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Family Equidae
Horses, asses, and zebras
Family Equidae
The equids are the most numerous and widely-distributed group of modern perissodactyls, although only one genus and seven species survive to the present day (an eighth, Equus quagga, was lost to extinction in 1883). The current distribution of the Equidae - open habitats of Eastern and Southern Africa and regions of Asia - is significantly reduced from former times, a pattern seen amongst all perissodactyls. Modern horses are well adapted to the grasslands, steppes, and deserts that they inhabit; their teeth are high-crowned to cope with coarse vegetation and their limbs are long and slender, with just a single toe, for increased running efficiency.

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.

 

The Equid Family Tree
Branch lengths are not proportional to time
(From Oakenfull, Lim, and Ryder, 2000)

 
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Perissodactyla
Equus ferus

Equus kiang

Equus hemionus

Equus africanus

Equus zebra

Equus grevyi

Equus quagga

Click on the species above to learn more,
or jump to the Equidae Species List
Literature Cited

Estes, R. D. 1991. The Behavior Guide to African Mammals: including hoofed mammals, carnivores, primates. Los Angeles: University of California Press.

Martin, R. E., R. H. Pine, and A. F. DeBlase. 2001. A Manual of Mammalogy, Third Edition. Boston: McGraw-Hill Publishing.

Nowak, R. M. [Editor]. 1991. Walker's Mammals of the World. Fifth Edition. Johns Hopkins University Press, Baltimore.

Oakenfull, E. A., H. N. Lim, and O. A. Ryder. 2000. A survey of equid mitochondrial DNA: Implications for the evolution, genetic diversity and conservation of Equus. Conservation Genetics; 1(4): 341-355.

Vaughan, T. A., J. M. Ryan, and N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia.