EARLY EOCENE CLIMATE OPTIMUM AND ITS INFLUENCE ON THE PALEOBIODIVERSITY OF SHALLOW-MARINE GASTROPODS OF THE WEST COAST OF NORTH AMERICA

Shallow-marine gastropods of the west coast of North America are known to be indicative of warm-water conditions during the Paleocene to early late Eocene (early Priabonian). The purpose of this present study is to determine when these conditions reached their peak. To achieve this goal required making the most comprehensive and taxonomically updated compilation of all the published literature regarding the geologic stage and taxonomic position of the genera of these gastropods found in southern Alaska to Baja California Sur, Mexico. The total number of genera is 228. Nearly all were cosmopolitan and most were newcomers. North of Washington, the fossils are, however, very scarce, and their post-Eocene tectonic history is very poorly known. Generic diversity (123) of the west-coast gastropods was highest during the early Eocene (50 to 52.5 m.y.a. = middle Ypresian Stage = provincial “Capay” “stage”). That is when the warm-greenhouse climate of the Early Eocene Climatic Optimum (EECO) prevailed. Globally it was the warmest time of the Cenozoic. The thermophilic neritid and cypraeoidean gastropods, both of which were present on the west coast during the early Cenozoic, had their maximum generic diversity during the EECO. Starting in the EECO, some thermophilic gastropod genera established west-coast lineages (e.g., Conus, Eocypraea, Galeodea, Priscoficus, Rimella); others did not (e.g., Athleta, Clavilithes, Eocernina, Gisortia, Lyriscapha, and Platyoptera). Five were endemic (Loxotrema, Olequahia, Tejonia, Umpquaia, and Whitneynella). Arrival of newcomer gastropods to the west coast during the EECO was most likely predominantly via a current system that emanated from the Old World Tethys Sea (especially Western Europe) and continued across the Atlantic, down the Gulf Coast into Mexico and eventually to the west coast. Genera of west-coast large-orbitoid foraminifera, hermatypic colonial corals, and irregular echinoids also reached their highest peaks during the EECO. The west-coast warm-water fauna largely disappeared, when cooler climate began globally during the late Eocene.