EVALUATING TAPHONOMIC BIASES ON DRILLING AND REPAIR SCAR FREQUENCIES FOR GLYCYMERIS FROM THE NORTH CAROLINA PLEISTOCENE

The combined analysis of taphonomy, drilling predation traces, and repair scars represents a powerful tool for understanding predator-prey relationships. We applied these techniques to 534 Glycymeris bivalve specimens target collected from the Pleistocene lower Waccamaw Formation near Old Dock, NC. Glycymeris were drilled frequently by predatory naticid gastropods. Repair scars likely caused by durophagous predators are also apparent. Each shell was assigned a taphonomic grade I – IV with grade I representing exceptional preservation and grade IV representing poor preservation. Shells were placed in four size categories: very small (5 – 10 mm length), small (>10mm - 30 mm), medium (>30mm – 60 mm), and large (>60 mm). For each taphonomic and size category, drilling frequencies (DF) were calculated as number of drilled valves divided by half the number of valves. Repair scar frequency (RF) was calculated as frequency of shells with at least one scar. DF and RF were compared among taphonomic and size categories using either χ² or Fisher exact tests depending on sample size.

Some workers claim that drill holes make shells more vulnerable to taphonomic destruction. We tested the hypothesis that drilled shells do not withstand exposure to taphonomic damage. DF of poorly preserved (taphonomic grade IV) shells is 0.46 and is greater than the overall DF of 0.36, suggesting no taphonomic bias exists against drilled shells.

The sample size of exceptionally preserved shells (taphonomic grade I) was too small for statistical analysis. The repair scar frequencies for grades II, III, and IV show a decreasing trend of 0.49, 0.32, and 0 respectively; differences were statistically significant. Repair scars are less recognizable on shells as preservation quality decreases. Therefore, a bias exists against RF on poorly preserved shells.

Frequency of predation traces varied with shell size. Naticid gastropods preferred to prey on Glycymeris specimens that were 10 – 30 mm in length; larger specimens may have exceeded naticid handling capabilities. Very small shells had significantly fewer repair scars than all other categories. Very small shells may have been too fragile to survive durophagous predation and are underrepresented in the fossil record. Alternatively, durophagous predators may have preferred Glycymeris >10 mm in length.