英語聽力：自然百科 撒哈拉的史前鱷魚化石

A National Geographic researcher and his team have uncovered fossilized remains of 5 ancient crocodile species that lived among dinosaurs some 100 million years ago. One of the crocs had an oversized set of canines, and another had a snout like a duck's bill.

They were discovered in the North African Sahara, where National Geographic Explorer-in-Residence Paul Sereno began expeditions 15 years ago.

During those expeditions, Sereno first discovered an ancient croc, popularly known as Supercroc. At 40 feet long and ten tons, it was big enough to take down a dinosaur.

These stubby teeth didn't even touch each other to snare a fish? No, they were hook-like, strongcylinders to grab onto a dinosaurs limb or neck and pull it into the water. We've begun to understand this animal as a hidden predator of the dinosaurs.

Sereno and his team soon discovered key fossils of previously unknown or poorly understood species, most of them walking upright.

I began to realize that we really have half a dozen new species. And one of the great things we found in the Sahara was this world of crocodilians that we had no idea existed when we set foot there.

Sereno assembled a team of experts to re-create what these crocs might have looked like with computers and elastic casts. One of the most beautifully preserved specimens, Sereno calls it

'little duck croc", is flat-billed with long-sleek leg bones and about 3-feet long.

What would a long-limbed crocodile be like? I mean, would it really be like a mammal? How would it have moved? How does this galloping crocodile do it?

And another of the new species, the 20-foot, triple fanged reptile dubbed Boar Croc, could gallop, too. It had a head and mouth that could bring down a dinosaur. In the National Geographic Special, When Crocs Ate Dinosaurs, Sereno travels to Australia to witness living crocodiles with similar traits to the now extinct fossilized ancestors he's found in Northern Africa.

The Australian galloping crocodile, known as the freshie, can sprint on land at high speed, using the up and down motion of a true terrestrial animal. But in the water, goes back to the sidewinder motion of a tail-propelled river creature. It helps Sereno and his team visualize how the ancient crocs, especially the boar croc, could have been a terror to the dinosaurs.

They would have pursued dinosaurs on land with agility. And then they would have turned around and dove into the water, and swam away like a freshie. That I really think is the secret to crocodile success.

The other ancient crocs discovered are: the Pancake Croc, 20 feet long with a thin, 3-foot long set ofjaws; Dog Croc was about 3-feet long and had a very large forebrain, possibly indicating higher intelligence, and likely was as comfortable on land as in the water; and Rat Croc was about 3-feet long with a pair of buckteeth in the lower jaw to flesh out small prey. When Crocs Ate Dinosaurs premieres November 21st, at 9pm Eastern on the National Geographic Channel.