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A brief history of the family Equidae

        The family Equidae has a 55-million year history which has been preserved in one of the most complete fossil records of any animal group. Although over three dozen genera of this family have been described, only one genus, Equus, with 6-8 extant species, has survived to the present day. Because of this, the family Equidae has historically and popularly been held as a classic example of orthogenesis, with a progressive trend toward larger size, fewer toes, and sturdier teeth. This historical view is inaccurate, however, as the family tree of Equidae is not a linear path with Equus at its end, but is complex and branching, and reflects the changing environmental conditions throughout the Cenozoic era.