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John Day Formation: By 32 million years ago the climate had become cooler, a warm temperate climate with deciduous forests. This formation spans more than 20 million years and more than 120 species of mammals have been found in it, including the "rhinoceros" (Diceratherium) seen in the center of the slide, Miohippus (a three toed horse), Sabre toothed "tigers" (one is seen in the tree) and early "dog" Mesocyon.