Equidae

Extinct equids restored to scale. Left to right: Mesohippus, Neohipparion, Eohippus, Equus scotti and Hypohippus

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, with many new species appearing. By this time, equids were more truly horse like, having developed the typical body shape of the modern animals.[5] 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.[6]

Skeletons

Protorohippus

Hipparion

Equus (Przewalski's horse)

• 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
      • Genus †Epihippus
      • Genus †Haplohippus
      • Genus †Eohippus
      • Genus †Minippus[7]
    • Subfamily †Propalaeotheriinae
      • Genus †Orohippus
      • Genus †Pliolophus
      • Genus †Protorohippus
      • Genus †Sifrhippus
      • Genus †Xenicohippus[7]
      • Genus †Eurohippus
      • Genus †Propalaeotherium?[8]
    • Subfamily †Anchitheriinae
      • Genus †Anchitherium
      • Genus †Archaeohippus
      • Genus †Desmatippus
      • Genus †Hypohippus
      • Genus †Kalobatippus
      • Genus †Megahippus
      • Genus †Mesohippus
      • Genus †Miohippus
      • Genus †Parahippus
      • Genus †Sinohippus
    • Subfamily Equinae
      • Genus †Merychippus
      • Genus †Scaphohippus
      • Genus †Acritohippus
      • Tribe †Hipparionini
        • Genus †Eurygnathohippus
        • Genus †Hipparion
        • Genus †Hippotherium
        • Genus †Nannippus
        • Genus †Neohipparion
        • Genus †Proboscidipparion
        • Genus †Pseudhipparion
      • Tribe Equini
        • Genus †Haringtonhippus[9]
        • Genus †Heteropliohippus[10]
        • Genus †Parapliohippus[10]
        • Subtribe Protohippina
          • Genus †Calippus
          • Genus †Protohippus
        • Subtribe Equina
          • Genus †Astrohippus
          • Genus †Dinohippus
          • Genus Equus (22 species, 7 extant)
            • Equus ferus Wild horse
              • Equus ferus caballus Domestic horse
              • †Equus ferus ferus Tarpan
              • Equus ferus przewalskii Przewalski's horse
            • †Equus algericus[11]
            • †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)[12]
              • †Equus andium
              • †Equus neogeus
              • †Equus insulatus
            • Subgenus Asinus
              • Equus africanus African wild ass
                • Equus africanus africanus Nubian wild ass
                • Equus africanus asinus Domestic donkey
                • †Equus africanus atlanticus Atlas wild ass
                • Equus africanus somalicus Somali wild ass
              • Equus hemionus Onager or Asiatic wild ass
                • Equus hemionus hemionus Mongolian wild ass
                • †Equus hemionus hemippus Syrian wild ass
                • Equus hemionus khur Indian wild ass
                • Equus hemionus kulan Turkmenian kulan
                • Equus hemionus onager Persian onager
              • Equus kiang Kiang
                • Equus kiang chu Northern kiang
                • Equus kiang kiang Western kiang
                • Equus kiang holdereri Eastern kiang
                • Equus kiang polyodon Southern kiang
              • †Equus hydruntinus European ass
              • †Equus altidens
              • †Equus tabeti
              • †Equus melkiensis
              • †Equus graziosii
            • Subgenus Hippotigris[lower-alpha 1]
              • Equus grevyi Grévy's zebra
              • †Equus koobiforensis
              • †Equus oldowayensis
              • Equus quagga Plains zebra
                • Equus quagga boehmi Grant's zebra
                • Equus quagga borensis Maneless zebra
                • Equus quagga burchellii Burchell's zebra
                • Equus quagga chapmani Chapman's zebra
                • Equus quagga crawshayi Crawshay's zebra
                • †Equus quagga quagga Quagga
                • Equus quagga selousi Selous' zebra
              • Equus zebra Mountain zebra
                • Equus zebra hartmannae Hartmann's mountain zebra
                • Equus zebra zebra Cape mountain zebra
              • †Equus capensis
              • †Equus mauritanicus
            • Subgenus †Parastylidequus
              • †Equus parastylidens Mooser's horse
            • †Subgenus Sussemionus[15]
              • †Equus ovodovi[16]
            • incertae sedis
              • †Equus simplicidens Hagerman horse
              • †Equus cumminsii
              • †Equus livenzovensis
              • †Equus sanmeniensis
              • †Equus teilhardi
              • †Equus numidicus
              • †Equus plicidens
              • †Equus cedralensis
              • †Equus stenonis group
                • †Equus stenonis Stenon zebra
                  • †Equus stenonis guthi
                  • †Equus stenonis senezensis
                  • †Equus stenonis pamirensis (Hippotigris pamirensis)
                  • †Equus stenonis petraloniensis
                  • †Equus stenonis vireti
                • †Equus sivalensis
                • †Equus stehlini
                • †Equus sussenbornensis
                • †Equus verae
                • †Equus namadicus
              • †subgenus Allozebra
                • †Equus (A.) occidentalis western horse
          • †Equus (A.) excelsus
            • • †subgenus Hesperohippus
                • †Equus (H.) pacificus
                • †Equus (H.) mexicanus
              • †Equus complicatus
              • †Equus fraternus[17]
              • †Equus major
              • †Equus giganteus
              • †Equus pectinatus
              • †Equus crenidens
          • Genus †Hippidion
          • Genus †Onohippidium
          • Genus †Pliohippus