Anatomy of a Regional Mass Extinction: Plio-Pleistocene Decimation of the Western Atlantic Bivalve Fauna

Abstract

The Early Pliocene marine faunas of the southeastern United States were distinct from those of the Bahamas and Caribbean, apparently being separatedfrom them by a zone of cool upwelling. Study of the fates of 361 Early Pliocene bivalve species reveals that a regional mass extinction occurred in the Eastern United States beginning in Late Pliocene time, when continental glaciers expanded, and continued into Early Pleistocene time, eliminating perhaps as many as 65% of the Early Pliocene species. Several patterns suggest that refrigeration during intervals of glacial expansion was the primary cause. The 57 bivalve species that have survivedfrom the tropical zone of Florida all range into nontropical zones today: the mass extinction operated as a thermal filter, eliminating all purely tropical species. Endemic Early Pliocene species experienced especially low survivorship (15%7o) and most of the casualities of these stenothermal forms came early, in Late Pliocene time. A larger percentage of eurythermal Early Pliocene species survived; and most of those that did not, died out relatively late, during cold Early Pleistocene glacial intervals when even Florida became nontropical. Several observations oppose the hypothesis that Pleistocene regressions would have caused heavy extinction even in the absence of refrigeration. Among these are (1) the fact that in the Eastern Pacific Pleistocene extinction was weak even for species endemic to the temperate-warm temperate shelf, which was areally smaller than the shelf of the southeastern United States, and (2) the fact that even Western Atlantic species that were small, abundant, and adapted to muddy conditions suffered heavy losses.