Uniformitarianism as a guide to rocky-shore ecosystems in the geological record

Abstract

Literature on Pleistocene and Neogene rocky shores is reviewed from a uniformitarian perspective to assess the fidelity of physical and biological information entrained in the geological record through present-day processes. Coverage by latitude spans tropical to subarctic shores. Variations in paleotopography are illustrated by unconformities between sedimentary accumulations and rocks of igneous or non-igneous origin. Former rocky shores occur across a range of geographic scales that include (i) spot localities, (ii) embayments with continuous transition from exposed outer to protected inner shores, and (iii) entire islands with windward and leeward shores. Differences in rate and magnitude of sea-level change affect rocky-shore physiography. Terrace deposits characterize the Pleistocene, but ramp deposits typify older Neogene shores. On average, species with hard parts account for about 50% of the modern rocky intertidal fauna on a regional basis, regardless of latitude. Biodiversity is highest in mid-latitudes. Fixed organisms with encrusting, wedging, or boring habits (corals, barnacles, oysters, other bivalves, and vermetid gastropods) show high potential for fossilization in growth position. Mobile organisms that cling to rocks (gastropods, crabs, and echinoids) suffer post-mortem transport but may be immured within neptunian dikes or interstices among cobbles and boulders. At best, localized fossil deposits from the Pleistocene reflect <30% of the biodiversity of skeletonized organisms tabulated from modern rocky shores on a regional basis. Uniformitarianism provides little insight into the diversity of soft-bodied organisms (excluding stromatolites) that joined the ecosystem in Precambrian times and the extent to which soft-bodied organisms dominated shore life through later times.