Their cecum is enlarged and sacculate, and in it some bacterial digestion of cellulose takes place. Early in the Tertiary, this was a dominant group that included 14 families and many species. Now, all that remain are 3 living families, with 18 species in all. Their decline accelerated during the Oligocene and coincided with the rise of another group of large herbivorous and cursorial mammals, the artiodactyls . Modern perissodactyls are native to Africa, south and central Asia, southern North America, and northern South America.
Most species are herbivorous.
Orders and Families of Recent Mammals of the World. Savage, R. J. G. and M. R. Long. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. Vaughan, T. A. 1986.