Equidae (commonly known as thehorse family) is thetaxonomicfamily ofhorses and related animals, includingasses,zebras, and many extinctspecies known only fromfossils. The family evolved more than 50 million years ago, in theEocene epoch, from a small, multi-toedungulate into larger, single-toed animals. Allextant species are in thegenusEquus, which originated in North America. Equidae belongs to the orderPerissodactyla, which includes the extanttapirs andrhinoceros, and several extinct families. It is more specifically grouped within thesuperfamilyEquoidea, the only other family being the extinctPalaeotheriidae.
The termequid refers to any member of this family, including anyequine.
The oldest knownfossils assigned to Equidae were found in North America, and date from the earlyEocene epoch, 54 million years ago. They were once assigned to the genusHyracotherium, but thetype species of that genus is now regarded as apalaeothere. 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.[3] 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, arehindgut fermenters. They have evolved specialized teeth that cut and shear tough plant matter to accommodate their fibrous diet.[4] Their seemingly inefficient digestion strategy is a result of their size at the time of its evolution,[5] as they would have already had to be relatively large mammals to be supported on such a strategy.
The family became relatively diverse during theMiocene epoch,[6] with many new species appearing. By this time, equids were more truly horse like, having developed the typical body shape of the modern animals.[7] 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 earlyPleistocene epoch, and spread rapidly through the world.[8]
OrderPerissodactyla (In addition to Equidae, Perissodactyla includes four species oftapir in a singlegenus, as well as five living species (belonging to four genera) ofrhinoceros.) † indicatesextinct taxa.
^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.
^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
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