CONTRASTING PATTERNS OF PLIOCENE AND PLEISTOCENE EXTINCTIONS OF MARINE MOLLUSKS IN WESTERN NORTH AND SOUTH AMERICA

The western margins of North and South America span different latitudinal belts and consequently experience different oceanographic and climatic regimes. Such distinctions can explain in part the contrasting hemispherical patterns of extinction in temperate marine molluscan faunas during the Pliocene and Pleistocene. The mid-latitude late Neogene littoral zone of California and the Pacific Northwest has been occupied by a suite of latitudinally mobile molluscan faunas that have undergone gradual rates of extinction and origination. In contrast, the mid- and low-latitude littoral zone of Peru and Chile has been home to one or two faunas that have long been geographically constrained by steep thermal gradients at 4°30'S. The South American fauna was subject to a two-step mass extinction event that eliminated 80 percent of species between 3 and 2 Ma. The first phase of extinction disproportionately removed taxa with warm-water Panamic affinities (e.g., species of Anadara, Dosinia, Macrocallista, Cancellaria, Terebra), whereas the second phase extinguished endemic taxa and taxa with wide-ranging geographic affinities (e.g., species of Chlamys, Panopea, Chorus, Herminespina, Concholepas) from quiet-water, mixed substrate environments. Late Cenozoic manifestations of the Peruvian Molluscan Faunal Province have been characterized by climatic uniformity (coastal aridity from northern Peru to central Chile; coastal upwelling extending cool isotherms into low latitudes). A physiographic uniformity (straight shorelines, often lined with cliffs; small rivers; insignificant embayments) was imposed during the late Pliocene as the result of subduction-induced coastal uplift. The reduction of habitat diversity in a region already lacking a diverse ecological landscape just after a period of late Pliocene ocean cooling may be responsible for the severity of the South American extinctions.