Leapin’ Lizards

Biomechanics suggests how a giraffe-size pterosaur took flight By Stuart Fox

For almost a century, scientists struggled to explain how the extinct reptiles called pterosaurs managed to get off the ground. In regard to the smaller pterosaurs, bird models sufficed; flapping from standstill or a running start could work. But for the larger pterosaurs, some of which had a 26-foot wingspan and weighed 200 pounds, scientists could not find a bird model that explained takeoff.

That is because they did not take off like birds, thinks Michael Habib, who studies functional anatomy and evolution at Johns Hopkins University. After analyzing the biomechanics of the creatures, Habib proposes that pterosaurs took flight by using all four limbs to make a standing jump into the sky, not by running on their two hind limbs or jumping off a height, as more widely assumed.

“I started as a bird researcher,” Habib says. “I became interested in mechanical limits in flying animals, and that naturally leads to pterosaurs.”

And pterosaurs such as Quetzalcoatlus sit firmly on the far end of those limits. Even with its birdlike hollow bones, Quetzalcoatlus weighed between 250 and 550 pounds and had about a 36-foot wingspan. By comparison, an albatross weighs about 18 pounds and has an 11-foot wingspan. It had to take off somehow, but no one had a good guess how.

By analyzing the shape of the pterosaur arm bones, Habib calculated that the forelimbs could withstand stresses far greater than those encountered during flight. But why evolve reinforced wings if they would never experience high stress? Habib then made the connection between the quadrupedal gait of the large pterosaurs and the jumping quadrupedal takeoff he had seen in vampire bats. If the large pterosaurs used all four limbs to get off the ground, that would explain both the superstrong forelimbs and solve the mystery of pterosaur takeoff.

But just because an animal could do something does not mean it did, and some paleontologists remain unconvinced that Habib’s data actually explain how pterosaurs
got off the ground. “When I read the manuscript, my first reaction was, ‘Hmm, that’s odd.’ But if you work on pterosaurs, you get used to odd things anyway,” remarks David Unwin, a paleontologist at the University of Leicester in England and author of the book The Pterosaurs: From Deep Time. “Large and giant pterosaurs pose a problem,” he explains, “because the flying speed they need to achieve is quite high, 30 or 40 miles per hour, and I have a hard time understanding how they get that fast from a standing jump.”

Paleontologist Kevin Padian of the University of California, Berkeley, also questions some of Habib’s conclusions. Padian says he believes the smaller pterosaurs (some were the size of sparrows) were bipedal and thus took off with two legs, not four. He also does not think Habib has covered every kind of relevant bone stress.

The divide between the pterosaur researcher and the researcher looking at
Pterosaurs is fairly common in this area of paleontology. According to both Unwin and Habib, pterosaurs, with their improbable size and ability to fly, draw in biomechanics experts more focused on physics than prehistoric biology. “Because of the bizarre nature of pterosaurs, they’ve attracted attention from outside paleontology,” Unwin says. “So we’ve had a disproportionate number of people come in from outside paleontology, lots of people ready to have a go at the aerodynamics who are not pterosaur researchers first.” Still, all agree that Habib brings up interesting points, and they are not just for biomechanics. Showing that the large pterosaurs could take off without having to jump off a cliff expands the range of places they could have lived, raising all kinds of questions about the ecology of large pterosaurs. Says Padian: “Every time we think we’ve figured them out, they throw us another curve.”

Stuart Fox, a freelance writer based in New York City, studied pterosaur biomechanics while working at the Field Museum of Natural History in Chicago.

Source of Information : Scientific American(2009-05)