As a group Perissodactyla arose during the late Palaeocene or early Eocene (Norman and Ashley, 2000); this group resulted from major adaptive shifts, specifically the ankle joint that limit side movement, and a change in dental formation, particularly their molar pattern along with the jaw muscles (Radinsky, 1969).
Norman, J.E. & Ashley, M.V. (2000). Phylogenetics of Perissodactyla and Tests of the Molecular Clock. J. Mol. Evol. 50, 11–21.
Randinsky, L. (1969). The Early Evolution of the Perissodactyla. Evolution, 23(2), 308-328.