Paleobiogeography of Miocene Equinae of North America: A phylogenetic biogeographic analysis of the relative roles of climate, vicariance, and dispersal

The horse subfamily Equinae underwent a major radiation during the Miocene in North America, diversifying from one species, Parahippus leonensis, to 70 species. This radiation has been linked to climatic and vegetation changes that occurred in North America during this time. However, the relationship between climate change and speciation has not previously been studied quantitatively using phylogenetic biogeography. Distribution and age-range data were collected for all North American species within eighteen equine genera through a literature review and use of the Paleobiology Database. Fitch parsimony analysis of the taxon-area cladogram indicate that speciation by dispersal (Allopatry model II) was more common in the evolution of the clade than speciation by vicariance (Allopatry model I). Distribution data were analyzed using the Lieberman-modified Brooks Parsimony Analysis (LBPA) to determine patterns of vicariance and geodispersal, using four constrained biogeographic regions within North America in the analysis: the Great Plains, the Southwest, the Gulf Coast and the Southeast. Resulting general area cladograms are congruent with geological events, such as uplift of the Rocky Mountains, and climatic conditions, such as the change from a warm and moist to cool and arid climate during the Miocene. Well supported vicariance and geodispersal trees derived from the LBPA analysis are largely congruent with each other, indicating that cyclical events, in particular, climate change during the Miocene influenced the radiation of the clade.