Family
Equidae
Horses, asses, and
zebras |
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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)
Click on the species above to learn
more,
or jump to the Equidae Species
List
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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.
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