Journal of Paleontology, 92(2), 2018, p. 183–188 Copyright © 2018, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.124


Barnacles, their molluscan hosts, and comparative ecology in the St. Mary’s Formation (late Miocene) of Maryland, USA


Geerat J. Vermeij, and Sara K. Ruch


Department of Earth and Planetary Sciences, University of California, 1 Shields Avenue, Davis, California 95616,USA⟨gjvermeij@ucdavis.edu⟩, ⟨skruch@ucdavis.edu⟩


Abstract.—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 Forma- tion (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.


Introduction


Fossils can inform the study of modern ecosystems by showing how species interactions in ancient communities compare with those of today. To reconstruct ancient interactions, it is essential to infer the modes of life of key players from data on the relationships among form, function, and habit in living counterparts, supplemented by evidence from the fossils themselves. To illustrate how independent sources of evidence can be


used to infer the habits and ecological relationships of common fossil species, and to evaluate how extinction is selective not only at the species level but also with respect to entire modes of life, we investigated two large mollusks in the St. Mary’s For- mation of late Miocene (Tortonian) age in Maryland, the buc- cinid gastropod Conradconfusus parilis (Conrad) (Fig. 1) and the scallop Chesapecten santamaria (Tucker) (Fig. 2). Both species are common at the Chancellor’s Point locality in Zone 24 of the Windmill Point Member (Petuch, 1988, 1993, 2004; Ward, 1992; Petuch and Drolshagen, 2010). Conradconfusus parilis is the youngest member of a lineage that extends back in the Chesapeake region to the early Miocene (Petuch, 1993) as Buccinofusus. Its taxonomic placement remains unsettled, but we tentatively assign it to the Buccinidae because of its morphological similarity to such genera as Kelletia, Penion and Troschelia. Snyder (2002) noted that the genus name Buccino- fusus, to which Conradconfusus parilis had been assigned by earlier authors, was preoccupied, and therefore established the


name Conradconfusus for this taxon. Chesapecten santamaria, a scallop that likely swamduring young stages but became more sedentary as an adult (Gould, 1971), likewise belongs to a Chesapeake lineage extending back to the early Miocene (Ward and Blackwelder, 1975). The life habits of Conradconfusus parilis have not been


investigated. Here we ask whether it was epifaunal, living and crawling on a hard surface, or infaunal, living and burying itself beneath the surface of soft sediments. Second, we determined what kind of predator Conradconfusus parilis was. In particular, we asked whether this gastropod consumed hard-shelled prey by wedging the shell’s outer lip between the valves of prey bivalves, or whether it used methods not involving its shell lip. Relationships of the animal to the sediment can be assessed


in two ways. First, the shells of epifaunal and infaunal gastro- pods can usually be distinguished on the basis of morphology. Elsewhere, Vermeij (2017) has established criteria, based on streamlining and other traits of form and shell sculpture, that are associated with active sand burial. These include a streamlined shape (fusiform or high-spired, with a gently tapering spire and without angulations or an open umbilicus), the presence of terraced features in which the leading edge of ridges or sutures is less steep than the trailing edge, and a pattern of bioerosion and epibiont cover in which the colonists affect only the dorsal side of the shell of a living gastropod. Second, epibionts can be used as additional evidence


for the habits of living gastropods. At Chancellor’s Point, Conradconfusus parilis is frequently colonized by the large


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