DIFFERENCES IN NORTH AMERICA XENARTHRAN DISTRIBUTION IN THE IRVINGTONIAN AND RANCHOLABREAN: AN APPROACH TO BETTER UNDERSTANDING THE GREAT AMERICAN BIOTIC INTERCHANGE

Originating in South America, members of the Xenarthra (sloths, anteaters, armadillos, pampatheres, glyptodonts), dispersed into North America starting in the late Miocene. The first appearance of different taxa outside of South America is used to define the stages of the Great American Biotic Interchange (GABI). Given the diversity of Xenarthrans in South America, only a small subset were able to move northward into tropical Central and subsequently into temperate North America. A related question is why different taxa dispersed at different times? Was their dispersal northward facilitated or hindered directly by patterns of climate change during the Plio-Pleistocene or secondarily by the impact of climate change on the local ecology and habitats used by the different Xenarthrans as dispersal routes during the GABI? Are these differences reflective of differences in the ecology between the different taxa or merely a series of random events?

One approach to identifying climatic versus ecological parameters that affected the ecology and hence the distribution of the xenarthrans that did participate in the GABI is to compare their distribution in the Irvingtonian and Rancholabrean. For some taxa there is no change, while for others, the differences between their Irvingtonian and Rancholabrean distributions are significant and clearly indicate that each taxon responsed differently to changing climatic and ecological parameters during the Pleistocene. The patterns in how each taxon's range changed permit allow us to extrapolate some of the factors that allowed them to disperse out of South America. For example distributional changes that occur with longitude might indicate a response to changing rainfall patterns while changes in latitudinal distribution or in elevation may indicate a response to temperature change.