PRESERVATION POTENTIAL OF ROCKY INTERTIDAL BIOTA IN TEMPERATE AND TROPICAL ENVIRONMENTS

Previous studies concerning the fossilization potential of rocky shore communities agree that inclusion in the rock record is an unusual occurrence and that most ancient rocky shores are represented in the geologic record on unconformity surfaces. Due to the paucity of examples, the effect of latitude on preservation potential of intertidal communities has not been addressed. The possibility of preserving intertidal species as parautochthonous assemblages is explored by analyzing the compositional fidelity obtained in a live-dead-dead comparison from in-situ communities, sandy sub-tidal flats, and beach shell accumulations. During summer 2012, over 6500 live specimens were obtained from transects distributed among 5 collecting localities on San Salvador, Bahamas. Transects spanned the entire intertidal vertically and represent all intertidal biozones from Caribbean rocky coasts. Bulk samples from sandy sub-tidal flats and beach shell accumulations yielded ca. 8000 shells and shell fragments. All specimens from these samples were assigned a taphonomic grade (0-3) based on the degree of fragmentation. This research found that on San Salvador, Bahamas, intertidal species accounted for only 14% of specimens in the death assemblages. The compositional fidelity of the tropical rocky intertidal communities, however, was high: 70% of intertidal species were represented in the death assemblages regardless if identifiable fragments were included or not.

Appledore Island, located in the Gulf of Maine and the site of Shoals Marine Laboratory, provides a useful comparison site to study the compositional fidelity of live-dead assemblages from temperate rocky intertidal coasts. The two locations are similar in that they both are small islands featuring rocky coastlines with abundant intertidal life. Multi-year transect data from Appledore Island is used in companion with beach and subtidal death assemblages. Out of 43 marine gastropod species possessing hard parts observed on the rocks and surrounding waters, 28% were recorded in the subtidal death assemblage. The proportional occurrence of intertidal species there differed greatly from that of the living community. The differences in biodiversity and compositional fidelity of the two rocky coast localities will be discussed in detail.