LATERAL VARIATION OF DRILLHOLE FREQUENCY WITHIN THE MIOCENE ST. MARY’S FORMATION, MARYLAND

Predation in the fossil record is an important indicator of the state of an ecosystem. Drill holes found in fossilized gastropod shells are evidence of successful predation. Although drill hole frequency (DHF) is often used to quantify predation through time, few have examined how variation in DHF though space may affect the perception of predation through time. Here, we examine how predation varies within one temporally-equivalent stratigraphic bed in order to determine if it would be sufficient to collect samples from one spot or if multiple lateral samples are required.

Six laterally-equivalent samples were collected from the Miocene St. Mary’s Formation of Maryland. Fossils were collected at 0, 1.7, 1.85, 2.1, and 2.25 km from the southwest most assessable point of the formation to examine if DHF varies between these samples. The fossils were cleaned, sorted by genus, and counted. We specifically targeted Ilyanassa because it was present in all of the samples taken, abundant, and easy to count. Ilyanassa was sorted into drilled, undrilled, and unusable fossils, and the DHF was calculated. We compared the DHF of each of the five samples to one another using two-proportion z-tests. Finally, we qualitatively compared gastropod richness and total gastropod abundance to DHF.

The two northern-most samples had DHFs of 0.25 and 0.265, and the two directly to the south had DHF of 0.34 and 0.365. These two pairs of samples were only 400 m away from each other and varied significantly in DHF (p < 0.001). These results suggest that when collecting samples for studying DHF through time, taking samples from too narrow a lateral distance could give inaccurate results. That two samples can vary significantly in DHF while only being 400 m apart implies that there may be considerable differences in predation and prey availability throughout the area. There was a negative relationship between gastropod richness and DHF as well as abundance and DHF; richness and abundance decreased and then increased from the south to the north. This may mean that predators had more prey options and therefore could be pickier about what organisms they preyed upon. When studying predation through time, researchers should collect several lateral samples from each stratigraphic bed to ensure variation in DHF does not unnecessarily distort the results of the study.