BIOGEOGRAPHIC CHANGES FROM THE PLIOCENE TO MODERN IN THE ATLANTIC COASTAL PLAIN

The Pliocene to Modern transition is important because Modern carbon dioxide concentrations are beginning to resemble those of the Pliocene (Pagani 2009; Jones 2013), the Plio-Pleistocene extinction imprinted modern biogeography and diversity (Hecht 1969; Stanley 1986), and modern taxa have begun to shift their geographic ranges in response to local climate changes (Pinsky et al 2013). Today, faunal composition and diversity change dramatically across Cape Hatteras, North Carolina (approximately 35ºN), which marks the boundary between sub-tropical and temperate taxa. Here we use fossil data from the Paleobiology Database and modern data from the Global Biodiversity Information Facility to determine how geographic range structure changed from approximately 25ºN to 40ºN from the Pliocene to the Modern.

To characterize these changes, we used cluster analysis, latitudinal range endpoints, and range-through latitudinal diversity. The Pliocene cluster shows two major groups, but neither is associated with position relative to Cape Hatteras. Both the Pleistocene and Modern clusters sort primarily by position north or south of Cape Hatteras indicating that the major source of faunal variation in these data occurs across this boundary. Pliocene northern-most range endpoints found near Cape Hatteras (from 34ºN to 36ºN) are no more than expected by chance (observed: 11%; expected: 6%-13%). However, Pleistocene and Modern northern-most range endpoints near Cape Hatteras far exceed random (Pleistocene observed: 28%; expected 10%-17%; Modern observed: 31%; expected: 15%-21%). Of those taxa that occur in both the Pliocene and Pleistocene, most shift their northern endpoint southwards (88%). Only about half of the taxa that occur in the Pleistocene and Modern shift south (52%). Diversity is relatively flat across the 25ºN to 40ºN corridor during the Pliocene, but diversity decreased markedly near 35ºN during the Pleistocene from a plateau south of Cape Hatteras, and this pattern is maintained today. Overall these patterns suggest that genera shifted southward from the Pliocene into the Pleistocene, and that a major biogeographic boundary of molluscan diversity emerged near Cape Hatteras starting in the Pleistocene.