Horse-family fossils begin fifty-four million years ago
Equidae, the horse family, holds horses, asses, zebras and many species known only as fossils. Over more than 50 million years it turned small, many-toed browsers into large animals standing on one toe, and every living member belongs to a single genus, Equus, which arose in North America.
The oldest fossils placed in the family come from early Eocene North America, about 54 million years ago. They were once filed under Hyracotherium, but the type species of that genus turned out to be a palaeothere, so the rest were moved to other genera. These first equids were about the size of a fox, with four toes on each front foot and three behind. They nibbled soft plants, were already built for running, and had brains complex enough to suggest alert, intelligent animals.
Later species lost toes and gained teeth that could grind grass and other tough food. Like their relatives, equids ferment food in the hindgut, slicing fibrous plants with shearing teeth. That seemingly wasteful digestion only works for fairly large mammals, so they must already have been sizeable when it evolved.
The family diversified in the Miocene, when many new species appeared with the familiar horse shape. Many carried their weight on the central third toe, while the side toes shrank until they barely touched the ground, if at all. Equus had evolved by the early Pleistocene and spread swiftly around the world.
The family sits in the order Perissodactyla, which today also includes four tapir species in one genus and five rhinoceros species in four genera. Within that order it forms the superfamily Equoidea with just one other family, the extinct palaeotheres. Any member of the family, living or extinct, may be called an equid.
Source: Equidae
