A COMPARISON OF ANALYSES OF DRILLING PREDATION ON FOSSIL BIVALVES USING DIFFERENT SAMPLING METHODOLOGIES: BULK SAMPLING VS. TAXON-SPECIFIC SAMPLING AND THE ROLE OF COLLECTOR EXPERTISE

Taxon-specific sampling has been used in many studies of drilling gastropod predation and is particularly useful for rare taxa. Whether predation metrics from such samples are biased compared to bulk samples, the most widely accepted method of collection for studies of predation, requires testing.

To test for bias, we compared analyses of predation in bulk samples and taxon-specific samples collected by a novice and veteran collector using the bivalves Lirophora, Astarte, Cyclocardia, and Glycymeris. The Early Pleistocene lower Waccamaw Formation was sampled at five localities in southeast North Carolina by taxon-specific collection and bulk collection as part of a Research Experiences for Undergraduates program. Variables included body size distribution, valve frequency, thickness, size selectivity (measured by correlating drillhole size and prey size), drilling frequency (DF), prey effectiveness (PE), and drillhole site selectivity using Kelley’s (1988) 9-sector grid. DF equals the number of valves with a complete drillhole divided by half the number of valves (as only one of an individual’s two valves is drilled). PE is defined as the number of incomplete drillholes divided by the total number of drilling attempts.

Preliminary results for Holloman Pit show no statistically significant differences in DF and PE between sampling methods or collector expertise level for Astarte or Cyclocardia. However, differences between sampling methods for Astarte were statistically significant for body size distribution, valve frequency, thickness, and size. Body size distribution, thickness, size, and site selectivity were different for Cyclocardia. There were no statistically significant differences between collectors. These differences best reflect shell size variability between the bulk samples and taxon-specific samples. Preliminary results for Williamson Pit also show no significant differences between expertise level for body size distribution, valve frequency, thickness, DF, size, and site selectivity. PE was not evaluated as no specimens with incomplete drillholes were collected. Results indicate no bias due to expertise level but some size-related differences between sampling methods in variables typically examined in studies of drilling predation. Work on additional sites is in progress.