Under-documented micro species and the end of Carolinian endemism: bivalves of the Waccamaw Formation (basal Pleistocene, S.E. USA)

ABSTRACT The Waccamaw Formation is a Gelasian (basal Pleistocene, 2.5–1.8 Ma) muddy shell-sand found in patches in southeastern North Carolina and northeastern South Carolina, USA. It has a diverse molluscan fauna but has never been monographed. Previous studies documented about 650 species of molluscs, including about 215 bivalves. Extensive sampling of three sites and examination of other unpublished collections has increased those figures to 1,250 and 335 species, respectively. In addition to numerous geographical and chronological range extensions, this study has also discovered 52 undescribed species of bivalves and 8 apparently undescribed genera of bivalves (two of those are extant and known from previous studies) and 9 families of bivalves new to the formation. Galeommatoidea has seen the greatest increase in documented diversity, going from 21 to 53 species known from the formation; the next most dramatic is Anomalodesmata, going from 14 to 32 species known to be present. The molluscan fauna of the Waccamaw Formation has a species richness more typical of tropical areas, but with a distinctive mix of subtropical shallow to mid-shelf taxa, and a few typically cold-water groups. The Recent fauna in the region with the most comparable average water temperature, South Carolina to northern Florida, has much lower diversity (about 775 species of molluscs, with about 280 of them bivalves) and lower endemism (almost no recent endemics) in a comparable amount of geographic area to the Waccamaw Formation deposits. A dramatic drop in typical winter water temperatures c. 1.8–1.7 Ma seems to be the most likely cause for these changes. Bivalve groups which have had the greatest drop in regional diversity are Noetiidae, Carditidae, Astartidae, Galeommatoidea, Ungulinidae and Pandoridae. Contrastingly, the local diversities of Arcidae, Mytiloidea, Lucinidae, Cardiidae and Pholadidae have all increased between the Gelasian and the recent, overwhelmingly through northwards range expansions.