12-3 Comparison of Drillhole Frequencies between Small and Large Size Fractions in Mollusks from the Miocene St. Marys Formation, Maryland

Session: Advances in ichnology: walking in Memphis and other records of behavior (Posters)

Poster Booth No.: 88

Presenting Author:

Haley Haulk

Authors:

Haulk, Haley Xylem¹, Gay, Alexis², Stafford, Emily S.³, Forcino, Frank Louis⁴

(1) Western Carolina University, Cullowhee, NC, USA, (2) Western Carolina University, Cullowhee, NC, USA, (3) Western Carolina University, Cullowhee, NC, USA, (4) Western Carolina University, Cullowhee, NC, USA,

Abstract:

Drilling predation is one of the most-studied interactions in the fossil record, but research rarely includes very small (<2mm) specimens, due to fragility and difficulty in collecting and handling. Small specimens are often ontogenetically younger and can provide insight into how predation affects populations at different life stages. We examined the bivalve Spisula and the gastropods Ilyanassa and Mangelia from the Miocene St. Marys Formation of Maryland, hypothesizing that naticid drillhole frequencies (DHF) would be similar between small and large size-fractions. Four bulk-sediment samples from a 4-meter stratigraphic range of the Little Cove Point Member were sieved into small (1-2mm) and large (2-12mm) fractions, subsampled, and counted. DHFs were calculated by dividing the number of whole shells with complete drillholes by total whole shells. Large and small-fraction DHFs were compared using chi-square tests. 

Spisula was the most abundant taxon in Samples 2-4 (small n=185,262,109; large n=124,1224,95). Small DHF (0.12,0.09,0.03) and large DHF (0.10,0.03,0.14) differed in Samp3 (p<0.001) and Samp4 (p=0.008) but not Samp2. Small DHF was higher than large in Samp2-3, but lower in Samp4. Spisula was nearly absent in Samp1.

Gastropod sample sizes were too low to statistically compare DHFs by sample, so Samp1-4 were combined. Large Ilyanassa (n=161, DHF=0.52) were considerably more drilled than small (n=48, DHF=0.04, p<0.001); this was consistent among the stratigraphic samples. Mangelia was much less abundant: while large (n=53, DHF=0.42) were more drilled than small (n=10, DHF=0.10), the difference was not significant.

Comparing large DHFs among genera and by sample, there were no obvious relationships or stratigraphic trends in DHF, prey genus abundances, or the abundance of naticid gastropods (n=1,33,18,14). Further research, including increasing sample sizes and examining drillhole site stereotypy, may help to find patterns and explain observed differences.

We found that DHFs are not reliably similar between the large and small size fractions of paleocommunities. Importantly, there was no consistency in which size-fraction was more heavily drilled. This suggests that it is important to examine size-fractions of the community separately to fully understand the impact of drilling predation. These findings add depth to our understanding of how small specimens can help us interpret the impact of drilling predation throughout Earth’s history.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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