Horses

My husband and I enjoy riding horses when we get the chance, which is not as often as we would like. We enjoy being able to go into back country areas which are not accessible to vehicles, or would take a human a long time to get to. We also enjoy being able to spend time with these large animals, who tolerate us silly humans so patiently, asking only for a scratch behind the ears, some hay, and maybe a treat or two.

The origin and development of horses through geologic time is a fascinating subject, and one that I am unable to cover here in any great detail. Many books exist on the subject of horse development, and for me to cover horse development at this site would require an on-line book! Instead, I will offer only a few highlights, and refer you to two wonderful web sites I have found that offer much more detailed descriptions of extinct horses, and also list some great books with even more information about these interesting creatures.

Horses are members of the Order Perissodactyl, which also includes rhinos and tapirs. They have hypsodont ("high crowned") teeth, enabling them to graze on grasses with a high silica content. Their teeth have selenodont dentition, distinguishing them from cows and other ruminant animals with lophodont dentition. See my article, "Teeth II: The Herbivores" for more information about the different types of teeth seen in herbivores, and what this information can tell us about the animals.

The first known fossil horse was Hyracotherium, formerly known as Eohippus, or "dawn horse". Hyracotherium lived in North America and north western Europe during the early Eocene Epoch (55 to 36 million years ago). It was a small animal, standing only 25 to 50 cm (10 to 20 inches) tall at the shoulder. Its forefeet each had four toes, and its hind feet each had three toes, which ended in small hooves. It had teeth with low crowns, more resembling the bunodont dentition that we see in humans, indicating that it probably ate soft vegetation with a low silica content.

During the later Eocene Epoch, the European Hyracotherium led to animals like Pachynolophus and Anchilophus, while the American Hyracotherium led to Orohippus, Epihippus and Mesohippus.

By the late Oligocene Epoch (36 to 22.5 million years ago), horse evolution was limited to North America, where Miohippus existed until the early Miocene Epoch (22.5 to 5 million years ago). The appearance of Parahippus coincided with the early spread of grasses, and lead to the development of high-crowned, hypsodont teeth. (See my article "Grasses and Grazers" for more information.)

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