It is hard enough to imagine a 500-pound, furry reptile the size of a giraffe soaring overhead. How the largest pterosaurs actually launched their prehistoric bulk into the air in the first place has proved to be a much trickier problem for biomechanics researchers. Now a study published January 6 in the journal Zitteliana seems to have settled the question. John's Hopkins University of Medicine researcher Michael B. Habib showed that pterosaurs didn’t flap their wings to take off from two legs as birds do, but stood on all fours and hurled themselves skyward in a leapfrog-style jump.

Paleontologists don't lose too much sleep over the question of how such giant creatures could remain airborne. Soaring, which would have been the most efficient flight strategy for a pterosaur-size animal, takes little energy. And once cradled in an updraft, large objects such as condors and hang gliders can remain aloft for long periods, given a large enough wingspan and relatively light weight. Having lightweight bones relative to their size is a characteristic these dinosaur-contemporaries shared with the dinosaurs' descendants, living birds. But other features of the two groups skeletons couldn't be more different.

Habib calculated the limb bone strengths of twenty bird species using computer scans and compared them to the bone strengths of three pterosaur species, calculated in previously published studies. He found that the flying reptiles would never have been able to leap into the air from two legs as birds do—their hindlimbs were too weak. The front limbs, or wings, however were structurally much stronger. His data suggested that not only did pterosaurs walk on all fours, but they employed a limb-over-limb vault to take flight. 

This helps answer the question of why birds have never approached the towering heights of the larger pterosaurs. Birds' bipedal gait and liftoff requires leg bones and muscles strong enough to support the body's full weight and to supply 80 to 90 percent of the force required to jump into the air. A body built this way can get only so large before the weight of the legs becomes too heavy for flight. (Think of ostriches or the extinct, flightless elephant bird that had legs like tree trunks.)  Pterosaurs allocated much of this weight-bearing function to the front limbs. Serving as flight equipment as well, the weight of muscles and bones contribute thrust and soaring ability in the air instead of merely adding weight. Examining the mathematical possibilities, Habib could find no way pterosaurs could have taken flight from just two legs. The limitation didn't slow them down though. He estimates that using the leapfrog takeoff strategy, pterosaurs such as the huge Hatzegotpteryx could jump into the air in one second, without the benefit of wind, cliffs, or any other pterosaur to leap over. “Pterosaurs had long, huge front limbs, so no partner was required," he says. "Then, with wings snapping out, off they’d fly.”

The Pterosaur Database

John's Hopkins University Center for Functional Anatomy and Evolution

Stanford University - National Geographic Pterosaur Replica Project

Zitteliana