99 Paleontology (Posters) II: Extinction and Environment

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

MOLLUSCAN DIVERSITY PATTERNS IN THE LOWER AND UPPER WACCAMAW FORMATION (PLEISTOCENE) OF SOUTHEASTERN NORTH CAROLINA AND NORTHEASTERN SOUTH CAROLINA

Paper number: 99-11
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

This study examined diversity of mollusc assemblages at three localities in the Waccamaw Formation: Snake Island Pit near Old Dock, Columbus Co., SE NC (lower Waccamaw), and Walkers Bluff (Bladen Co., SE NC) and the IntraCoastal Waterway near North Myrtle Beach (Horry Co., NE SC) representing the upper Waccamaw. Bulk samples were collected as part of a Research Experiences for Undergraduates study and sieved through a quarter-inch mesh. Students in an invertebrate paleontology class at University of North Carolina Wilmington picked bivalves with beaks and gastropods with apices, sorted and identified specimens to genus level. We analyzed diversity using Holland’s Analytic Rarefaction (http://www.uga.edu/~strata/software/) and PAST (http://folk.uio.no/ohammer/past/). Richness was expected to decline from the lower to upper Waccamaw as part of the Plio-Pleistocene extinctions in the western Atlantic.

The Snake Island sample included 36 bivalve genera (651 specimens) and 15 gastropod genera (118 specimens). At Walkers Bluff, 40 genera (1453 specimens) of bivalves and 18 genera (421 specimens) of gastropods were sampled. Three North Myrtle Beach samples yielded 45 bivalve genera (3422 specimens) and 24 gastropod genera (222 specimens). The Walkers Bluff sample showed the greatest dominance for bivalves (0.33 vs 0.12 and 0.21) and gastropods (0.66 vs. 0.18 and 0.33). When rarefied to the same sample size, the sites did not differ in richness for bivalves, but for rarefactions without the dominant genus, the upper Waccamaw at Walkers Bluff was richer than the others (38 vs. 35 genera among 520 specimens). In contrast, North Myrtle Beach was significantly richer in gastropods (18 vs. 11 and 14 rarefied genera), but all three sites had equivalent richness when rarefactions excluded the dominant genus (14 genera). Thus the expected decline in richness was not observed. When these assemblages were compared to five previously sampled lower and two upper Waccamaw sites, results are ambiguous. Bivalve and gastropod rarefied richness of each new sample overlaps in 95% confidence intervals with previous sites from both the upper and lower Waccamaw. However, average rarefied richness of all six lower Waccamaw sites exceeded that of the four upper Waccamaw sites for bivalves (40 vs 32 genera) and gastropods (18 vs 12 genera).