在线英语听力室_免费在线英语听力学习网站

This is Scientific American's 60-Second Science. I'm Karen Hopkin. This will just take a minute.

A bird in flight is a thing of beauty. Even their takeoffs and landings usually look effortless. But pterodactyls? Well, that’s another story. Scientists have long assumed that pterodactyls, also called pterosaurs, flew just like birds, and launched themselves using their hind¹ legs. Now a biologist at Johns Hopkins says that can’t be true. Instead, he says that pterosaurs needed all four limbs for liftoff.

Modern birds use their legs to launch and their wings to stay aloft. Once they’re in the air, their hind limbs are essentially² payload, carried along for the ride. That arrangement presents a problem. A bird’s legs have to be muscular enough to get Tweety off the ground, but not so big that they drag him down. That limits how big a bird can be. Some pterosaurs, on the other hand, were the size of a giraffe. And looking at the bones of three different pterodactyl species, the Hopkins scientist concludes that there’s no way those legs were strong enough to get that bulk airborne. Instead, he says a pterosaur used all four limbs, leapfrogging forward on its knuckles³, to propel itself into the air. Maybe not a thing of beauty. But it must have been something to see.

Thanks for the minute for Scientific American's 60-Second Science. I'm Karen Hopkin.