Scans of fossils have cracked the mystery of pterosaur power. The biggest animals ever to fly drew their energy from bird-like lungs that they evolved 70 million years before the first birds took to the air.
Leathery-winged pterosaurs evolved 220 million years ago, from the same group of reptiles that gave rise to crocodiles, dinosaurs, and later birds. Yet how pterosaurs powered their flight had been a mystery because their ribcages were thought to be inflexible, making their breathing inefficient.
Leon Claessens of the College of the Holy Cross in Worcester, Massachusetts, realised that view might be mistaken after pterosaur specialist Dave Unwin at the University of Leicester, UK, showed him a fossil Rhamphorhynchus with a beautifully preserved ribcage.
Computed tomography (CT) scans of this specimen – as well as other pterosaur fossils – revealed that the bones had been filled with air sacs that connected to lungs. It is likely that, as in living birds, flexing of the body caused the air sacs to push and pull fresh air through the lungs, allowing them to extract oxygen from the air more efficiently than mammalian lungs.
The air sacs bring an added bonus in that they reduce the weight of the bones. In both birds and pterosaurs, the volume of the air space inside the bones increases with the size of the animal. In the largest pterosaurs, whose wing spans could exceed 10 metres, even the outermost "finger" bones of the wings were hollow.
Intriguingly, the sacs connected with a pneumatic system under the skin, which pterosaurs might have been able to inflate to change the shape of their wings, Claessens says.
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