How pterosaurs took flight

CLEVELAND — Extinct flying reptiles known as pterosaurs — especially large ones, which had wingspans as broad as a single-engine plane — may have taken flight by leapfrogging into the air, new analyses suggest.

Pterosaurs, a group that includes pterodactyls, lived in the era of the dinosaurs. Like modern birds and many other animals, pterosaurs are tetrapods, or creatures with four limbs. While birds on the ground use only two of those limbs to get around, trails of fossilized footprints indicate that pterosaurs walked on all fours, says Michael Habib, a biomechanicist at the Center for Functional Anatomy and Evolution at Johns Hopkins University School of Medicine in Baltimore. Most previous studies have presumed that pterosaurs spring into the air from a bipedal stance, like modern birds do, but analyses of pterosaur anatomy don’t support that idea, Habib reported October 17 at the annual meeting of the Society of Vertebrate Paleontology.

In most modern birds, the breaking strength of the major bone in the wing — analogous to the one in a human’s upper arm — is about the same as the breaking strength of the bone in the upper part of its leg. Nevertheless, between 80 and 90 percent of the power that a bird on the ground uses to get into the air comes from its legs and not its wings, field tests suggest. In large pterosaurs, however, the major bone in the wing can be as much as 40 times stronger than its leg bones, Habib says. Pterosaurs, he speculates, probably used that wing power to their advantage when taking off, and did not rely on their legs as modern birds do.

Simply leaping into the air and then flapping away probably wasn’t an option, considering some pterosaurs were the size of giraffes and had wingspans the size of a small aircraft (SN: 6/21/08, p. 7). Also, the largest of the pterosaurs may have weighed about 250 kilograms, Habib estimates, and therefore would have been too heavy to accelerate quickly from a standstill.

But with a leapfrog-like launching technique — a vault forward onto its clawed hands and then a quick push-up to spring into the air — the pterosaur could gain enough forward speed and altitude to flap away at a speed of about 30 miles per hour (48 kilometers per hour) in just a second or two, Habib’s calculations suggest. That’s more than quick enough to get away from a ground-based predator, he adds, noting that this technique would work even on flat terrain and without any wind.

The leapfrogging technique is “certainly plausible,” says David Unwin, a vertebrate paleontologist at the University of Leicester in England. “I first thought this idea was a little odd, but the more I think about it, the more I like it,” he adds. A leapfrogging pterosaur wouldn’t have to gain much altitude with its hop, he notes, because fossils suggest that most large species didn’t flap their wings below horizontal.

Although Habib’s idea remains just speculation, evidence to support the notion of leapfrogging pterosaurs might be preserved in fossilized footprints, Unwin says. Researchers would need to look for pterosaur trackways in which the last set of handprints were unusually deep, among other characteristics.