In fact, pterosaurs -- believed to have died off 65 million years ago in the same cataclysm that killed off the dinosaurs on land and plesiosaurs at sea -- had much stronger "arms" than legs, which is the opposite of the way birds are built, Michael Habib, a researcher at the Johns Hopkins University medical school's Center for Functional Anatomy and Evolution, wrote in a study published in the German journal Zitteliana.

"The difference between pterosaurs and birds with regard to critical mechanical properties is very, very large, especially when you're talking about the big pterosaurs," Habib said in a statement.

"As the size gets bigger, the difference gets bigger too," he said.

His model indicated that Quetzalcoatlus pterosaurs, for instance -- one of the largest known flying animals of all time, with a 35-foot wingspan -- couldn't possibly launch themselves using their hind legs alone.

But by folding their wings and balancing on their "knuckles," they could push themselves off, almost like a leap-frogging long-jump, he said.

"Then, with wings snapping out, off they'd fly," he said.

"Using all four legs, it takes less than a second to get off of flat ground, no wind, no cliffs," Habib said.

"This was a good thing to be able to do if you lived in the late Cretaceous period and there were hungry tyrannosaurs wandering around," he said.

Habib's research was funded by the Jurassic Foundation, a non-profit organization providing funding for research on dinosaur paleobiology.

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