The ancient pterosaur was a slow flier that coasted on light air currents and could soar for hours. Colin Palmer, a graduate student at the University of Bristol, arrived at this conclusion by employing his expertise as a turbine engineer to carry out first-of-a kind tests on models of pterosaur wings in a wind tunnel.
Pterosaurs were enormous reptiles (but not dinosaurs) that lived and flew until 65 million years ago. Fossil records show that their unique limbs could have supported flight, but unlike bat wings or bird wings, they were made of a living membrane reinforced with muscle and tissue, stretched like a sail over a single long bone. Without a living analogue, the mechanics of pterosaur take-off, flight and landing, have been part conjecture and part theory. A study published November 15 in PLoS ONE filled in some pieces of the puzzle, and offered one explanation for how the animals took off: Pterosaurs launched off the ground using all four limbs, reached a very high speed in half a second, and quickly gained altitude. Now, Palmer's wind tunnel tests with models of the pterosaur wing are a second chapter to this story, filling out the full picture for how these reptiles used their unique limbs to stay in the air. The results are detailed online in the November 24 issue of the Proceedings of the Royal Society B: Biological Sciences.
Palmer designed wings from a combination carbon fiber and epoxy resin in thin curved sheets, for a wingspan of about 20 feet, simulating the structure of the pterosaur wing based on fossil evidence. He then measured drag and lift under different wind conditions, varying the shape of the cross section of the supporting bone. His results showed that the pterosaur wing was very sensitive to thermal lifts. Launching on four legs, the pterosaur would have flapped its wings till it caught these small pockets of warm air rising from ocean or hot land, and then coasted easily on these for several hours. For the larger pterosaurs, soaring would use less energy than flapping flight. Palmer's study also suggests that the floppy pterosaur had a hard time navigating strong winds, unlike albatrosses that plunge into storms and surf strong breezes.
"I think the overall membrane dynamics he's looking at are very good," says Michael Habib, a pterosaur expert at Chatham University who co-developed the quadruped launch theory for pterosaur take-off. "But I'm a little skeptical of their extreme vulnerability to turbulence and strong winds," says Habib. A living wing membrane, layered with tissue and muscle, would be able to tense and relax in sections and control flight better, Habib says, though these were mechanical limitations expected from the reconstructions that Palmer was testing. "I do actually think there's probably more control in the [living] membrane than he's allowing for."
According to Palmer's reconstruction, pterosaur flight was slow but well-controlled, and pterosaurs could circle ominously in one area for hours, like a hawk or an eagle, perhaps waiting for prey to emerge from hiding. "I think that's kind of awesome and kind of frightening," says Habib, "because it's one thing if you're talking about a predator with a 4-foot wingspan and another if it has a 35-foot wingspan."
The slow-soaring pterosaur would have landed slowly as well, which might have helped preserve its flight-adapted light bones, unsuited for high impacts. "Like airliners you don't have to be efficient when you're landing and taking off; you just need to be slow so you don't break anything," says Palmer. The real wing, a much more complicated version of his reconstruction, would have loosened or tightened like a boat sail. "Physics is physics whether you're a pterosaur or a sailing boat," Palmer says.
This study also offers a slice of pterosaur life history that is out of reach of fossil evidence, suggesting that the reptiles lived within easy access of warm thermal wind currents near open spaces of land or near the ocean. "Fossils show you where the animals died, or where they washed up," says Habib. "Having something like Palmer's work tells you where they liked to spend their time when they were alive."