UK researchers examine 150 million years of flight evolution

The pterosaurs, more popularly known as pterodactyls, were the first vertebrate animals to evolve powered flight, according to most scientists. They are thought to have achieved sky dominance almost 80 million years before modern birds. 

Pterosaurs died out in the sudden mass extinction at the end of the Cretaceous, generally believed to have been caused by an asteroid strike on the Earth about 66 million years ago.

An article published in the Oct. 28 edition of "Nature," as well as recently in Cold Spring Harbor Laboratory's journal bioRxiv, examines the pterosaurs' flight efficiency as it evolved over the 150 million years before these dinosaurs went extinct.

The two lead writers among the five United Kingdom-based researchers of the article are Chris Venditti, professor and evolutionary biology lecturer at the University of Reading, and Stuart Humphries, professor of evolutionary biophysics at the University of Lincoln.

The pterosaurs' ancestors were small, probably bipedal early archosaurs who were well adapted to terrestrial locomotion, according to the "Nature" article. Sometime during the Triassic Era, more than 220 million years ago, pterosaurs took to the skies where they "appear to have become capable and efficient flyers," the article said.

"However, the evolutionary processes that led to this efficiency remain enigmatic," the article continued. "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."

How early pterosaurs overcame the steep energetic hill of transitioning from locomotion on the ground to flying remains unclear, but once over the top "flight must have provided some offsetting fitness benefits," the article said.

"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," the article continued. "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."

Once they'd taken to the skies, pterosaurs continued to increase their flight efficiency over the 140 million years they existed on the planet. Through natural selection, their body size and wing area evolved "to sculpt these enigmatic creatures from what might have been inefficient flyers that only took to the air for only short spells, to creatures that could fly long distances over extended periods of time," the article said.

Pterosaurs also continued with the bigger-is-better approach even as they began to compete with Mesozoic birds, which "had a profound effect on pterosaurs, increasingly driving smaller species to extinction," the "Nature" article said.

"Why the birds had a competitive advantage is not clear, but further investigation into the relative flight efficiency of birds and pterosaurs at small sizes is likely to be enlightening," the article continued. "Our results highlight that after the appearance of pterosaur flight, there was still significant room for improvement in terms of efficiency."

Over the centuries, pterosaurs consistently adapted better and the study found "no evidence of a slowdown in the rate of efficiency increase," the article said.

"It is interesting to consider whether this process of fast evolutionary change to a new mode of locomotion, followed by a long but consistent process of fine tuning of efficiency, is a characteristic of all such evolutionary transitions," the article concluded. "With that question in mind, our approach demonstrates the power of combining biophysical models and phylogenetic statistical methods with the fossil record to understand the evolution of flight in pterosaurs. In doing so we offer a blueprint to objectively study functional and energetic changes through geological time at a far more nuanced level than has ever before been possible."