MARINE HARD SUBSTRATE COMMUNITIES THROUGH TIME: PATTERNS AND PROCESSES

Hard surfaces in the sea are colonized by a wide variety of animals and plants, often forming diverse communities across otherwise relatively barren areas of the sea floor. Many of these organisms have fossilizable hard parts or excavate borings that can be preserved as trace fossils. Furthermore, in-situ preservation of the constituent organisms in hard substrate communities provides unique opportunities for studying ancient interactions between individual organisms and with their substrates. Data from a recent review of hard substrate palaeoecology (Taylor, P. D and Wilson, M. A. 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62, 1-103) reveals variations in the numbers of published studies through time, a rough proxy for the abundance of hard substrate communities in the geological record. The overall pattern resembles the Sepkoski curve of Phanerozoic marine invertebrate diversity. Rather few studies have been undertaken in the Cambrian but there is a steep rise in the Ordovician mirroring the ‘Great Ordovician Biodiversification Event’ when epifaunal suspension feeders underwent major radiation. An appreciable decline in the numbers of studies is evident for the Carboniferous and particularly the Permian. The low number of studies for the Triassic is not unexpected in view of the sustained effect of the end-Permian mass extinction on marine biotas. The Jurassic, Cretaceous and Cenozoic are each characterised by a greater number of studies than any of the periods of the Palaeozoic. Interpreting these temporal patterns is complicated by the fact that they reflect both the availability of hard substrates for colonization, and the presence of fossilizable organisms able to colonize them. Secular changes in sea-water chemistry and hardground formation undoubtedly influenced the former, providing a possible explanation for the late Palaeozoic decline.