The Evolution of a Horse's Pentadactyl Limb
Unlike most mammals who have the original five bones of the
pentadactyl limb, the horse does not. Over the course of 60 million years the
horse has grown from 0.4 metres to 1.6 metres. They have also changed
dramatically in the way they look. For example 60 million years ago the horse
had four toes, the bone structure being similar to a dog’s foot. Twenty million
years later the four toes became three toes and the size of the horse increased
by 0.2 metres. Thirty million years ago the three toed foot became more hoof
like, with the two extra toes were higher up the leg. Now at ten million years
ago the horse has a single hooved leg that has the same original five bones but
incorporated in the leg and not extruding.