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150 million years of sustained increase in pterosaur flight efficiency

By Chris Venditti, Joanna Baker, Michael J. Benton, Andrew Meade, Stuart Humphries

Posted 22 Jun 2017
bioRxiv DOI: 10.1101/153957 (published DOI: 10.1038/s41586-020-2858-8)

The long-term accumulation of biodiversity has been punctuated by remarkable evolutionary transitions that allowed organisms to exploit new ecological opportunities, and often resulted in large radiations of species. The Mesozoic flying reptiles — pterosaurs — which dominated the skies for over 150 million years (Myr), were the product of such a transition. The ancestors of pterosaurs were small and likely bipedal early archosaurs, which were certainly well adapted to terrestrial locomotion. More than 220 Myr ago, at some point in the Triassic, pterosaurs took flight and subsequently appear to have become capable and efficient flyers. However, the evolutionary processes that led to this efficiency remain enigmatic. Given the lack of proto-pterosaurs it is difficult to study how flight first evolved in this group, but we can test hypotheses about evolutionary changes to the energetics of locomotion following the transition to flight. Early pterosaurs were challenged by the costs of transitioning between forms of locomotion — from terrestrial to aerial. This imposed a steep energetic hill to climb which means that flight must have provided some offsetting fitness benefits. If the initial transition resulted in a form that was very well adapted to flight we would expect to see no directional change in flight efficiency throughout the history of pterosaurs. Alternatively, the transition may have produced a form that was able to fly but, was not under strong selection for efficiency owing to many benefits conferred by the lack of competition in the novel environment. In the latter case the evolutionary signal of natural selection acting to increase efficiency over millions of years should be detectable. Novel phylogenetic statistical methods and biophysical models combined with information from the fossil record mean we now have the opportunity to test this hypothesis.

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