterosaur from the late Jurassic period, in near-perfect condition. "Parts of the rib cage were so well preserved that they basically answered a lot of questions we had on how pterosaurs breathed," says Claessens. For comparison, the researchers examined more than 100 pterosaur fossils and made x-ray films of crocodiles and birds breathing. They concluded that the pterosaur rib cage was not immobile, as was previously thought, but expanded and contracted. They say this air circulation let pterosaurs exchange enough carbon dioxide and oxygen to keep up their high metabolism.
The researchers also studied the air spaces in pterosaur bones and concluded that they were associated with air sacs, arranged in patterns similar to those seen in modern birds. The bigger the pterosaur, the more air sacs, just like in modern birds. The air spaces help oxygen circulate and probably also made bones light enough for flight.
But matching anatomy in pterosaurs to modern animals may be misleading, says Jaap Hillenius, a functional morphologist at the College of Charleston in South Carolina. Pterosaurs left no descendants and are only distantly related to birds. It's possible that the new study is correct, Hillenius says, but he's skeptical. For example, he thinks the model of rib-cage movement doesn't allow enough air for active flight, and that the sternum was not strong enough to support such movement. "Until we find a living pterosaur," there's no way to know for sure—"and that's not going to happen."