RELATIONSHIP OF DRILLING PREDATION TO MOLLUSCAN ESCALATION AS INDICATED BY ANTI-PREDATORY CHARACTERISTICS IN THE CHOPTANK AND ST. MARYS FORMATIONS, MARYLAND MIOCENE

Escalation is an evolutionary process by which species adapt to the pressures of their enemies (organisms that can potentially cause harm or compete for resources). We examined molluscan escalation and its relationship to frequency of naticid gastropod predatory drilling. Surface ornamentation, thick shells, large size, and tight valve closure have been considered anti-predatory adaptations because they interfere with capture, manipulation, and drilling of prey items by the predator. We tested the hypothesis that escalation level, as indicated by shell surface ornamentation, thickness, size, and tightness of valve closure, has an inverse relationship with drilling frequency. We sieved bulk samples through 6mm mesh and identified specimens to genus level. Our samples contained 40 genera and 1,465 specimens from four localities representing “zones” 17- 24 in the Choptank and St. Marys formations (Miocene) of Maryland. To quantify surface ornamentation, a numerical value was assigned for each species: 1 being least ornamented (i.e., smooth shell) and 4 being most ornamented (e.g., prominent ribs, spines). Thickness was measured with digital calipers at a consistent point along the ventral margin for bivalves and at the aperture for gastropods. Relative thickness was calculated by dividing thickness by the geometric mean of length and height. Tightness of valve closure was determined by the presence of a crenulated or overlapping margin among bivalve species. 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 (less escalated) and those that have ornamentation levels 3 and 4 (higher levels of escalation). Chi-squared tests were also conducted to compare drilling frequency between bivalves with and without tight valve closure (i.e., overlapping or crenulated margins) and for bivalve and gastropod specimens that differ in size and relative shell thickness. Chi-squared tests showed a significant inverse relationship of drilling frequency to bivalve thickness and gastropod size, and an unexpected positive relationship between drilling and degree of gastropod surface ornamentation at Little Cove Point in the St. Marys formation. No other tests at any localities yielded significant results.