"Today, the middle of Wyoming is a vast desert, and a few antelope and deer are all you see," said lead author Michael Woodburne, honorary curator of geology at the Museum of Northern Arizona. "But 50 million years ago, when temperatures were at their highest, that area was a tropical rainforest teeming with lemur-like primates, small dawn horses and a number of small forest rodents and other mammals. In fact, there were more species of mammals living in the western part of North America at that time than at any other time."By the end of the Oligocene, all of the North American primates had died out or moved south because of the shifting temperatures.
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