This drawing depicts a pterosaur called Darwinopterus hunting a small feathered dinosaur called Anchiornis. Researchers said their analysis of pterosaur fossils confirms the reptiles were strong flyers, contrary to some theories that they were flightless. ((Mark Witton, University of Portsmouth))

Giant prehistoric flying reptiles took off by vaulting themselves into the air with their arms, U.S. and British researchers propose.

The theory published this week in the journal PloS One counters other theories that pterosaurs — winged reptiles that died off 65 million years ago — must have been flightless as they seemed too heavy to launch themselves into the air with their legs.

"The problem with launch for any flying animals is you're basically starting at zero speed," researcher Michael Habib said in an interview scheduled to air Saturday on CBC Radio's Quirks & Quarks.

"Wings don't actually work particularly well when they're moving slowly," added Habib, a biologist at Chatham University in Pittsburgh who co-wrote the study with Mark Witton at the University of Portsmouth in England.

In fact, large flyers have to get up to speed far more quickly than small flyers.

That posed a difficult problem for pterosaurs, which could grow as tall as giraffes, stretched their leathery wings up to 10 metres (similar to a small airplane) and weighed 250 kilograms — 10 times as much as the largest flying bird on record.

Large birds typically run or jump into the air to take off, but the researchers discovered that, compared with birds, pterosaurs had weak hind limbs.

"It turns out that the arms are much stronger than the legs," Habib said.

The researchers propose that pterosaurs took off more like bats, which also have strong arms. They suggest the reptiles prepared to flight by squatting slightly on all fours, then kicked their legs off the ground.

"That's going to pivot them, sort of like a pole vault, over the arms," Habib said.

At that point, they would push off their arms, going into a giant leap, then raise and unfold their wings.

The researchers came up with their theory after carefully examining the fossils of pterosaurs, estimating their size and weight, and using the data to calculate their bone strength and flying performance.

"All the indications are they're very good at flying," Habib said.

In fact, based on comparison with migrating birds, he proposes some pterosaurs could keep flapping for 10 days straight, provided they fattened up to store enough energy beforehand.

"They should be able to go halfway around the world non-stop."

Animation showing pterosaur take-off (Direct Dimensions/University of Portsmouth)