I. Overview
The perissodactyls (odd-toed ungulates) are an order of ? land mammals that comprise, in modern times, horses ■ (including zebras and asses), rhinoceroses, and tapirs. Perissodactyls are mainly characterized by their feet. There is an odd number of toes, and the axis of symmetry of each fore-and hindfoot runs through the middle (third) digit (this is called mesaxony). The third digit is the largest in all perissodactyls and is the only digit in horses. The toes immediately adjacent to the third digit (the second and fourth) are similar to each other and smaller than the central toe. Rhinos have three toes per foot and tapirs have three on their hindfeet and four on the forefeet. The fourth toe in a tapir is medial to and smaller than any other toe. All toes bear a hoof in perissodactyls, just like in the even-toed ungulates (artiodactyls), where there are two or four toes per foot and the axis of symmetry runs between the third and the fourth digit (paraxony).
II. Relation to Marine Mammals
The earliest perissodactyl fossils known date from the early Eocene, approximately 52 million years ago, and are close temporally to the earliest cetaceans and sirenians. Perissodactyls are related to marine mammals. Studies of morphological data (Domning, 1992; Fischer and Tassy, 1993) commonly find that perissodactyls are close relatives to sirenians, proboscideans, and hyracoids. Molecular data disagree with that, instead finding that certain insectivores are related more closely to sirenians (Stanhope et al., 1998). A minority opinion is that perissodactyls are closely related to whales, but most evidence instead points to a close relationship between whales and artiodactyls.
