Census Assemblages in Hard-Bottom Coastal Communities: A Case Study from the Plio-Pleistocene Mediterranean

Census assemblages from hard-bottom coastal environments have scarcely been reported in the fossil record. Late Pliocene-Pleistocene sdiments from the marginal Mediterranean Almeria-Nijar Basin (SE Spain) contain fossils with exceptional preservation following catastrophic burial (obrution deposits). The assemblage, dominated by barnacles, bryozoans, coralline algae, mussels and other mollusks, colonized debris-flow boulders of a fan delta in a protected, shallow subtidal environment, probably a bay. The minimum time for paleocommunity development isestimated at 17 years based upon coralline growth-rates. Reactivation of fluvial processes in the area caused anastrophic burial and instantaneous death; the biota remained in situ and permanently isolated from post-burial reworking. Taphonomic bias due to organismic escape from burial and sediment mixing by bioturbation is negligible. Dissolution of skeletal aragonite and loss of non-skeletonized organisms seem to be the main post-burial biases altering the original community. The biotic composition of hard-bottom coastal communities has remained almost unchanged since the Late Oligocene, at least at high taxonomic level of skeletonized organisms, indicating coordinated stasis. This represents an ecological-evolutionary unit for the Neogene and Quaternary