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any of various extinct hoofed mammals that first appeared in the Paleocene Epoch (which began about 65.5 million years ago) and died out during the Pleistocene Epoch (which ended about 11,800 years ago). The order was restricted to South America, but in many ways, the evolution of the litopterns paralleled that of hoofed mammals in the Northern Hemisphere. Two distinct lineages of litoptern evolution are discernible in the fossil record.