Horses move onto the grasslands

During the Miocene, horses in North America diversified from a handful of forest browsers into dozens of species. A group that includes Merychippus, Parahippus and later relatives developed teeth with tall crowns covered in cement, tough enough to grind abrasive grass and grit. Their legs lengthened, and most lost side toes, relying on a larger central toe for speed on open ground. Some species were still three-toed and lived alongside one-toed forms.¹²

Grass spread widely across the continent as the climate dried and cooled after the warm Miocene. Grass leaves are full of silica, which wears teeth quickly, and open ground favours fast flight from predators, so both teeth and legs were under strong selection. Horse evolution is often drawn as a ladder from small to large, but it was really a branching bush with many contemporary forms. A study of tooth wear in 2011 suggested that tall teeth appeared before horses switched to a mainly grass diet, so the story is not simple. By the late Miocene North American horses were among the most varied hoofed animals, and they set the stage for the modern genus Equus.²³