VALIDATION OF TAXON-SPECIFIC SAMPLING FOR STUDYING DRILLING PREDATION ON FOSSIL BIVALVES

Although taxon-specific (targeted) sampling has been considered biased compared to bulk sampling for studying predation in the fossil record, Ottens et al. (2012) demonstrated that targeted sampling, even by a trained novice, can yield results comparable to bulk sampling. The present study examines whether the previous novice was atypical or if other trained novices could produce results comparable to those of experienced collectors and to those from bulk samples.

Four additional novices and two additional veteran collectors used targeted sampling in the lower Waccamaw Formation (Lower Pleistocene) at a site previously studied, Register Quarry near Old Dock, NC. Each collected five replicate taxon-specific samples, retrieving every specimen visible within ~ 1 – 4 m² areas for the bivalves Astarte concentrica, Cyclocardia granulata, Lirophora latilirata and Glycymeris. Shell length and thickness were measured and position and size of drillholes determined. Frequency of left valves, drilling frequency (number of valves with a complete drillhole divided by half the number of valves), prey effectiveness (% of drillholes that were incomplete), and size selectivity (correlation of drillhole size and prey size) were determined for each taxon replicate.

To date, we have compared results for the novice collectors to results from bulk samples and to those for the previous veteran collector for Astarte, Cyclocardia, and Lirophora for all variables except drillhole site. Of 165 comparisons between bulk samples and replicates collected by the additional novices, 88% showed no significant differences between targeted and bulk samples. Of 234 tests comparing results from the additional novices and the previous veteran, 93% showed no significant differences. Data have not yet been standardized for size; nearly all statistically significant differences occurred in the size-related variables length and thickness (18 of 20 and 15 of 17 significant comparisons for collecting method and collector expertise respectively). Astarte and Cyclocardia had more significant differences between bulk and targeted samples than Lirophora, for which sample sizes were larger, but results varied little among students. Previous results were not anomalous; use of targeted sampling in studying drilling predation is again validated.