IS THERE PREDATORY DRILLHOLE SITE STEREOTYPY IN THE MIOCENE ST. MARY’S FORMATION?

This study explores predatory drillhole patterns in the Little Cove Point Member of the Upper Miocene St. Mary’s Formation, Maryland (~8 Ma). We examined predatory drillholes on three gastropod taxa: Ilyanassa,Mangelia, and the family Naticidae (the genera Euspirella and Neyerita combined for analysis). Naticids were the predators, preying upon Ilyanassa and Mangelia and cannibalizing fellow naticids. We looked at drillhole site stereotypy, in which holes occur in non-random locations on the prey shell. Stereotypy can reflect behavioral patterns in the predator-prey interaction. To test for drillhole site stereotypy, we identified complete naticid drillholes, noted whether the hole was on the apertural or abapertual side of the shell, and noted which whorl the hole was on.

Drillhole frequencies (DHF) were similar among the three taxa: Ilyanassa (n=958) had a DHF of 0.46, Mangelia(n=415) had a DHF of 0.42, and Naticids (n=143) had a DHF of 0.41. Both Ilyanassa and naticids were preferentially drilled on the abapertural side on the second whorl. Previous research has found that drillhole location may depend on the size and prey shell and the predator’s handling behavior. Drilling on the apertural side may be preferred since the predator’s foot seals off the prey aperture, blocking escape. This may be the case with Ilyanassa and naticid prey. Mangelia tends to be smaller than the other prey taxa. The abapertural hole locations may indicate that the naticid predators had to grapple Mangelia differently than larger prey, perhaps because the small Mangelia was more able to escape from the apertural position.