Equidae

Equidae (horses; order Perissodactyla, suborder Hippomorpha) A family that includes the modern horses, asses, and zebras (all of which are placed in a single genus Equus, divided into six or seven species, depending on the classification used) and many extinct forms, the earliest being known from the late Palaeocene. Sufficient of these are believed to be ancestral to modern equids for the evolution of the family to have been traced in considerable detail. The domestic horse (E. caballus) is probably not descended from the only true living wild horse, Przewalski’s horse (E. ferus przewalskii), but from a progenitor closely related to it. Modern equids are adapted for rapid movement: they have a short humerus and femur, and a long radius and tibia; the ulna is reduced and fused to the radius, and the fibula is also reduced. Only the third toe is developed. All the incisors are present, canines are present in males, and the cheek teeth are hypsodont, with complex grinding surfaces. The brain is large and senses of sight, smell, touch, and hearing are highly developed. Equids are migratory grassland animals, graze mainly on grasses, and have complex social organization. Wild species are found in Africa and parts of central and western Asia. The horses are first represented in the fossil record by Hyracotherium, which diverged from a condylarth predecessor in the Palaeocene. Numerous evolutionary lines subsequently appeared from this fox-sized prototype. Most of the evolutionary advances occurred in the New World, although the Equidae were to survive the Pleistocene only in the Old World. The conquistadores reintroduced horses to the Americas in the 16th century.