EXPLORATION OF POTENTIALLY PRESERVED AND TRANSPORTED ROCKY INTERTIDAL COMMUNITIES

Investigations of ancient rocky intertidal communities have largely focused on in-situ fossil assemblages. The possibility of preserving intertidal species as transported assemblages was explored by analyzing the compositional fidelity (presence-absence and rank abundance) obtained through live-dead comparisons from living in-situ communities, sandy sub-tidal flats, and beach shell accumulations. Over 6,500 live specimens were obtained from 15 transects distributed among 5 collecting localities. Transects spanned all intertidal biozones from rocky coasts. Thirty bulk samples from sandy sub-tidal flats and beach shell accumulations yielded ca. 8,000 shells and shell fragments. All specimens from these samples were assigned a taphonomic grade (0-3) based on the degree of fragmentation. Death assemblages were dominated by shallow sub-tidal bivalve and gastropod fragments; almost 3,000 (37%) of these specimens were represented by complete valves or shells. Further analysis revealed that beach samples contained more complete shells than sub-tidal samples. Intertidal species accounted for only 14% of specimens from both the sub-tidal and beach death assemblages. Yet the compositional fidelity of the rocky intertidal communities was high: 70% of intertidal species were represented in the death assemblages. This holds true whether identifiable fragments were included or not. However, the rank of the 10 most abundant species was altered when fragments were removed, with complete limpet valves becoming the most abundant. Moreover, approximately 90% of rocky intertidal species found in sandy sub-tidal samples were also represented in beach shell accumulations. Results suggest that the composition of rocky intertidal communities may be preserved with high fidelity in sub-tidal fossil assemblages.