|Extant and subfossil genera|
For fossil genera and classification see text
Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. 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.
The term equid refers to any member of this family, including any equine.
The oldest known fossils assigned to Equidae were found in North America, and date from the early Eocene, 54 million years ago. They used to be assigned to the genus Hyracotherium, but the type species of that genus now is regarded to be not a member of this family. The other species have been split off into different genera. These early equids were fox-sized animals with three toes on the hind feet, and four on the front feet. They were herbivorous browsers on relatively soft plants, and already adapted for running. The complexity of their brains suggest that they already were alert and intelligent animals. Later species reduced the number of toes, and developed teeth more suited for grinding up grasses and other tough plant food.
The equids, like other perissodactyls, are hindgut fermenters. They have 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, with many new species appearing. By this time, equids were more truly horse-like, having developed the typical body shape of the 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, if at all. The sole surviving genus, Equus, had evolved by the early Pleistocene, and spread rapidly through the world.
- Order Perissodactyla (In addition to Equidae, Perissodactyla includes four species of tapir in a single genus, as well as five living species (belonging to four genera) of rhinoceros.) † indicates extinct taxa.
- Family Equidae
- Subfamily †Eohippinae
- Subfamily †Propalaeotheriinae
- Subfamily †Anchitheriinae
- Subfamily Equinae
- Genus †Merychippus
- Genus †Scaphohippus
- Genus †Acritohippus
- Tribe †Hipparionini
- Tribe Equini
- Genus †Haringtonhippus
- Genus †Heteropliohippus
- Genus †Parapliohippus
- Subtribe Protohippina
- Subtribe Equina
- Genus †Astrohippus
- Genus †Dinohippus
- Genus Equus (22 species, 7 extant)
- Equus ferus Wild horse
- †Equus algericus
- †Equus alaskae
- †Equus lambei Yukon wild horse
- †Equus niobrarensis
- †Equus scotti
- †Equus conversidens Mexican horse
- †Equus semiplicatus
- Subgenus †Amerhippus (this subgenus and its species are possibly synonymous with E. ferus)
- Subgenus Asinus
- Equus africanus African wild ass
- Equus hemionus Onager or Asiatic wild ass
- Equus kiang Kiang
- †Equus hydruntinus European ass
- †Equus altidens
- †Equus tabeti
- †Equus melkiensis
- †Equus graziosii
- Subgenus Hippotigris[a]
- Equus grevyi Grévy's zebra
- †Equus koobiforensis
- †Equus oldowayensis
- Equus quagga Plains zebra
- Equus zebra Mountain zebra
- †Equus capensis
- †Equus mauritanicus
- Subgenus †Parastylidequus
- †Equus parastylidens Mooser's horse
- †Subgenus Sussemionus
- incertae sedis
- †Equus simplicidens Hagerman horse
- †Equus cumminsii
- †Equus livenzovensis
- †Equus sanmeniensis
- †Equus teilhardi
- †Equus numidicus
- †Equus plicidens
- †Equus cedralensis
- †Equus stenonis group
- †subgenus Allozebra
- †Equus (A.) occidentalis western horse
- †Equus (A.) excelsus
- Genus †Hippidion
- Genus †Onohippidium
- Genus †Pliohippus
- Family Equidae
- "PBDB". paleobiodb.org. Retrieved 2021-07-18.
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