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"This small dog-sized animal is the oldest found horse ancestor that lived about 55 million years ago.”
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"The earliest evidence of this “little horse” is found in the middle Eocene of Wyoming, about 2 million years after the first appearance of Hyracotherium. The two genera coexisted during the Eocene, although Orohippus fossils are not as numerous or as geographically widespread as those of Hyracotherium. Fossils of Orohippus have been found in Eocene sediments in Wyoming and Oregon, dating from about 52-45 million years ago."
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"The "middle horse" earned its name. Mesohippus is intermediate between the eohippus-like horses of the Eocene, (which don't look much like our familiar "horse") and more "modern" horses. Fossils of Mesohippus are found at many Oligocene localities in Colorado and the Great Plains of the US (like Nebraska and the Dakotas) and Canada. This genus lived about 37-32 million years ago."
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"Species of Miohippus gave rise to the first burst of diversity in the horse family. During the Miocene, over a dozen genera existed. m about 32-25 million years ago."
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"Parahippus appears to be the evolutionary “link” between the old forest-dwelling horses and the modern plains-dwelling grazers. Species in this genus lived from 24-17 million years ago."
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"Merychippus represents a milestone in the evolution of horses. Though it retained the primitive character of 3 toes, it looked like a modern horse.. Species in this genus lived from 17-11 million years ago."
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"Grandfather" to the modern horse, Pliohippus appears to be the source of the latest radiation in the horse family. Species in this genus lived from 12-6 million years ago."
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"Dinohippus is believed to be the closest relative to Equus, the genus that includes the living horses, asses and zebras. Dinohippus fossils are found in the Upper Miocene of North America and date from 13 - 5 million years ago."
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"Merychippus represents a milestone in the evolution of horses. Its long legs allowed it to escape from predators and migrate long distances to feed. It had high-crowned cheek teeth, making it the first known grazing horse and the ancestor of all later horse lineages. Species in this genus lived from 17-11 million years ago."
Sources: For more information: visit http://chem.tufts.edu/science/evolution/HorseEvolution.htm
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