99 Paleontology (Posters) II: Extinction and Environment

Monday, 10 October 2011
9:00 AM - 6:00 PM
Hall C

EFFECT OF PLEISTOCENE EXTINCTIONS ON PALEOECOLOGY OF LOWER AND UPPER WACCAMAW FORMATION MOLLUSCAN ASSEMBLAGES IN SOUTHEASTERN NORTH CAROLINA AND NORTHEASTERN SOUTH CAROLINA

Paper number: 99-10
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Ecological relationships within three mollusc assemblages from the Waccamaw Formation were examined by students in an invertebrate paleontology class at University of North Carolina Wilmington. The lower Waccamaw was collected at Snake Island Pit near Old Dock, Columbus Co., SE NC. The upper Waccamaw was collected at Walkers Bluff (Bladen Co., SE NC) and North Myrtle Beach (Horry Co., NE SC). Research Experiences for Undergraduates students collected and processed the samples by sieving through ¼” mesh; UNCW students picked mollusc specimens with beaks (> 5500 bivalves) and apices (~800 gastropods), sorted and identified them to genus level. Life modes were assigned using the NMITA molluscan life mode database, including diet, substrate relation, attachment and mobility for bivalves and diet for gastropods. Data on predation by shell-drilling gastropods were also collected. Results were compared for the lower and upper Waccamaw to determine the effects of Pleistocene extinctions on paleoecology.

Most bivalve genera were infaunal (71-79%), actively mobile (74-82%), and suspension feeders (67-80%); these percentages did not differ significantly between the lower and upper Waccamaw. However, the lower Waccamaw had significantly more suspension-feeding bivalve individuals (97% vs. 85-91%), epifaunal individuals (53% vs. 9 – 22%), and significantly fewer actively mobile individuals (68% vs 76 – 92%) than the upper Waccamaw. The lower Waccamaw also had more suspension-feeding gastropod individuals (53% vs 7 – 28%) and fewer predatory gastropod individuals than the upper Waccamaw (45% vs 60 – 91%), though percent of genera did not vary significantly among localities. Among the predatory gastropods, shell-drilling naticids were uncommon, and drilling frequency (DF) on gastropods was relatively low (3% for Walkers Bluff and ~10% for the other sites). Bivalves were more frequently attacked, with DF ~27% for Snake Island and Walkers Bluff and 15% for North Myrtle Beach. Incomplete (failed) drillholes were rare (0 – 8%).

The drop in abundance but not diversity of suspension feeding bivalves and gastropods also occurred in the Caribbean Plio-Pleistocene, where it was attributed to productivity decline. However, the Caribbean showed fewer predators post-extinction and more epifaunal taxa, unlike in the Carolinas.