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Equus simplicidens is a species of early horse that first appeared in the Pliocene (5.3 million years ago). It lived until the end of the Pleistocene approximately ten thousand years ago. Equus simplicidens was named by Cope in 1892, although it has changed names many times throughout the years. This is because Equus simplicidens is a transitional fossil and represents one of the first horses to fall within the genus Equus. This is the genus that includes modern horses, zebras, and donkeys (Howe, 1970).
Sometimes called the Hagerman Horse, Equus simplicidens has been recovered from fossil beds that span the continental United States. It is commonly found in sites in California, Florida, Nebraska, and Texas. The most common of these sites is in Idaho, where a large number of fossils have been excavated since the early twentieth century (Idaho Museum of Natural History).
Equus simplicidens stood about 1.3 meters tall at the shoulder and weighed about kilograms. This indicates that it would have been nearly identical in size to modern zebras. In fact, it is believed that the Hagerman horse more resembled the zebra than the modern horse, and many artists’ recreations depict them with stripes (National Park Service).