NOTOUNGULATA

extinct group of mammals found as fossils, mostly in South America, in deposits of the later Paleocene Epoch (66.4 to 57.8 million years ago); the oldest forms seem to have an East Asian origin. The peak of notoungulate evolution occurred during the Oligocene Epoch (36.6 to 23.7 million years ago). By the Pliocene Epoch (5.3 to 1.6 million years ago) they had been reduced in numbers and diversity; the reason may have been climatic. In South America the notoungulates evolved and diversified free from competition from more advanced forms that were evolving elsewhere. When North America and South America were rejoined late in the Pliocene, the notoungulates, apparently unable to compete with more efficient forms from North America, became extinct. In their time the notoungulates included a variety of hoofed animals whose development paralleled the evolution of more advanced forms elsewhere. One group, the toxodonts, was clumsily built and rather massive; Toxodon itself stood about 2 m (6 feet) high at the shoulder. Other notoungulates developed along lines similar to the rabbits and rodents.