The evolution of horses in the Cenozoic-the last 65 million years-is one of the best-studied prolonged vertebrate radiations. Their evolution is in part thought to be related to changes in available food, and the abrasiveness of that food, as grasslands expanded. Mihlbachler et al. (p. 1178) used measures of the wear on fossil horse teeth, which have an excellent fossil record, to trace these changes. Most horse populations show relatively low wear, suggesting that dietary pressures were overall low. However, there are several episodes of higher wear in some species, especially during the Miocene, 23 to 5 million years ago, before the appearance of a group of horses with a markedly high crown.
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