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extinct group of hoofed mammals found as fossils, mostly in South America, although the oldest forms seem to have originated in East Asia. Notoungulates lived from the late Paleocene Epoch (about 57 million years ago) to the beginning of the Pleistocene Epoch (about 1.8 million years ago) and were most diverse during the Miocene Epoch (23-5.3 million years ago). By the Pliocene Epoch (5.3-1.8 million years ago) their numbers and diversity were reduced, perhaps owing to changes in climate and geography. In South America, notoungulates evolved and diversified in isolation; they became extinct after the land connection between North and South America was reestablished about 3.5 million years ago.