The late appearance of the single toed horse, Pliohippus, in the northern Great Basin provides an opportunity for study of its environmental requirements in contrast to those of the 3-toed horse, Hipparion, which occupied the region earlier and contemporaneously. Many characteristics of vegetational change in the northern Great Basin of the late Tertiary coincide with the historical biogeography of these and other horses in this region. This conclusion leads to possible answers to related problems of migration, homotaxis, community change, and morphology in equids.

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