Caroli Linnaei...Systema naturae per regna tria naturae, 1758 (p.73)

The establishment of universally accepted conventions for the naming of organisms was Linnaeus’ main contribution to taxonomy—his work marks the starting point of consistent use of binomial nomenclature. During the 18th century expansion of natural history knowledge, Linnaeus also developed what became known as the Linnaean taxonomy; the system of scientific classification now widely used in the biological sciences. ( from Wikipedia ) In Linnaeus’ Scala Naturae (1758)

Family Equidae Gray, 1821

The Equidae family fossil record begins with the dawn horse or Hyracotherium in the Eocene epoch about 60-45 million years ago. This ancient equid, also commonly known as Eohippus was said to inhabit forests and woodlands and is believed to be a browser that foraged primarily on soft leaves as well as some fruits, nuts and plant shoots.

Hyracotherium or Eohippus?

The first fossils of this genus were found in England and described by the paleontologist Richard Owen in 1841. Suspecting that his species was a hyrax due to its teeth, but lacking parts of the skeleton, Owen called it a “Hyrax-like beast” and placed it in the new genus Hyracotherium. In 1876 in America Othniel C. Marsh found a full skeleton, which he placed in another new genus Eohippus, from the Greek ηώς (eōs, “dawn”) and ιππος (hippos, “horse”), meaning “dawn horse”. When it became apparent that the two genera were likely one and the same, Eohippus for a time became a synonym of Hyracotherium, the genus with the earlier date of publication. ( from Wikipedia )

For more information on equid evolution you can check Horse Evolution by Kathleen Hunt also check The Branching Bush of Horse Evolution by Brian Switek

Cutting a very long story a bit short it is plausible to consider that the main trends in Equidae evolution up until the extant Equus, were an increase in body size with a lengthening of the legs, a reduction in the number of toes up until the surviving single-toed hoof, increasing height and complexity of crown patterns in the cheek teeth, lengthening and deepening of the skull to accommodate the high-crowned cheek teeth, and an increase in the size and complexity of the brain.

The mainstream of equid evolution occurred in North America and during the Pleistocene, when the modern genus Equus appeared and spread to Eurasia and Africa. In North America, there were approximately eight equid species. However, during the late Pleistocene, there was a mass extinction of mammals in North America and all the equid species disappeared. In Eurasia and Africa, seven species of equids survived. Current taxonomy identifies seven species in this genus. However, taxonomy is an evolving science and questions remain concerning evolutionary and genetic relationships and whether some species should be split or combined.

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An Understanding of Horse – Intro

Large social non-ruminant grazing herbivores

Bibliography

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