During the middle Miocene an explosive adaptive radiation resulted in the advent of grazing horses with high-crowned teeth in North America. New Sr isotopic, paleomagnetic, and biostratigraphic evidence from the Miocene marine and nonmarine sequence of the Florida panhandle calibrates the base of this adaptive radiation. The transition from the primitive outgroup species "Parahippus" leonensis to the most primitive high-crowned horse, "Merychippus" gunteri occurred after about 17.7 Ma. After this event, the lowest known stratigraphic level at which diversification (i.e., presence of two or more sympatric species) of grazing merychippine horses occurs is about 16.2 Ma, or within the early part of Chron C5BR. Although this currently is the only sequence where the parahippine-merychippine transition is directly calibrated, biochronologic evidence from other important, contemporaneous localities in Texas, Nebraska, and California indicate that diversification occurred rapidly throughout North America between about 15 and 16 Ma.