Taphonomy of modern and fossil intertidal gastropod associations from Isla Santa Cruz and Isla Santa Fe, Galápagos Islands

Abstract

Examination of modern gastropod associations from the low intertidal zone of Isla Santa Cruz suggests that fossil rocky intertidal deposits from this tropical locality will be taphonomically compromised in three ways: (1) Marine hermit crabs, by their use of empty gastropod shells, will mix the shells from varying tidal heights and habitats, thus facilitating mixed associations of such shells in the fossil record, (2) encrusting organisms on crab-inhabited shells are abundant, while boring organisms are almost non-existent, indicating possible differences in postmortem shell retention, and (3) intertidal shells are further taphonomically altered by terrestrial hermit crabs, which transport the shells onto land as well as physically modify the shells. Gastropod fossils from beach and terrace deposits on Isla Santa Fe are interpreted to be a mixed assemblage of rocky intertidal assemblage with few shells indicating influence from marine hermit crabs. Modification of the shell by marine and terrestrial hermit crabs was also evident. A unique polish to the shells at one locality is attributed to the marine iguanas and is only found in the terrace site biologically bulldozed by egg-laying iguanas. Few studies exist on modern rocky intertidal associations in the Galapagos, and the fossil record of rocky shores may provide a baseline for future studies in how community structure has changed over since the advent of humans.Galapagos, Coenobita Compressus, gastropods, humans, Gulf of California, bionts, nutrients. Sally E. Walker, Department of Geology, The University of Georgia, Athens, Georgia, USA; 8th September, 1994; revised 28th June, 1995.