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Equidae in the Miocene

Merychippus, a Miocene equid

        The Miocene was a time of great diversification of Equidae, and as many as twelve species have been found fossilized in single localities from this time (MacFadden 1994).  This diversification may have been a result of the intense climate changes and the increased seasonality seen during the Miocene (MacFadden 1994).  The most dramatic change seen in the family was the development of teeth suitable for grazing, an adaptation to the changing vegetation of the time (MacFadden 1994).  However, this change was not seen in all species of Equidae: different species ate different types of vegetation, and some species continued to be mainly browsers (Hunt 1995). An increase in body size and in length of the limbs also occurred during this epoch, either for better escape from predators in the open grasslands, or to facilitate migration due to seasonal changes in vegetation (MacFadden 1994). Accompanying this change was a reduction or complete loss of the two remainin side toes on the fore- and hindlimbs (MacFadden 1994).  Near the end of the Miocene, many species of Equidae died out, possibly because of the arid climate of the time (MacFadden 1994).  

browser vs. grazer teeth

Browser teeth, above, belonging to Hyracotherium.  These have short crowns and relatively smooth surfaces, and are suitable for eating soft vegetation such as fruit and leaves (Draper 1996, Benton 2000)

Grazer teeth, below, belonging to Equus.  Grasses are abrasive (MacFadden 1994), so the teeth of grazers must have high crowns that will take a long time to wear down (Benton 2000).  Their grinding surfaces are also more complex than those of browsers' teeth, to better break down the tough grasses (Benton 2000).  

Picture modified from MacFadden 1992.

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