Mollusc life and death assemblages on a tropical rocky shore as proxies for the taphonomic loss in a fossil counterpart

• Comparison of rocky shore life and associated death assemblages
• Testing the fidelity in reconstruction of fossil rocky shore faunas
• High environmental fidelity between the life and death assemblages
• High environmental fidelity between the life and a hypothetical fossil assemblage
• Change from a gastropod-dominated life to a bivalve-dominated fossil assemblage

Comparison of a modern rocky shore mollusc life assemblage from Thailand with the associated death assemblage, and interpretation of the fossilization potential of the latter, are used to investigate the fidelity in reconstruction of ancient analogues. The fauna from the death assemblage represents species from the rocky shore and the associated sandy pocket beaches, and only a few exotic species from other, completely different habitats are present. The environmental fidelity between the life and death assemblage is thus high, with the majority of species from the death assemblage representing the intertidal to shallow subtidal rocky shore environment, from which the life assemblage was sampled, and the associated sandy beach environment. The life assemblage should in principle have a high fossilization potential because only two out of 67 species are without a calcareous shell, but it actually has a lower taxonomic agreement to the death assemblage than found in previous published studies. Rocky shore life and death assemblages thus appear to show lower taxonomic agreement compared to muddy or sandy shelf assemblages due to the mix after death with the sandy beach assemblage. A hypothetical fossil assemblage is constructed from the death assemblage, assuming that only calcitic and infaunal aragonitic species have a chance to be preserved in the geological record. The hypothetical fossil assemblage is not taxonomically similar to the life assemblage but shows high environmental fidelity to the rocky shore and sandy pocket beach. In terms of species richness the life assemblage is dominated by epifaunal, herbivorous gastropods, whereas the hypothetical fossil assemblage is dominated by infaunal, suspension feeding bivalves. The hypothetical fossil assemblage thus represents a faunal community from an intertidal to shallow subtidal rocky shore environment and the associated sandy pocket beach environment, and shows marked differences in faunal composition, modes of life, richness and density patterns compared to the rocky shore life assemblage due to the mix between the two associated habitats. This serves to illustrate the significance of integrating the taphonomic influence on a fossil faunal assemblage before attempting to reconstruct an ancient ecosystem and environment.