But that did not completely answer the question of what caused the animals' extinction, says Stanley. To Stanley, the data suggested that a change in the horses' diet was causing their teeth to wear down faster. Whatever the change was, it apparently did not harm the horses with very long teeth, who had enamel to spare, but it wore down the teeth of the horses with only long teeth. These horses apparently did not get the nutrition they needed, says Stanley. Perhaps they became malnourished and weakened, more prone to predators or disease.
Plants use silica compounds as protection against predators.
Perhaps, Stanley hypothesized, C 4 plants have more silicates than C 3 plants. But he turned out to be right. On average, C 4 plants have about three times as much silica as C 3 plants. And C 4 grasses prefer drier conditions. So from extra silica to horse extinction, there is the connection, says Stanley.