The first equid - Hyracotherium from the early Eocene - was a small, unspecialized, forest-dwelling ungulate. As grasses evolved and began dominating open territory, some equids moved from forests to grasslands to take advantage of this new food source, upon which they would become highly successful. The principal radiation of the Equidae occurred in the New World during the Miocene; fossils show a distinct trend towards a cursorial existence (longer legs with fewer digits) and increased adaptations to grazing. However, the diversity of Miocene horses (up to 20 genera have been described) did not continue into the Pliocene, potentially due to the rise of the ruminants. The modern genus Equus first appeared around 2 million years ago in North America, and is now the last remnant of this family. Due to the completeness of the fossil record and the presence of many intermediate forms, horses are frequently used to demonstrate the principles of evolution.
With just a single hoofed toe on each foot, horses represent an extreme in cursorial adaptation. The general form of equids - including a large, blockish head, sturdy neck, and long legs - is easily recognizable. A characteristic bristly mane is found on the nape of the neck, and the tail has a long tassle. The orbit and temporal fossa of the skull are completely separated by a post-orbital plate. The teeth are high-crowned and have complex enamel grinding surfaces, enabling horses to consume the coarsest vegetation. The dental formula is I 3/3, C 0-1/0-1, P 3-4/3, M 3/3 x 2 = 36-42. The canines, usually present only in males, are small and spade-shaped.
(From Oakenfull, Lim, and Ryder, 2000)
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