Fossils can inform the study of modern ecosystems by showing how species interactions in ancient communities compare with those today and how extinction is selective not only with respect to species but also with respect to entire modes of life. We studied the life habits and pattern of occurrence of the barnacle Chesaconcavus chesapeakensis Zullo, 1992 on the shells of the gastropod Conradconfusus parilis (Conrad) and the bivalve Chesapecten santamaria (Tucker) from Chancellor's Point in the Windmill Point Member of the St. Mary's Formation (late Miocene, Tortonian) of Maryland. Using several criteria, we show that the barnacle occupied living hosts only. The 59% incidence of the barnacle on Conradconfusus parilis is high compared to known living associations between barnacles and gastropods. Although Conradconfusus parilis with and without barnacles do not differ in size, suggesting that the barnacle had little effect on this gastropod, there is some indication that Chesapecten santamaria with barnacles are somewhat smaller than those without and may therefore have been adversely affected by the presence of barnacles. On the basis of morphology and the low (15%) incidence of repaired scars, Conradconfusus parilis was a predator that did not use its shell lip to subdue prey. No ecological equivalents of Conradconfusus and Chesaconcavus have existed in the temperate northwestern Atlantic between Cape Cod and Cape Hatteras for the past three million years.