THE EFFECT OF CLIMATE ON THE PALEOBIOGEOGRAPHY OF THE SUBFAMILY EQUINAE DURING THE MIOCENE AND PLIOCENE IN NORTH AMERICA

In North America, the subfamily Equinae underwent a radiation during the Miocene to Pliocene, from a single ancestral taxon to eighteen genera. By the middle of the Pliocene, however, all genera but three 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 Miocene species within eighteen equinid genera through a literature review and use of the Paleobiology Database. 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). Species analyzed were distributed between four biogeographic regions within North America. Speciation by dispersal occurred in these areas more frequently than vicariant speciation in the evolution of this clade. LBPA analysis resulted in recovery of one single most parsimonious tree for both vicariance patterns and geodispersal patterns. The vicariance tree indicates that a division between the Great Plains and Southwest occurred most recently. Before then, a boundary rose between these two areas and Florida and ancestrally, a boundary rose with the Gulf Coast. The geodispersal tree indicates taxa dispersed in two major areas of North America: between the Great Plains and Southwest or the Gulf Coast and Florida. Congruence between the geodispersal and vicariance trees indicates that fluctuating climatic conditions repeatedly joined and divided taxa inhabiting the four areas. The general pattern of biogeographic relationships of the clade as a whole appears to be more strongly affected by climatic rather then tectonic events.