​This page covers the family Equidae (superfamily Equoidea, Order Perissodactyla), which is a sister clade to the Ceratomorpha (rhinoceroses and tapirs). The Equidae comprise the horses, zebras and asses.

A huge number of fossils of the horse stem group have been found, and many phylogenetic trees have been published (e.g. Froehlich, 2002; Bai et al, 2014; Cantalapiedra et al, 2017; Bai et al, 2018; Barrón-Ortiz et al, 2019; McHorse et al, 2019; Cirilli et al, 2021; and MacLaren, 2021). These trees are difficult to compare because they tend to contain different sets of fossil species, but the one by Maclaren (2021) may be considered a representative example. It is shown as a time tree below, but should not be taken to be a complete phylogeny of the stem-Equidae:
​Figure 1. Time tree of the stem-Equidae
The oldest known member of the equid stem group is Sifrhippus sandrae, described from the earliest Eocene (Ypresian) Willwood Formation at Sand Coulee in the Clark’s Fork Basin, Wyoming (Froehlich, 2002). This species is illustrated below, together with all other species shown on Figure 1 for which images are available in the public domain (for larger view, click on image):
​Figure 2. Images of stem-group equids
The images shown above are placed in left-to-right order from most basal towards the crown group. They all appear horse-like, but a gradual progression can be observed towards animals that appear very like modern horses.

The phylogeny of the crown-Equidae is still uncertain. The genus Equus, to which belong all modern horses, zebras and asses, is not confined to the crown group (see Figure 2 of Cirilli et al, 2021). As expressed by the latter authors, "Currently, there is no consensus on either the taxonomic content nor phylogeny of Equus." However, both Froehlich (2002) and Barrón-Ortiz et al (2019) place two extinct members of the genus, E. francisci and E. conversidens, in the crown group. If this is correct, these two species represent the oldest crown-Equidae. A skeleton and a life restoration of Equus conversidens are shown below:
​Figure 3. Images of one of the oldest known members of the crown-Equidae
​The above time tree (Figure 1) indicates that the equid stem group developed from Early Eocene to Pleistocene time, representing a stem-to-crown transition of between 53 and 56 million years.

References

Bai, B., Wang, Y., Meng, J., Li, Q., & Jin, X. (2014). New Early Eocene basal tapiromorph from Southern China and Its phylogenetic implications. PLoS One, 9(10), e110806.

Bai, B., Wang, Y. Q., & Meng, J. (2018). The divergence and dispersal of early perissodactyls as evidenced by early Eocene equids from Asia. Communications Biology, 1(1), 115.

Barrón-Ortiz, C. I., Avilla, L. S., Jass, C. N., Bravo-Cuevas, V. M., Machado, H., & Mothé, D. (2019). What is Equus? Reconciling taxonomy and phylogenetic analyses. Frontiers in Ecology and Evolution, 7, 343.

Cantalapiedra, J. L., Prado, J. L., Hernández Fernández, M., & Alberdi, M. T. (2017). Decoupled ecomorphological evolution and diversification in Neogene-Quaternary horses. Science, 355(6325), 627-630.

Cirilli, O., Pandolfi, L., Rook, L., & Bernor, R. L. (2021). Evolution of Old World Equus and origin of the zebra-ass clade. Scientific reports, 11(1), 10156.

Froehlich, D. J. (2002). Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla). Zoological Journal of the Linnean Society, 134(2), 141-256.

McHorse, B. K., Biewener, A. A., & Pierce, S. E. (2019). The evolution of a single toe in horses: causes, consequences, and the way forward. Integrative and Comparative Biology, 59(3), 638-655.

MacLaren, J. A. (2021). Biogeography a key influence on distal forelimb variation in horses through the Cenozoic. Proceedings of the Royal Society B, 288(1942), 20202465.
Image credits – stem-Equidae
Header (wild horses in France): Fitindia, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Sifrhippus sandrae, fossil): Jonathan Chen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Sifrhippus sandrae, life restoration): Eduard Solà, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Eurohippus parvulus, fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Eurohippus parvulus, life restoration): Haplochromis, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Eohippus angustidens, fossil): James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Eohippus angustidens, life restoration): Heinrich Harder (1858-1935), Public domain, via Wikimedia Commons
  • Figure 2 (Propalaeotherium hassiacum): H. Zell, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Propalaeotherium sp.): Grube_Messel_fg14.jpg: Fritz Geller-Grimmderivative work: Xvazquez, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons
  • Figure 2 (Mesohippus bairdi, fossil): James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Mesohippus bairdi, life restoration): Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Plagiolophus sp.): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Palaeotherium magnum, fossil): Shadowgate from Novara, ITALY, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Palaeotherium magnum, life restoration): ДиБгд, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Miohippus sp.): mark6mauno, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
  • Figure 2 (Hypohippus osborni, fossil): Osborn, Public domain, via Wikimedia Commons
  • Figure 2 (Hypohippus osborni, life restoration): Juandertal, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Anchitherium sp.): PePeEfe, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Archaeohippus sp.): National Park Service, Public Domain
  • Figure 2 (Parahippus tyleri): Smithsonian Institution, public domain (free of copyright restrictions
  • Figure 2 (Protohippus simus): Ryan Somma, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
  • Figure 2 (Acritohippus isonesus, fossil): James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Acritohippus isonesus, life restoration): James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Pliohippus sp., fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Pliohippus sp., life restoration): Juandertal, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Pseudhipparion retrusum): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Neohipparion leptode): Jonathan Chen, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Neohipparion affine): Robert Bruce Horsfall, Public domain, via Wikimedia Commons
  • Figure 2 (Cormohipparion occidentale): James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Hippidion sp., fossil): Emőke Dénes, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Hippidion sp., life restoration): Emőke Dénes, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Hippotherium primigenium): H. Zell, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Dinohippus leidyanus): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
  • Figure 2 (Equus simplicidens): Daderot, CC0, via Wikimedia Commons
  • Figure 3 (Equus conversidens, fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 3 (Equus conversidens, life restoration): No machine-readable author provided. Karkemish assumed (based on copyright claims)., CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons