Air-filled bones helped prehistoric reptiles take first flight

New international research involving the University of Leicester published today sheds new light on how prehistoric reptiles took to the air. In the Mesozoic Era, 70 million years before birds first took wing, pterosaurs dominated the skies with sparrow to aeroplane-sized wingspans. Scientists already knew, on the basis of fossil evidence from the wings, that these extinct reptiles were able to power their flight through flapping, but had little understanding of how pterosaurs met the high energetic requirements for flight.

The new research published in the journal PLoS ONE by researchers from the University of Leicester (UK), Ohio University (USA) and College of the Holy Cross (USA) explains how balloon-like air sacs, which extended from the lungs throughout the body, hollowing out many of the bones in the pterosaur skeleton, provide evidence for a remarkably efficient breathing system in these ancient beasts. As an important bonus, the pneumatised skeletal system and air sacs reduced the density of pterosaurs, allowing the evolution of the largest vertebrates ever to take flight, some reaching 10 metres in wingspan.

'We have identified the breathing system of a pterosaur. It's a surprisingly efficient mechanism with the same essential structure of a modern bird's lung apparatus - except 70 million years earlier' said study co author Dave Unwin, a palaeobiologist in the Department of Museum Studies at the University of Leicester.

Leon Claessens, an assistant professor of biology at the College of the Holy Cross and Patrick O'Connor, an assistant professor of biomedical sciences in Ohio University's College of Osteopathic Medicine, were inspired to conduct the study after viewing an extraordinarily well preserved rib cage of a pterosaur in Berlin in 2003. The fossil was shown to them by Dave Unwin of the Department of Museum Studies, University of Leicester, who was then curator at the Museum fuer Naturkunde in Berlin. Examining this specimen the three palaeobiologists realised that it could finally solve the mystery of how pterosaurs were able to power sustained flapping flight.

'The shape and size of the rib segments that articulate with the sternum (sternal ribs) show that, contrary to what was previously thought, the rib cage was mobile' said Claessens. Moreover, unique and previously unrecognised projections on the sternal ribs provided important leverage for the muscles that moved the ribcage.

Fossils rarely preserve soft tissue and to get a better understanding of the relationship between air sacs, lung structure and the skeleton the research team compared pterosaur specimens with modern bird skeletons by using CT scans and X-rays to take a deeper look at the bones. Many pterosaur bones had cavities that were invaded by air sacs. 'Amazingly, the air sacs extend all the way to the tips of the wings which opens up a wide range of possibilities for the use of air sacs during flight and for social behaviours' the team explained.

Further comparison to living creatures such as pelicans shows strong similarities in the relationship between size and density of the bones. 'We found a direct relationship between the proportion of the skeleton invaded by air sacs and the absolute body size of an animal' the team said. Smaller birds, such as songbirds, have few pneumatised bones because they weigh less and expend less energy in flight. Conversely, most bones in larger birds, as well as pterosaurs, are pneumatised which significantly reduces bone density and resolves a major problem with sustaining flight: the energetic cost of keeping a heavy body up in the air.

'Active flapping flight is extremely expensive in terms of energy expenditure' O'Connor explained. To keep up with the enormous need for energy, the team posit that the pterosaurs' pulmonary systems took up much of their bodies. Such a pulmonary air sac system would have facilitated the necessary gas exchange to enable sustained activity.

The research was funded by the University of Leicester, the Museum fuer Naturkunde, Berlin, the National Science Foundation (USA), Harvard University and Ohio University College.