Proto-horses: The teeth and skull
These changes in the limbs were accompanied by others changes. The grasses of the plains were becoming tougher to chew and contained within their leaves tiny sharp crystals of silica which wore teeth badly. So the proto-horses changed their rounded molars into bigger and bigger grinders with hard ridges of dentine in them. One of the problems of the grazing life is that an animal, with its head on the ground for such long periods, cannot keep a good lookout for predators. The higher the eyes are placed on the head the better the visibility. This requirement, together with the necessity to provide room for the enlarged molars, resulted in a considerable elongation of the skull. So the early horses evolved into the forms we know today (e.g. Equus). They spread across the plains of America and eventually, at a time when the Bering Strait was dry and connected North America with Asia, they reached Europe. From there they spread south and colonised the plains of Africa. Later, they died out in North America and only reappeared when they were introduced by European man. In Europe and Africa, they flourished as horses (Equus), donkeys (Equus asinus) and zebras (Equus burchelli).