MOLLUSCAN DIVERSITY OF THE YORKTOWN FORMATION (PLIOCENE) IN VIRGINIA: AN UPDATE

Seventeen years have passed since Pliocene Molluscs From The Yorktown and Chowan River Formations In Virginia (L. Campbell, 1993) documented 572 Lower and Middle Pliocene species. Subsequent shorter publications, collections and manuscripts provided additional species or analysis. The earliest deposits of Zone 1, Sunken Meadow beds, had a fauna of 105 species, with a preponderance of cold-temperate species, documentable in 1993. M. Campbell (thesis, 1998) added 115 species with mainly warm-temperate to subtropical ranges, suggesting pulsed alterations of marine provincial affinities during that interval. The earliest post-Sunken Meadow fauna had only 5 calcitic species documented. Rediscovery of this fauna, marked by the bivalve Puberella cortinaria, at Kings Mill (Camp Wallace, Virginia of earlier collectors), contributed over 60 species. Subsequent work added 12 species to the 507 species documented from the six stratigraphic units of younger Yorktown strata (L. Campbell, 1993). In North Carolina Bailey (1977) documented 116 Chowan River species and Ward and Gilinsky (1993) listed 214 species, while L. Campbell (1993) added 108 from Yadkin, Virginia deposits.

The faunal totals are Sunken Meadow beds 220 species; P. cortinaria beds 60 species; and younger Yorktown strata collectively 519 species. The aggregated Chowan River fauna probably exceeds 250 species. Combined Yorktown and Chowan River faunas now approach 650 species. For all of their diversity, the assemblages retain their warm-temperate provincial signature, becoming fully subtropical only south of the Neuse River. Extinction levels were 86 percent for the Sunken Meadow fauna, about 80 percent for the younger Yorktown faunas, and about 70 percent for the Chowan River faunas. These high levels of extinction reflect elimination of the warm-temperate province and cooling of Western Atlantic waters during repetitive Pliocene and Pleistocene regressions.