184
Journal of Paleontology 92(2):183–188 Chesaconcavus chesapeakensis also colonized other sub-
Figure 1. Buccinofusus parilis, CAS 90301, length 80mm, with a single large Chesaconcavus chesapeakensis on its dorsal side. Scale bar=10mm.
strata, including the shells of the large pectinid Chesapecten santamaria. Here again we asked whether barnacles settled on living scallops or on shells after the hosts’ death. In the former case, barnacles should be confined to the outer shell surface and to grow to but not beyond the valve margins. The feeding habits of some predatory snails can be inferred from the contour of the outer lip. In busyconines and fasciolariids that prey on bivalves by wedging the lip between the valves or chipping the valve edges by striking the edge of the lip against the valve margins, the predator’s outer lip has a convex sector where force is con- centrated (Dietl, 2003a, b, 2004; Dietl et al., 2010; Durham et al., 2012; Vermeij, 2015). Convex-lipped species often show repeated repaired damage resulting from attempts to wedge or chip prey valves. The predator’s outer lip often sustains damage when the prey shuts its valves in defense. The damage is rarely lethal and is easily and rapidly repaired. Species that do not use the outer lip to subdue molluscan or barnacle prey tend to have a planar outer lip. We place our investigation of the barnacle, the gastropod,
and the bivalve in the broader context of comparative paleoe- cology by asking how the Miocene community at Chancellor’s Point differs from modern shallow-water marine communities in the same geographical area, the northwestern Atlantic coast of the United States between Cape Cod, Massachusetts, and Cape Hatteras, North Carolina. We also consider other studies of barnacles on shells to assess how our findings compare with those in other fossil and living communities.
Materials and methods
Figure 2. Chesapecten santamaria, CAS 90302 (right valve) with a cover of Chesaconcavus chesapeakensis. Note the barnacles at the edge going toward but not beyond the shell margin. Scale bar=10mm.
barnacle Chesaconcavus chesapeakensis Zullo, 1992. If barna- cles settled on living hosts, they should be concentrated on the upper and lateral surfaces of the shell, and no barnacle should extend its base beyond the edge of the shell’s outer lip or grow on the shell’s interior surfaces. Furthermore, many barnacles should curve in the direction of the host’s spiral growth, indi- cating that they grew in size as the gastropod host added shell material at the outer lip in a spiral direction. Barnacles settling on empty shells or on shells occupied by hermit crabs would not be subject to spatial restrictions and would not be expected to curve in the direction of growth because empty and hermit-crab- occupied shells cannot grow. Interior shell surfaces of empty or crab-occupied shells would normally be colonized by epibionts, whereas they would remain smooth in living snails (Walker, 1988, 1992).
We studied mollusks from Chancellor’s Point, St. Mary’s County, Maryland, collected by Roy K. Kropp in the late 1980s when the site was still accessible. Material comes from what Petuch (2004) called the Chesapecten santamaria community, a shallow subtidal assemblage in pale silty sand from the Windmill Point Member (Zone 24) of the St. Mary’s Formation of late Miocene (early Tortonian) age. Besides the focal species Conradconfusus parilis and Chesapecten santamaria, the assemblage contains such common large mollusks as the gastropods Coronafulgur coronatus (Conrad), Turrifulgur fusiformis (Conrad, 1840), Gradiconus diluvianus (Green), Ecphora gardnerae Wilson, Mercenaria tetrica (Conrad), Dosinia thori Ward, 1992, Dallarca idonea Conrad, and Chesacardium laqueatum (Conrad) (for taxonomy, see Ward, 1992; Petuch, 1993, 2004; Petuch and Drolshagen, 2010). The many endemic lineages in this assemblage existed under warm- temperate conditions (Ward, 1992). We have no information on the method of collection, but
the presence of numerous small shells in the collection indicates that material was collected nonselectively with respect to size, epibiotic cover, or other characteristics relevant to this paper. Nevertheless, given that our study deals with specimens from only one locality, we have avoided over generalizing our results and adding undue significance to them. For Conradconfusus parilis and Chesapecten santamaria,
we examined each specimen for the presence and position of barnacles and other epibionts, with particular attention to whether the barnacles would have interfered with the activity of
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