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6 minute read
J. Madeleine Nash

There was nothing modest about the beast named Quetzalcoatlus. This winged creature–the largest flying machine nature ever constructed–was the size of a small airplane. It was nearly 20 ft. long; its wings stretched 40 ft. across; and it boasted a toothless, 6-ft.-long beak that tapered to the width of chopsticks. What on earth, scientists have long wondered, did such a big animal eat?

That was one of the questions researchers tried to answer last week at the annual meeting of the Society of Vertebrate Paleontology in New York City, which featured a symposium focusing on the ancient winged creatures known collectively as pterosaurs. Were flying giants such as Quetzalcoatlus carrion eaters, like outsize vultures, as researchers once proposed? Or were they–as Thomas Lehman, of Texas Tech University, and Wann Langston Jr., of the Texas Memorial Museum, convincingly argued last week–more like humongous storks, probing the lake bottoms for tasty tidbits and snaring them with their lancelike beak?

When dinosaurs ruled the earth, Quetzalcoatlus and its cousins dominated the skies. Yet ever since their fossils were first discovered in the 1700s and mistaken for strange marine creatures or bats, pterosaurs–literally, winged lizards–have remained a perplexing enigma. Did these extraordinary beasts take off by running on the ground or by dropping from a tree? Did they energetically flap their wings or deploy them as passive sails? Did they, like seabirds, nurture their young in large colonies, or did they lead a solitary life?

As last week’s meeting made clear, pterosaurs continue to confound. These bizarre animals, the first vertebrates that truly flew, are a “biological oxymoron,” says paleontologist Kevin Padian of the University of California, Berkeley. “Apart from the fact that they flew, there isn’t a thing that all the experts agree on. How can animals that are so familiar to generations of schoolchildren be so confusing to the people who study them?”

In fact, scientists have learned quite a bit about pterosaurs. For example, they know that the first pterosaurs shared a common ancestor with the dinosaurs and appeared on earth at roughly the same time. The earliest pterosaurs sported long, bony tails that functioned as dynamic stabilizers. By contrast, later models looked more like birds, with their tails shrunken to stubs and their necks and heads greatly elongated. They lacked feathers, but their bodies were probably wrapped in a thick, furlike cloak. The range in size of pterosaurs was enormous. Some were as small and sprightly as robins, while others–like Quetzalcoatlus–were giants five times the size of a whooping crane, the largest bird in North America today.

Like birds, which evolved independently 70 million years later, pterosaurs had bones that were hollow and lightweight. (One scientist refers to pterosaur skeletons as “Styrofoam and mailing tubes.”) But of all the trademarks of a pterosaur, one of the most peculiar was its hand, which boasted three clawlike fingers of normal size and a fourth digit that was outlandishly long. It was this fourth finger that provided structural support for the wings. Made of a skin-like membrane, the wings were supported by thousands of microscopic fibers that acted rather like the ribs of a folding umbrella, creating a flexible structure that was stiff enough to be aerodynamic.

So far, so good. The trouble starts when scientists try to extrapolate patterns of behavior and locomotion from the fossil evidence. At last week’s meeting, for example, scientists debated whether pterosaurs walked on two legs, like birds, or crawled on all fours, like bats. Hundreds of footprints discovered at dozens of sites in the U.S. and Europe over the past few years, argues Martin Lockley of the University of Colorado at Denver, strongly support the latter conclusion. The pattern of these footprints, which range in size from 1 in. to 5 in., suggests that pterosaurs held their bodies in a semierect position, with their long wings folded back so that their clawlike fingers gripped the ground.

Berkeley’s Padian, on the other hand, contends that pterosaurs did not have to walk on their wings, but were agile two-legged runners. He also disagrees with the explanation University of Bristol paleontologist David Unwin offers for the long fifth toe that juts out from pterosaur hind limbs. Unwin believes this toe served as the attachment site for a second skinlike membrane that stretched between the animals’ hind limbs. “Why else would the fifth toe have been so elongated?” he asks. Padian responds that Unwin’s membrane does not make anatomical sense: among other things, it would have hampered pterosaurs’ ability to move on the ground.

Endless debate also swirls around the question of how pterosaurs managed to become airborne. Some scientists think the beasts launched themselves from a running start, while others believe they were so clumsy on the ground that they would have had to drop from cliffs or trees to attain a flightlike glide. Lockley, for one, argues that pterosaurs had to be capable of birdlike takeoffs and landings if only because so many pterosaur footprints come from mudflats along the seashore. If they were incapable of flying off after landing in such areas, he says, they would have quickly died out.

What did pterosaurs eat? Because pterosaur fossils and footprints have been found primarily in marine and freshwater environments, paleontologists believe many pterosaurs had tastes similar to modern sea- and shorebirds. For example, the ancient salt lakes in southwestern Texas in which Quetzalcoatlus was found contained lots of crustacean burrows but no bones from larger animals like crocodiles that might have fed a carrion eater. Some pterosaurs had beaks shaped like those of spoonbills. Pterodaustro had a mouthful of strainer-like teeth that it probably used to filter microscopic plankton from the water. Pteranodon is thought to have scooped up its prey and stashed it in a pelican-like throat pouch.

What researchers don’t know about pterosaurs still far outweighs what they do know, but at least some of the dark corners of pterosaur life are beginning to divulge their secrets. A group of fossils discovered in Chile, for example, hints that pterosaurs may have nested in colonies. Some pterosaurs, like some birds, were crowned with dramatic crests that probably played a role in sexual display. But reconstructing their lost world is made difficult by the vagaries of fossil preservation and by the fact that these winged reptiles were evolutionary dead ends and left no descendants. Pterosaurs, for this reason, will forever remain wondrously strange. “In the living world,” says Peter Wellnhofer of Munich’s Bavarian State Collection for Paleontology and Historical Geology, “there’s nothing really comparable to a pterosaur.”

–Reported by Andrea Dorfman/New York

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