IS NATICID CANNIBALISM PREFERRED IN THE MIOCENE ST MARY’S FORMATION?

Predation in the fossil record provides information on ecological relationships and evolutionary trends through geologic time. Fossilized gastropods with drillholes in their shells show an unquestionable record of predation. These drillholes caused by Naticids are found both on prey taxa as well as Naticids themselves. Here, we compared the DHF among four gastropod taxa from the Miocene St. Mary’s Formation to evaluate if predator Naticid gastropods are more likely to cannibalize themselves or non-predator taxa (Turritella, Ilyanassa, Mangelia).

We collected 11 samples at the Calvert Cliffs along the western Chesapeake Bay. The sediment was separated from the fossils, and they were sorted by size until we were left with >2mm fossils. The four taxa were identified and counted, recording the presence or absence of drillholes. We then calculated the drillhole frequency (DHF) of each taxon and compared the shell size of each taxon to DHF.

Ilyanassa had a DHF of 0.42 and a mean drilled height of 6.2mm with 3736 total drilled and undrilled fossils. The DHF of Mangelia was 0.40 (n = 935) with a mean drilled height of 6.2mm and undrilled height of 6.1mm. The DHF of these two taxa helped to support our original hypothesis of non-cannibolistic predation being more common than cannibalism. Naticids had a DHF of 0.31 ( n = 377) and their mean drilled width was 4.8mm with undrilled height of 4.4mm. Turritella's DHF was 0.21 (n = 2858) with a mean drilled height of 14.8mm and undrilled height of 12.3mm. Our prediction was refuted by the outcome of Turritella having a lower DHF than Naticids. The DHF of Ilyanassa and Mangelia were in line with the hypothesis of predation being more common than cannibalism. Our hypothesis did not hold for Turritella.

These results could be explained by a size preference caused by Naticids attempting to be energy efficient by optimizing their foregoing strategy. This type of predation relies on prey being a desirable size that provides enough energy for the predator to attack, but the prey isn’t too large to be a danger to the predator.