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Equus (horse)

Mammalia - Perissodactyla - Equidae

Discussion

The genus Equus includes many extinct species in addition to the living domestic horse, zebras, and asses. It originated in North America and emigrated to the Old World during the Pliocene. In the 1920s a spectacular mass mortality assemblage of the primitive species Equus simplicidens was found at the Gidley Horse Quarry in Idaho. Before the end-Pleistocene megafaunal mass extinction several species of horses lived in North America. Pleistocene horses fossils are very common, but their taxonomy is still poorly resolved because many of the older species names were based on poor type specimens.

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Taxonomy

Equus was named by Linnaeus (1758) [type Eurasian E. caballus]. It is extant. It is the type genus of Equoidea.

It was assigned to Bellua by Linnaeus (1758); to Perissodactyla by Owen (1848); to Solipedia by Leidy (1860); to Equinae by Hay (1902), Gidley (1907), Koufos (1992); to Pliohippina by Prado and Alberdi (1996); to Equini by Quinn (1955), Quinn (1957), Prothero and Schoch (1989), Kelly (1995), MacFadden (1998), Alberdi et al. (2014); and to Equidae by Leidy (1873), Cope (1881), Scott (1913), Oliver Schneider (1926), Schultz and Howard (1935), Stirton (1940), Hibbard (1955), Kurten and Anderson (1980), Thurmond and Jones (1981), Carroll (1988), Capasso Barbato and Gliozzi (1995), Wilson and Reeder (2005), Cisneros (2005), Salles et al. (2006), Eshelman et al. (2018).

Species

Asinus africanus, E. (Amerhippus), E. (Asinus) cumminsi, E. (Asinus) kiang, E. (Dolichohippus) enormis, E. (Dolichohippus) grevyi, E. (Hemionus), E. (Hesperohippus), E. (Parastilidequus), E. (Sussemionus), E. africanus, E. alaskae (syn. E. lambei), E. asinus (syn. Asinus africanus aegyptiacus), E. australis, E. caballus (syn. E. (Amerhippus) neogeus, E. curvidens, E. rectidens, Hipphaplus bravardi, E. hippagrus), E. calobatus, E. capensis, E. cedralensis, E. coliemensis, E. complicatus (syn. E. eous, E. americanus, E. major), E. conversidens (syn. E. barcenaei, Onager zoyatalis), E. eisenmannae, E. excelsus (syn. Onager arellanoi, Onager hibbardi, Asinus aguascalentensis), E. francisci (syn. E. quinni, E. (Asinus) calobatus, E. arrelanoi), E. fraternus, E. fromanius, E. giganteus, E. holmesi, E. intermedius, E. intermedius, E. jubatus, E. laurentius, E. leidyi, E. mexicanus (syn. E. occidentalis, E. pacificus, E. scotti, E. bautistensis), E. nalaikhaensis, E. nearcticus, E. nevadanus, E. niobrarensis (syn. E. hatcheri), E. nobisi, E. parastylidens, E. pectinatus, E. przewalskii, E. pseudaltidens (syn. Onager altidens), E. quagga, E. sanmeniensis, E. santaeelenae, E. semiplicatus, E. sivalensis, E. tabeti, E. taeniopus, E. yunnanensis, E. zebra, Hippotherium princeps, Hippotigris grevyi, Hippotigris robustus, Hippotigris stenonis, Onager onager

Species lacking formal opinion data

E. algericus, E. altidens, E. bressanus, E. ferus, E. germanicus, E. helmei, E. helmei, E. hydruntinus, E. koobiforensis, E. livenzovensis, E. marxi, E. mauritanicus, E. mosbachensis, E. namadicus, E. oldowayensis, E. petraloniensis, E. robustus, E. simionescui, E. stehlini, E. valeriani, E. verae

Synonyms

Equus (Megacephalon) was named by Hilzheimer (1912) [homonym of a bird]. It is extant.

It was replaced with Equus (Grevya) by Hilzheimer (1912); it was replaced with Equus (Ludolphozecora) by Griffini (1913); it was synonymized subjectively with Equus (Dolichohippus) by Skinner (1972).

It was assigned to Equus by Hilzheimer (1912).
Equus (Grevya) was named by Hilzheimer (1912). It is extant.

It was synonymized subjectively with Equus (Dolichohippus) by Skinner (1972).

It was assigned to Equus by Hilzheimer (1912).
Equus (Ludolphozecora) was named by Griffini (1913). It is extant.

It was synonymized subjectively with Equus (Dolichohippus) by Skinner (1972).

