TEMPORAL AND SPATIAL VARIATION OF DRILLHOLE FREQUENCY WITHIN THE MIOCENE ST. MARY’S FORMATION, MARYLAND

The Miocene St. Mary’s Formation offers an abundance of fossil gastropod and bivalve genera. Analysis of these predator and prey species can give insight into predation in the fossil record, which is a widely utilized indicator of ecosystem health. Complete drillholes found in fossilized gastropod shells are evidence of successful predation. Drillhole frequency (DHF) is often used to quantify predation through time. Variation in DHF through space may affect the perception of predation through time. Here, we first examined how DHF changed through time in ten stratigraphic samples of the St. Mary’s Formation. Then, we compared the temporal variation in DHF to the variation in DHF through space as indicated by seven samples collected from one stratigraphic horizon.

We employed the gastropod Ilyanassa because it was present and abundant in all of the samples with clear predatory drillholes. We calculated DHF (number of drilled shells divided by total usable shells) for samples with at least 100 individuals, resulting in seven laterally equivalent samples and nine additional stratigraphic samples. We then compared variation in DHF across space to the variation observed through time.

The range of DHFs for all of the samples was 22% to 53%. Lower stratigraphic samples generally had a lower DHF, and the upper stratigraphic samples had the highest DHFs. This demonstrates an ecological change through the St. Mary’s Formation. The DHFs of the laterally equivalent samples (the oldest aged samples) ranged from 23% to 37%, with an average of 32%. There was no significant difference between the seven spatial samples DHFs and the DHFs for the complete stratigraphic extent (p=0.07). There is still a noticeable difference between DHFs in the nine separate horizons. These findings suggest that the DHF variation observed within the laterally-sampled horizons would not affect the interpretation of the stratigraphic patterns.