Comparative biogeography of North American turtle faunas: Neogene regionalization

Abstract

North America harbors substantial species diversity in non-marine turtles (includes tortoises and terrapins), much of which arose in the Neogene Period (Miocene and Pliocene epochs) within Kinosternon , Emydinae, Trachemys , Pseudemys , Graptemys , and Gopherus . This diversity is distributed among 16 biogeographical provinces, but natural, hierarchical relations among provinces are unresolved. We used three-item analysis to identify such relations among provinces for these clades, following a recent, relatively complete phylogenetic reconstruction. The final three-item analysis identified 53,353 taxon-area cladograms — free of paralogy and multiple areas on a single branch. The final intersection tree has a retention index of 73.6% and a completeness index of 75.4%, both indicating moderately strong congruence in patterns among turtle clades. All six turtle clades support some nodes on the intersection tree, which is divided into eastern and western forks. The crown group on the eastern fork includes four provinces almost entirely east of the Mississippi River drainage, whereas the western fork is split into wet tropics and aridlands sub-forks. The east-west transition zone spans the Mississippi River drainage and Great Plains. Our summary of divergence estimates and geological history suggests that although phylogenetic synchrony existed between select taxa, there was general asynchrony because provincial turtle faunas developed over an extended period (Neogene). Temperature-sensitive distributions of various taxa responded to climatic cycles by expanding during periods of warming, but contracting during periods of cooling. Grassland expansion and geomorphic change in the Neogene created provincial boundaries that, although sometimes crossed under favorable climates, more commonly acted as dispersal barriers. Despite asynchrony in faunal assembly, existence of shared patterns reveals natural relations among provinces, producing a regionalization for North American turtles, useful for understanding their evolution as well as the biogeography of North America.