It was assigned to Equus by Griffini (1913).
Neohippus was named by Abel (1913).

It was synonymized subjectively with Equus by Prothero and Schoch (1989), Rincón et al. (2006).
Tomolabis was named by Cope (1892).

It was synonymized subjectively with Equus by Hay (1902).
Equus taeniopus was named by Heuglin (1861). It is extant.

It was recombined as Asinus taeniopus by Noack (1884); it was recombined as Equus asinus taeniopus by Lydekker (1916); it was considered a nomen dubium by Groves and Smeenk (2007).
Asinus africanus was named by Fitzinger (1858) [non Asinus africanus Heuglin and Fitzinger 1866]. It is extant.

It was considered a nomen nudum by Groves and Smeenk (2007).
Kraterohippus was named by van Hoepen (1930).

It was synonymized subjectively with Equus by Prothero and Schoch (1989).
Kolpohippus was named by van Hoepen (1930).

It was synonymized subjectively with Equus by Prothero and Schoch (1989).
Sterrohippus was named by van Hopen (1930).

It was synonymized subjectively with Equus by Prothero and Schoch (1989).
Haringtonhippus was named by Heintzman et al. (2017). It was considered monophyletic by Heintzman et al. (2017).

It was synonymized subjectively with Equus by Barrón-Ortiz et al. (2019).

It was assigned to Equini by Heintzman et al. (2017).
Allozebra was named by Trumler (1961).

It was synonymized subjectively with Equus by Mooser and Dalquest (1975).
Amerhippus was named by Hoffstetter (1950). Its type is Equus andium.

It was reranked as Equus (Amerhippus) by Skinner (1972), Zurita et al. (2004), Rincón et al. (2006); it was considered an invalid subgroup of Equus by Orlando et al. (2008).

It was assigned to Equus by Skinner (1972), Zurita et al. (2004), Rincón et al. (2006).
Asinus was named by Gray (1822) [type African Equus asinus, senior synonym of Asinus vulgaris Gray 1822; also said to be 1824]. Its type is Equus asinus.

It was reranked as Equus (Asinus) by Lydekker (1916); it was synonymized subjectively with Equus by von Meyer (1832), Sclater (1884), Thomas (1884), Pavlow (1890), Scott (1895), Osborn (1929), Schultz (1938), Popov (1939), McGrew (1944), Edinger (1948), Azzaroli (1966), Groves and Willoughby (1981), Prothero and Schoch (1989).

It was assigned to Equidae by Gray (1822), Gray (1825); to Equus by Lydekker (1916); and to Equini by Quinn (1955), Quinn (1957).
Dolichohippus was named by Heller (1912) [type African Equus grevyi].

It was synonymized subjectively with Hippotigris by Quinn (1955) and Trumler (1961); it was reranked as Equus (Dolichohippus) by Skinner (1972); it was synonymized subjectively with Equus by Lydekker (1916), Gazin (1936), Kurten and Anderson (1980), Groves and Willoughby (1981), Prothero and Schoch (1989), Downs and Miller (1994).

It was assigned to Equus by Skinner (1972).
Equus (Parastilidequus) was named by Mooser and Dalquest (1975).

It was considered an invalid subgroup of Equus by Downs and Miller (1994).

It was assigned to Equus by Mooser and Dalquest (1975).
Equus leidyi was named by Hay (1913). Its type specimen is Wagner Free Institute of Science 4086, a tooth, and it is a 3D body fossil.

It was synonymized subjectively with Equus fraternus by Sellards (1940), Lundelius (1972), Hulbert and Morgan (1989); it was considered a nomen vanum by Savage (1951).
Equus parastylidens was named by Mooser (1959). Its type locality is Cedazo, which is in an Irvingtonian terrestrial horizon in the Tacubaya Formation of Mexico.

It was recombined as Equus (Parastilidequus) parastylidens by Mooser and Dalquest (1975); it was considered a nomen dubium by Downs and Miller (1994).
Equus semiplicatus was named by Cope (1893).

It was synonymized subjectively with Equus intermedius by Matthew (1903); it was recombined as Onager semiplicatus by Quinn (1957); it was synonymized subjectively with Equus conversidens by Kurten and Anderson (1980); it was considered a nomen dubium by Dalquest and Schultz (1992), Hulbert (1995).
Hippotherium princeps was named by Leidy (1890).

