"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."Note as always there are dissenting opinions:
"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."".
The study itself:
John Ruben et al. (1997, 1999, 2003, 2004) disputed this and suggested that dinosaurs had a "tidal" respiratory system (in and out) powered by a crocodile-like hepatic piston mechanism – muscles attached mainly to the pubis pull the liver backwards, which makes the lungs expand to inhale; when these muscles relax, the lungs return to their previous size and shape, and the animal exhales. They also presented this as a reason for doubting that birds descended from dinosaurs.
Critics have claimed that, without avian air sacs, modest improvements in a few aspects of a modern reptile's circulatory and respiratory systems would enable the reptile to achieve 50% to 70% of the oxygen flow of a mammal of similar size, and that lack of avian air sacs would not prevent the development of endothermy. Very few formal rebuttals have been published in scientific journals of Ruben et al.’s claim that dinosaurs could not have had avian-style air sacs; but one points out that the Sinosauropteryx fossil on which they based much of their argument was severely flattened and therefore it was impossible to tell whether the liver was the right shape to act as part of a hepatic piston mechanism. Some recent papers simply note without further comment that Ruben et al. argued against the presence of air sacs in dinosaurs.
A 2009 study showed that pterosaurs had a lung-air sac system and a precisely controlled skeletal breathing pump, which supports a flow-through pulmonary ventilation model in pterosaurs, analogous to that of birds. The presence of a subcutaneous air sac system in at least some pterodactyloids would have further reduced the density of the living animal.Like modern crocodilians, pterosaurs appeared to have had a hepatic piston, seeing as their shoulder-pectoral girdles were too inflexible to move the sternum as in birds, and they possessed strong gastralia. Thus, their respiratory system had characteristics comparable to both modern archosaur clades.