Fossil horses from the Ashfall Fossil Beds in Nebraska, a site whise richness of mammal fossils is attributed to a large Yellowstone eruption. Horse remains are plentiful, with five genera known to be present. They are depicted above in the following order: Pseudohipparion, Neohipparion, Cormohipparion, Protohippus, and Pliohippus. The first three genera are three-toed, while the last two bear single hooves.
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