STUCK IN THE NEW WORLD TROPICS? EARLY MIOCENE BIOGEOGRAPHY AND DIVERSITY OF CENTRAL AND NORTH AMERICAN MAMMALS

The “Out of the Tropics” hypothesis (OTTH) for the origin of modern latitudinal biodiversity gradients states that tropical taxa should exhibit higher diversification and dispersal rates than extra-tropical taxa and is supported by studies including such disparate clades as bivalves and mammals. The scope of these studies, however, span the Great American Biotic Interchange (GABI), a substantial perturbation of New World Tropics (NWT) ecological dynamics that may hinder our ability to test process-based hypotheses concerning the origin of NWT biodiversity. Efforts of the Panama Canal Project have extensively added to our knowledge of the early Miocene Centenario Fauna (Panama Canal Basin) that, combined with the rich mammalian record of North America, allows us to test the OTTH in a paleoecological context prior to the GABI. We analyzed the geographic distribution of first and last appearances in mammalian genera during the early Hemingfordian North American Land Mammal Age (He-1 NALMA subdivision, ~19-17.5 Ma) using occurrence data from North and Central America, including new data from the Panama Canal. Cluster analysis of faunal similarity among regional collections of He-1 mammals resulted in a statistically significant agglomerative coefficient and two distinct faunal provinces with a boundary of 33°N, clearly defining a biologically-meaningful boundary between tropical and extra-tropical mammalian communities during He-1. Taking sampling intensity into account, our results show a significantly higher proportion of first appearances in the tropical province relative to the extra-tropical province with no significant difference in last appearances, supporting a hypothesis that higher origination rates lead to greater mammalian biodiversity in the NWT. The temporal resolution of our data precludes assessment of dispersal rates among the fossil mammalian taxa under study. However, the OTTH predicts extra-tropical regions should disproportionately accumulate older taxa over geologic time due to relatively lower origination rates and high immigration sourced from tropical regions. The average longevity of range-through and last appearance He-1 genera in the two provinces are not significantly different, suggesting minimal dispersal of tropical mammalian taxa prior to and during He-1.