Large Size Didn't Keep Pterosaurs Grounded

Until a couple of years ago, few researchers doubted that pterosaurs, the giant winged reptiles from the age of the dinosaurs, could fly. Then two papers appeared to ground at least the largest pterosaur species. Now a new study has put pterosaurs back in the air.

The first major challenge to pterosaur flight came from a team of Japanese and French researchers in 2009. Based on evidence from modern sea birds, the group concluded that any animal weighing more than 41 kilograms would have trouble staying aloft—a problem for larger pterosaurs, which tipped the scales at several hundred kilograms. "That paper irritated the heck out of most people who work on pterosaurs," says David Unwin, a paleobiologist at the University of Leicester in the United Kingdom.

But the criticism didn't end there. Earlier this year, a study by Canadian paleontologist Donald Henderson reached a similar verdict. He estimated that the pterosaur Quetzalcoatlus northropi, previously labeled the largest flier of all time, tipped the scales at 544 kilograms and thus could never become airborne. Like other skeptics, Henderson assumed that the winged beasts must have lost their flight capabilities somewhere during their evolution, forcing them to walk.

That sent paleontologists Mark Witton of the University of Portsmouth in the United Kingdom and Michael Habib of Chatham University in Pittsburgh, Pennsylvania, back to the fossil record, scant as it was. The duo analyzed bones from Texas, Romania, and Kansas. The record is so scarce, they didn't have much to work with beyond fragments of wing, leg, and neck bones, roughly about 5% of the body, though Witton says they got "the important bits" that could tell them about flying.

Taking flight. A reconstructed skeleton model shows how an extinct pterosaur could take off using all four of its legs.
Credit: Julia Molnar

The researchers found that the humerus bone in the arm of several pterosaurs was at least two times stronger than would be predicted for a bird of the same size. This finding lent support to one of Habib's suspicions: Unlike birds, which take off using only their puny hind legs, pterosaurs took off with all four of their limbs (see video). With the added force of their much more powerful forelimbs, even giants like Quetzalcoatlus could leave the ground, the team reports online today in PLoS ONE. "It's very difficult to explain why pterosaurs went to such lengths to have such powerful bones ... if they weren't doing anything with them," says Witton.

And once pterosaurs got airborne, they stayed aloft. Some of the fossils of Pteranodon, a pterosaur with an estimated 7-meter wingspan, were found in sediment that, at the time the animals lived, would have been far out in the ocean, hundreds of kilometers from the nearest shoreline. They're too well-preserved too have been washed there, says Witton. That leaves only one other possibility: Pteranodon must have flown.

For other pterosaur experts, the paper brings a mixture of relief and disappointment. "I'm absolutely confident that they're correct that these animals could fly," says Unwin. But on the other hand, "I don't think it's as effective a response as it could have been." Part of the reason, Unwin says, is that the authors didn't show all of their work. Some of the data and calculations they used to get their figures on bone strength and wingspan are missing from the paper. Also, many of their calculations depend heavily on the mass estimates for the pterosaurs. The authors assumed a mass of 250 kilograms for the giant Quetzalcoatlus when in reality, says Unwin, such mass estimates vary widely.

Henderson, who wrote the 2010 paper arguing that Quetzalcoatlus was too enormous to fly, also questions the authors' estimates of mass and bone strength. "They're doing an awful lot of extrapolation," he says. Until scientists find a complete skeleton of a giant pterosaur—or even half of one—Henderson says they won't have the evidence they need to say anything definitive. In the meantime, he says, "be cautious and wait for better fossil material. It will come."