Modern horses, rhinos, and tapirs belong to a biological group, or order, called Perissodactyla. Also known as "odd-toed ungulates," animals in the order have, as their name implies, an uneven number ...

Perissodactyla, Artiodactyla and Primates are extant mammalian orders that appeared abruptly at the beginning of the Eocene across the Holarctic continents 1, with little indication of their source.