It was recombined as Hipparion princeps by Roger (1896); it was synonymized subjectively with Equus complicatus by Hay (1902); it was recombined as Neohipparion princeps by Gidley (1907); it was synonymized subjectively with Equus fraternus by Osborn (1918); it was considered a nomen dubium by Lucas (1896), Simpson (1930), Macdonald (1992).
Hippotigris was named by Hamilton-Smith (1841) [type African Equus zebra]. Its type is Hippotigris grevyi.

It was reranked as Equus (Hippotigris) by Skinner (1972); it was synonymized subjectively with Equus by Sclater (1900), Trouessart (1905), Lydekker (1916), Lundholm (1952), Ellerman et al. (1953), Azzaroli (1966), Groves and Willoughby (1981), Prothero and Schoch (1989).

It was assigned to Equini by Quinn (1955); to Equidae by Mooser (1958); and to Equus by Skinner (1972).
Onager was named by Brisson (1762) [attributed to Brisson 1780; credited as author because Brisson is unavailable]. It is extant.

It was synonymized subjectively with Equus by Groves and Willoughby (1981), Prothero and Schoch (1989).

It was assigned to Equini by Quinn (1957).

Synonymy list

Year	Name and author
1758	Equus Linnaeus p. 73
1762	Onager Brisson
1822	Asinus Gray
1825	Asinus Gray p. 342
1841	Hippotigris Hamilton-Smith
1846	Asinus Owen
1848	Equus Owen p. 139
1858	Asinus africanus Fitzinger
1860	Equus Leidy p. 100
1861	Equus taeniopus Heuglin
1873	Equus Leidy p. 321
1881	Equus Cope p. 400
1884	Asinus Noack
1884	Asinus taeniopus Noack
1890	Hippotherium princeps Leidy p. 182
1892	Tomolabis Cope p. 125
1893	Equus semiplicatus Cope p. 80
1896	Hippotherium princeps Leidy and Lucas p. 49
1896	Hipparion princeps Roger
1901	Equus semiplicatus Gidley p. 129 fig. 20
1902	Equus Hay p. 621
1902	Equus semiplicatus Hay p. 624
1907	Equus Gidley p. 867
1907	Neohipparion princeps Gidley p. 910
1912	Dolichohippus Heller
1912	Equus (Grevya) Hilzheimer
1912	Equus (Megacephalon) Hilzheimer
1913	Neohippus Abel
1913	Equus (Ludolphozecora) Griffini p. 382
1913	Equus leidyi Hay p. 572
1913	Equus Scott p. 291
1916	Equus (Asinus) Lydekker
1916	Equus asinus taeniopus Lydekker
1926	Equus Oliver Schneider p. 149
1930	Kolpohippus van Hoepen
1930	Kraterohippus van Hoepen
1930	Sterrohippus van Hopen
1935	Asinus Dollman
1935	Equus Schultz and Howard p. 285
1940	Equus Stirton p. 194
1942	Asinus Hopwood
1944	Hippotigris McGrew
1945	Asinus Harper
1955	Equus Hibbard p. 51
1955	Hippotigris Quinn p. 46
1955	Asinus Quinn p. 52
1955	Equus Quinn p. 58
1957	Onager Quinn p. 10
1957	Onager semiplicatus Quinn p. 10
1957	Asinus Quinn p. 27
1957	Equus Quinn p. 27
1958	Hippotigris Mooser p. 360
1958	Asinus Quinn
1959	Asinus Mooser
1959	Equus parastylidens Mooser
1961	Allozebra Trumler
1961	Asinus Trumler
1961	Hippotigris Trumler
1972	Equus (Amerhippus) Skinner p. 118
1972	Equus (Dolichohippus) Skinner p. 118
1972	Equus (Hippotigris) Skinner p. 118
1975	Equus (Parastilidequus) Mooser and Dalquest p. 807
1975	Equus (Parastilidequus) parastylidens Mooser and Dalquest p. 807
1980	Equus Kurten and Anderson p. 285
1980	Equus parastylidens Kurten and Anderson p. 287
1981	Equus Thurmond and Jones p. 185
1988	Equus Carroll
1989	Equus Prothero and Schoch p. 532
1992	Equus Koufos
1995	Equus Capasso Barbato and Gliozzi p. 246
1995	Equus leidyi Hulbert, Jr.
1995	Equus Kelly p. 19
1996	Equus Prado and Alberdi p. 676
1998	Equus MacFadden p. 552
2004	Equus (Amerhippus) Zurita et al.
2005	Equus Cisneros p. 249
2005	Equus Wilson and Reeder
2006	Equus (Amerhippus) Rincón et al.
2006	Equus Salles et al. p. 8
2014	Equus Alberdi et al.
2017	Haringtonhippus Heintzman et al. p. 5
2018	Equus Eshelman et al. p. 8

