英语听力：探索发现 2012-11-07 美洲大平原 American Serengeti—5

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Every now and then,new clues surface, hinting at what else might lie beneath. ln this dried-up pond in South Dakota, known as Hot Springs, scientists unearthed¹ great piles of bones. What kind of creature died here? The bones reveal it stood four metres tall and weighed more than 10 tonnes. There's nothing fitting that description living here today.

 

Here's the give-away, a pair of tusks² two metres long, the trademark³ of a Columbian mammoth⁴, the biggest animal to roam the ice age plains. By comparing it to elephants in Africa today, can we shed light on how those ice age elephants lived and what they lived on? 

 

These are mammoth teeth, huge molars the size of bricks. They have deep ridges⁵ very similar to those of modern elephants, suggesting mammoths, too, survived by grinding vast amounts of grass.

 

Plant fragments trapped between the ridges can still be identified today. Thousands of years after this mammoth died, we know exactly what it ate for its last meal. Grass is a tough, abrasive⁶ food. Even with protective enamel⁷ ridges, these teeth would gradually have worn down. But just like modern elephants, the mammoths had evolved a way to deal with this. As one set of teeth was eroded⁸, another grew up to take its place. The evidence suggests they had six sets in all, to last a lifetime, up to 60 years.

 

The South Dakota mammoth didn't make it to old age and it was not alone. The site turned out to be a mammoth graveyard⁹, hiding more than 50 skeletons, all from animals in their prime. There's no sign they were killed by hunters, so how did so many healthy mammoths die?

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