Equidae
Taxonomic family of horses, asses, zebras, and extinct relatives.
Equidae, commonly known as the horse family, is the taxonomic family of horses and related animals, including asses, zebras, and many extinct species known only from fossils. The family evolved more than 50 million years ago, in the Eocene epoch, from a small, multi-toed ungulate into larger, single-toed animals. All extant species are in the genus Equus, which originated in North America. Equidae belongs to the order Perissodactyla, which includes the extant tapirs and rhinoceros, and several extinct families. It is more specifically grouped within the superfamily Equoidea, the only other family being the extinct Palaeotheriidae.
- Evolved
- more than 50 million years ago, in the Eocene epoch
- Oldest known fossils
- 54 million years ago, early Eocene epoch, North America
- Extant genus
- Equus (22 species, 7 extant)
- Order
- Perissodactyla
- Superfamily
- Equoidea
- Key adaptation
- hindgut fermenters, specialized teeth for tough plant matter
Lore & Background
The oldest known fossils assigned to Equidae were found in North America, dating from the early Eocene epoch, 54 million years ago. They were once assigned to the genus Hyracotherium, but the type species of that genus is now regarded as a palaeothere, with other species split into different genera. These early equids were fox-sized animals with three toes on the hind feet and four on the front feet, herbivorous browsers on relatively soft plants, already adapted for running. The complexity of their brains suggests they were alert and intelligent animals. Later species reduced the number of toes and developed teeth more suited for grinding grasses and other tough plant food.
Reader's Guide
Equidae, as hindgut fermenters, evolved specialized teeth that cut and shear tough plant matter to accommodate their fibrous diet. Their seemingly inefficient digestion strategy is a result of their size at the time of its evolution, as they would have already had to be relatively large mammals to be supported on such a strategy. The family became relatively diverse during the Miocene epoch, with many new species appearing. By this time, equids were more truly horse-like, having developed the typical body shape of modern animals. Many of these species bore the main weight of their bodies on their central third toe, with the others becoming reduced and barely touching the ground. The sole surviving genus, Equus, had evolved by the early Pleistocene epoch and spread rapidly through the world. The family's classification includes three subfamilies—Eohippinae, Anchitheriinae, and Equinae—with numerous extinct genera and tribes such as Hipparionini and Equini. The term 'equid' refers to any member of this family, including any equine.
Did You Know?
- The oldest known Equidae fossils date from 54 million years ago in the early Eocene epoch, found in North America.
- Early equids were fox-sized with three toes on hind feet and four on front feet.
- All extant species of Equidae belong to the genus Equus, which originated in North America.
- Equidae are hindgut fermenters, a digestion strategy linked to their size at the time of evolution.
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