Equidae (sometimes known as the horse family) is the
taxonomicfamily of
horses and related animals, including the extant horses,
asses, and
zebras, and many other
species known only from
fossils. The family evolved
around 50 million years ago from a small, multi-toed
ungulate into larger, single-toed animals. All
extant species are in the
genusEquus, 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 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. 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.[2] 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.[3] Their seemingly inefficient digestion strategy is a result of their size at the time of its evolution,[4] 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,[5] with many new species appearing. By this time, equids were more truly horse like, having developed the typical body shape of the modern animals.[6] 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 epoch, and spread rapidly through the world.[7]
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.
^Although the plains and mountain zebras have been placed in Hippotigris and Grévy's zebra in Dolichohippus, Vilstrup et al. (2013) finds that the plains zebra and Grévy's zebra are more closely related.[14] Groves and Bell (2004) place all three species in the subgenus Hippotigris.[15]
^Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 255.
ISBN1-84028-152-9.
^Cantalapiedra, Juan L.; Sanisdro, Oscar L.; Cantero, Enrique; Prado, Jose Luis; Alberdi, Mª Teresa Luis (4 August 2023).
"Evolutionary Radiation of Equids". In Prins, Herbert H.; Gordon, Ian J. (eds.). The Equids, A Suite of Splendid Species. Fascinating Life Sciences. Springer. pp. 27–45.
doi:
10.1007/978-3-031-27144-1_2.
ISBN978-3-031-27143-4 – via www.springer.com.
^Remy, Jean A.; Krasovec, Gabriel; Marandat, Bernard (2016). "A new species of Propalaeotherium (Palaeotheriidae, Perissodactyla, Mammalia) from the Middle Eocene locality of Aumelas (Hérault, France)". Palaeovertebrata. 40 (2): e1.
doi:
10.18563/pv.40.2.e1.
^Hay, Oliver P. (1915). "Contributions to the Knowledge of the Mammals of the Pleistocene of North America". Proceedings of the United States National Museum. 48 (2086): 535–549.
doi:
10.5479/si.00963801.48-2086.515
^Aouraghe, Hassan; et al. (1999). "Les équidés du Pléstocène supérior de la grotte Zouhrah à El Harhoura, Maroc". Quaternaire (in French). 10: 283–292.
doi:
10.3406/quate.1999.1649.
^Groves, C. P.; Bell, C. H. (2004). "New investigations on the taxonomy of the zebras genus Equus, subgenus Hippotigris". Mammalian Biology. 69 (3): 182–196.
doi:
10.1078/1616-5047-00133.