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About the Woolly Mammoth:
Unlike the case with most dinosaurs, paleontologists know exactly what the prehistoric elephant Mammuthus primigenius (better known as the Woolly Mammoth) looked like, thanks to the discovery of complete specimens buried in Arctic permafrost. About the size of modern elephants, Woolly Mammoths were distinguished by the camel-like humps on their shoulders, their long, woolly hair, and their enormous, curved tusks. (See 10 Facts About the Woolly Mammoth.)
Unlike its leaf-eating relative, Mammut (better known as the Mastodon), the Woolly Mammoth grazed on grass, like an enormous cow. Thanks to various cave paintings, we know that the Woolly Mammoth (like the other mammalian megafauna of the last Ice Age) was hunted to extinction by the prehistoric human settlers of Eurasia, who likely coveted this beast's shaggy coat and impressive horns as much as its ample meat. (Amazingly, despite the ongoing carnage, a pygmy species of Mammuthus managed to survive until 4,000 B.C. on Wrangel Island, in the Arctic Ocean.)
Scientists think it may yet prove possible to "clone" a Woolly Mammoth, by scraping together enough preserved genetic material to constitute an entire genome and then implanting the fertilized egg in a female elephant, where it can gestate. Unfortunately, this same technique won't work for dinosaurs, since there's no reasonable prospect of assembling a complete dinosaur genome (much less finding a living relative close enough to reliably complete the gestation!).