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G. Equus Linnaeus 1758 [horse]

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†Equus (Asinus) cumminsi Cope 1893

Subg. Equus (Hemionus) Stehlin and Graziosi 1935

Equus (Hemionus) hemionus Pallas 1775 [onager]

Subg. †Equus (Hesperohippus) Hibbard 1955

†Equus (Hesperohippus) mexicanus Hibbard 1955

Invalid names: Equus midlandensis Quinn 1957 [synonym], Onager oviedoi Mooser 1959 [synonym]

Subg. †Equus (Sussemionus) Eisenmann 2010

†Equus (Sussemionus) suessenbornensis Wüst 1901

Equus africanus Heuglin and Fitzinger 1866 [African wild ass]

Equus africanus africanus Heuglin and Fitzinger 1866 [Nubian wild ass]

Equus africanus somaliensis Noack 1884 [Somali wild ass]

Invalid names: Equus asinus aethiopicus Denman 1957 [synonym]

Invalid names: Asinus asinus dianae Dollman 1935 [synonym]

†Equus alaskae Winans 1989

Invalid names: Equus lambei Hay 1917 [synonym]

Equus asinus Linnaeus 1758 [donkey]

†Equus australis Moreno 1888

Equus caballus Linnaeus 1758 [horse]

Equus caballus ferus Boddaert 1785

Invalid names: Equus (Amerhippus) neogeus Lund 1840 [synonym], Equus hippagrus Smith 1841 [synonym]

†Equus calobatus Troxell 1915

†Equus capensis Broom 1909

†Equus cedralensis Alberdi et al. 2014

†Equus coliemensis Lazarev 1980

†Equus complicatus Leidy 1858

Invalid names: Equus americanus Leidy 1847 [replaced], Equus eous Cope 1895 [synonym], Equus major DeKay 1842 [synonym]

†Equus conversidens Owen 1869

Invalid names: Equus barcenaei Cope 1884 [synonym], Onager zoyatalis Mooser 1959 [synonym]

†Equus eisenmannae Qiu et al. 2004

†Equus excelsus Leidy 1858

Invalid names: Asinus aguascalentensis Mooser 1959 [synonym], Onager arellanoi Mooser 1959 [synonym], Onager hibbardi Mooser 1959 [synonym]

†Equus francisci Hay 1915

Invalid names: Equus (Asinus) calobatus Troxell 1915 [synonym], Equus arrelanoi Mooser 1958 [synonym], Equus quinni Slaughter et al. 1962 [synonym]

†Equus fraternus Leidy 1858

†Equus fromanius Repenning et al. 1995

†Equus giganteus Gidley 1901

†Equus holmesi Hay 1920

†Equus intermedius Rütimeyer 1877

†Equus intermedius Cope 1895

†Equus jubatus Hay 1927

†Equus laurentius Hay 1913

†Equus mexicanus Hibbard 1955

Invalid names: Equus occidentalis Leidy 1865 [synonym], Equus pacificus Leidy 1868 [synonym], Equus scotti Gidley 1900 [synonym]

†Equus nalaikhaensis Kuznetsova and Zhegallo 1996

†Equus nearcticus Blake 1863

†Equus nevadanus Hay 1927

†Equus niobrarensis Hay 1913

†Equus niobrarensis alaskae Winans 1989

Invalid names: Equus hatcheri Hay 1915 [synonym]

†Equus nobisi Pichardo 2004

†Equus pectinatus Cope 1901

Equus przewalskii Poliakov 1881 [Przewalski's horse]

†Equus pseudaltidens Hulbert, Jr. 1995

Invalid names: Onager altidens Quinn 1957 [replaced]

Equus quagga Boddaert 1785 [plains zebra]

Equus quagga boehmi Matschie 1892

Equus quagga borensis Lönnberg 1922

Equus quagga burchelli Gray 1824 [Burchell's zebra]

Equus quagga crawshayi De Winton 1896

Equus quagga quagga Boddaert 1785

Equus quagga chapmani Layard 1865

†Equus sanmeniensis Teilhard de Chardin and Piveteau 1930

†Equus santaeelenae Spillmann 1938

†Equus sivalensis Falconer and Cautley 1849

†Equus tabeti Arambourg 1970

†Equus yunnanensis Colbert 1940

Equus zebra Linnaeus 1758 [mountain zebra]

Equus zebra hartmannae Matschie 1898 [Hartmann's mountain zebra]

