MIOCENE-PLIOCENE PALEOBIOGEOGRAPHY OF NORTH AMERICAN EQUINAE

In North America, the subfamily Equinae underwent a radiation during the Miocene to Pliocene, from a single ancestral taxon, Parahippus leonensis, to fifteen genera. By the end of the Pliocene, however all genera but one, Equus, became extinct. Due to the extensive collection of fossil horse material in North America, this radiation can be studied to determine the evolution of the subfamily, including the effect of climate and geological events on biogeographic patterns. More specifically, we can measure how dispersal and vicariance were involved in the evolution and distribution of the subfamily.

Distribution and age-range data were collected for all species within fifteen Equinid genera through a literature review and use of the Paleobiology Database (PBDB). Distribution data were analyzed using the Lieberman Modified Brooks Parsimony Analysis (LBPA) to determine patterns of vicariance and geodispersal following the protocol of Lieberman (2000). These patterns were compared to climatic and geological changes during the Miocene-Pliocene.

Species analyzed were distributed between six biogeographic regions within North America. Speciation by dispersal occurred more frequently than vicariant speciation in the evolution of this clade. LBPA analysis resulted in recovery of a well-resolved vicariance pattern and moderately resolved geodisperal tree. Congruence between the vicariance and geodisperal trees indicates that fluctuating climatic conditions repeatedly joined and divided taxa inhabiting Florida, Coastal Texas and the Great Plains. Inconsistencies between the vicariance and geodisperal trees indicate that geological events also contributed to the speciation of Equinae in other areas.