DEVELOPMENT OF A BIOGEOGRAPHIC BOUNDARY: PLIOCENE TO RECENT IN THE WESTERN ATLANTIC COASTAL PLAIN

The Plio-Pleistocene extinction occurred at approximately 2.0 Ma and resulted in the elimination of approximately 70% of Western Atlantic mollusk species. The extinction apparently coincided with the development of a strong Gulf Stream current and the formation of a major biogeographic boundary at Cape Hatteras, North Carolina separating southern, subtropical taxa from northern, temperate taxa. This situation presents an opportunity to investigate the formation of a biogeographic boundary where latitudinal differences in selectivity of extinction and recovery led to modern differences in diversity.

The goal of this study is to determine how taxa responded geographically after the Plio-Pleistocene extinction. This work builds on previous study (Hecht 1969) by using updated stratigraphic nomenclature and larger data sets from the Paleobiology Database and other published sources for the fossil data, and the Global Biodiversity Information Facility for modern data. We sort fossil and modern data into latitudinal bins from Florida to Virginia and calculate the similarity between adjacent bins, the number of range endpoints in a bin, and perform cluster analyses of all bins for each time interval.

Preliminary results confirm previous estimates of extinction magnitude; nearly 67% of species go extinct across the boundary. For genera, Jaccard similarity decreases near Cape Hatteras during the Pleistocene and slightly farther south during the Modern, but not during the Pliocene, indicating greater faunal turnover near Cape Hatteras after the Pliocene. The number of range endpoints increases slightly at Cape Hatteras in the Pleistocene and greatly in the Modern, indicating that many genera reach the northern-most limit of their range at that latitude. Finally, cluster analyses show that the fairly smooth gradient of turnover during the Pliocene is disrupted during the Pleistocene, and is further driven towards a faunal break at Cape Hatteras during the Modern. Taken collectively, these results suggest that the biogeographic boundary at Cape Hatteras formed after the Pliocene, but took time to evolve into the sharp faunal boundary seen today.