You can tell a lot about what a horse eats from its teeth--even if the teeth are fossils. The first horses all had short, broad chewing teeth, like ours. Later horses had teeth three times longer. Why?
Short teeth are fine if you're grazing on soft leaves, like the Hypohippus shown eating in the forest. But grazing on tough grasses would quickly wear short teeth down to nothing. For prairie horses like Dinohippus, evolution favored longer teeth that could handle the grind of grazing--as a tooth wears down, more emerges.
Glass In Grass?
Grazing is hard on teeth for two reasons. Grasses contain bits of the mineral silica that resemble glass and wear teeth away like sandpaper. Chomping grass close to the ground also picks up gritty soil that wears teeth away. Grass-eating horses evolved longer teeth that could withstand this wear.
Until recently, scientists thought that all horses with long teeth grazed on grass. But new evidence shows that some long-toothed species also grazed on leaves. How do scientists know? Tiny scratches on fossil teeth, and chemicals preserved in the teeth, provide clues about what the horses ate.