Invalid names: Equus matschiei Zukowsky 1924 [synonym], Equus penricei Thomas 1900 [synonym]

Equus zebra zebra Linnaeus 1758 [Cape mountain zebra]

Invalid names: Equus campestris Gray 1852 [synonym], Equus indica Trouessart 1898 [synonym], Equus montanus Burchell 1822 [synonym], Equus zebra frederici Trouessart 1905 [synonym]

Invalid names: Allozebra Trumler 1961 [synonym], Asinus Gray 1822 [synonym], Asinus africanus aegyptiacus Heuglin and Fitzinger 1866 [synonym], Dolichohippus Heller 1912 [synonym], Equus (Amerhippus) Hoffstetter 1950 [invalid subgroup], Equus (Parastilidequus) Mooser and Dalquest 1975 [invalid subgroup], Equus africanus somalicus Sclater 1884 [synonym], Equus leidyi Hay 1913 [nomen vanum], Equus parastylidens Mooser 1959 [nomen dubium], Equus semiplicatus Cope 1893 [nomen dubium], Equus taeniopus Heuglin 1861 [nomen dubium], Haringtonhippus Heintzman et al. 2017 [synonym], Hippotherium princeps Leidy 1890 [nomen dubium], Hippotigris Hamilton-Smith 1841 [synonym], Kolpohippus van Hoepen 1930 [synonym], Kraterohippus van Hoepen 1930 [synonym], Neohippus Abel 1913 [synonym], Onager Brisson 1762 [synonym], Sterrohippus van Hopen 1930 [synonym], Tomolabis Cope 1892 [synonym]

Diagnosis

Reference	Diagnosis
J. H. Quinn 1955	Large horses with complex permanent upper cheek teeth; styles of ectoloph high and thick, valleys deep and concave; protocones elongate and trans- versely compressed, internally concave but not as markedly as in Asinus; lower per- manent teeth with long, crescentic meta- conids and metastylids, intervening valleys U-shaped; lower molars with short median valleys not penetrating between the reen- trants of the flexids as deeply as in Hippo- tigris. In Old World species the median valley reaches the reentrants of the flexids but does not penetrate as deeply as in Hip- potigris; lower milk dentition without parastylids and hypostylids.
J. H. Quinn 1955 (Asinus)	Lower milk dentition with parastylids and hypostylids weak or absent; lower permanent dentition with anterior border of metaflexid produced outward in a deep sharp angle exceeding either Equus or Hippotigris; metaconid directed inward and then forward, elongate and flattened internally, metastylid deflected, restricted, and rounded, inter- vening valley V-shaped as in the zebras but less deeply so; entoconid rounded and restricted; hylpoconulid prominent and with a hypostylid; pli caballinid present; median valley of molars variably shortened and not protruding beyond the reentrants of the flexids which form a short narrow commissure; upper permanent teeth with small fossettes; small enamel inflections; protocones excessively flat- tened and concave internally in late species; metapodials slender.
J. H. Quinn 1957 (Asinus)	Upper permanent teeth with square outline, small fossettes, protocones shorter than in Onager and teeth less hypsodont; lower permanent dentition, metaconid tends to be deflected inward, then forward, metastylid deflected and short tending to be nearly rounded; inter- vening valley V-shaped as in Hippotigris but less deeply so, no parastylid, pli caballinid present, hypoconulid prominent and with a hypostylid, median valley of molars not protruding between re-entrants of the flexids (except in very early stage of wear); lower milk dentition with parastylids and hypostylids weak or absent; metatarsals short and stout but less so than in Equus
J. H. Quinn 1957 (Onager)	Size range equal to Equus; pattern of upper cheek teeth more complex, protocones elongated, hypoconal groove of premolars tends to be partly closed, third molar less modified than in Equus or Asinus; lower molars with median valley shorter than in most species of Equus; upper and lower teeth equalling or sur- passing Equus in hypsodonty and appreciably more hypsodont than Asinus; metapodials long and more slender than in Equus, longer and more slender than in A sinus.
P. D. Heintzman et al. 2017 (Haringtonhippus)	Haringtonhippus is the sister genus to Equus (equid crown group), with Hippidion being sister to the Haringtonhippus-Equus clade (Figure 1). Haringtonhippus can be differentiated from Equus and Hip- pidion by 178 synapomorphic positions in the mitochondrial genome, including four insertions and 174 substitutions (Appendix 1—Table 2 and Appendix 1—table 2—source data 1). We caution that these synapomorphies are tentative and will likely be reduced in number as a greater diversity of mitochondrial genomes for extinct equids become available.