ANALYSIS OF ORNAMENTATION AS AN ASPECT OF PREY ESCALATION AND ITS RELATIONSHIP TO DRILLING PREDATION IN MIOCENE MOLLUSCS OF MARYLAND

Escalation is an evolutionary process by which species adapt and respond to the pressures of their enemies (any organism that can potentially cause harm or compete for resources). This study examines molluscan escalation and its relationship to frequency of naticid gastropod drilling. Naticids are infaunal, predatory marine gastropods that typically prey on infaunal gastropods and bivalves. The naticid engulfs the prey with its foot in a preferred position for drilling. Drilling involves both mechanical rasping with the radula and chemical dissolution of the prey shell. Surface ornamentation has been considered an anti-predatory adaptation because it either acts to thicken the prey shell or because spines and prominent ribs interfere with manipulation or drill hole placement by the predator. We tested the hypothesis that escalation level, as indicated by ornamentation strength, has an inverse relationship with the frequency of drill holes. In these preliminary data, three bulk samples (1215 specimens) from “zones” 21, 22, and 23 of the St. Marys Formation at Little Cove Point, Maryland, were sieved through 6 mm mesh and identified to species level. Escalation level of molluscs was determined using surface ornamentation (e.g., rib patterns, spines, etc.). To quantify these characteristics, a numerical value was assigned to the level of ornamentation for each species: 1 being least ornamented (i.e., smooth shell) and 4 being most ornamented (e.g., prominent ribs, spines). Chi-squared tests were conducted to compare the frequency of complete naticid drill holes between bivalve and gastropod specimens with ornamentation levels 1 and 2 (lower levels of escalation) and those that have ornamentation levels 3 and 4 (higher levels of escalation). None of the Little Cove Point samples exhibited a significant relationship between level of escalation and drilling frequency. These data are preliminary; more samples will be included from Miocene and Pleistocene locations that exhibit high and low assemblage-level drilling frequencies in the fossil record. Presence of additional anti-predation characteristics (e.g., overlapping/crenulated valve margins) will be compared to frequency of complete and incomplete (failed) drill holes to analyze the relationship between escalation and drilling gastropod predation.