PHOTOS: 7 Major "Missing Links" Since Darwin

PHOTOS: 7 Major
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Thrinaxodon: The Emerging Mammal

Identified from fossils in South Africa and Antarctica, this archaic proto-mammal emerged on a reptile-ruled Earth some 245 million years ago.

Why it matters: An almost perfect intermediate between mammals and reptiles, Thrinaxodon has played a key role in unveiling the evolution of mammals. Descended from a reptile group called cynodonts, Thrinaxodon was a cat-size burrower that had scales and laid eggs. But, like mammals, it had whiskers, warm blood, and, scientist suspect, a fur coat.

"Thrinaxodon shows mammal-like features beginning to kick in," said paleontologist Paul Barrett of the Natural History Museum in London. "The origin of mammals is exceptionally well understood, and there is a whole series of fossils forming a nice transformation series that shows how mammals evolved a bit at a time."

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