Orohippus

Orohippus evolved from Eohippus about 50 million years ago. The only major differences in the two were the absence of the vestigial toes that were present in Eohippus and that the last premolar changed into a molar, giving Orohippus another grinding tooth. Also the crests on the teeth were more pronounced, indicating that Orohippus probably fed on tougher plants.

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Epihippus

Epihippus, who arose from Orohippus about 47 million years ago, had further developments with its teeth. Now the last two premolars were like molars, giving Epihippus a total of five grinding teeth.

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Miohippus

Miohippus, who lived during the Oligocene period some 25 to 40 million years ago, branched off from Mesohippus, and the two coexisted for about four million years. Although Mesohippus had died out by the mid-Oligocene, Miohippus grew rapidly.

Miohippus was larger than Mesohippus and had a slightly longer skull. Its facial fossa became deeper and more expanded, and the ankle joint changed subtly. Miohippus also had a variable extra crest on its upper cheek teeth, which is now a typical characteristic of the teeth of later equine species.

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Parahippus

About twenty million years ago, during the Miocene period, the earth was drastically changing. Great plains were developing, forests were thinning out, mountains were forming, and swamps dried up. This brought about many changes in the horse. Since leafy food had become scarce, it had to eat the grass of the plains. The molar teeth developed high crowns and a hard covering for grinding the grass, which was covered with dust and sand. Also, their body size increased and their legs and face lengthened. The bones in the legs fused. This, along with muscle development, allowed the horse to move with forward-and-back strides, causing flexible leg rotation to be eliminated. Most importantly, the horses began to stand on their middle toe, which made them run faster, instead of walking on pads; their weight was supported by ligaments under the fetlock to the big central toe. Their side toes seldom touched the ground.

Parahippus was one of the first to display these characteristics. Parahippus was very similar to Miohippus, but was slightly larger. It had three toes and was beginning to show the ligaments under the foot. The extra crest that was variable in Miohippus became permanent in Parahippus. Also, other teeth were beginning to form a series of tall crests with higher crowns. Later fossils of this horse were so similar to Merychippus that it was difficult to distinguish the two.

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Dinohippus

The fossils of Dinohippus, another one-toed horse, were recently discovered. Its foot structure, skull, and teeth are extremely similar to those of the modern equine; it is almost certain that Equus descended from it.

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