Echinoids as substrates for encrustation : review and quantitative analysis

The existing literature, including records of both fossil and extant echinoid encrustation, is quantitatively analysed and reviewed. This shows that echinoid encrustation (number of encrusted echinoid taphocoenoses) has increased nearly continuously and dramatically to the present day, as confirmed by linear regression values of more than 85 per cent. It also demonstrates that current levels of echinoid fouling stabilised by the Miocene, while there has been a more or less continuous record of echinoid encrustation since the Late Cretaceous. Several increases have been identified since echinoid encrustation first noted occurrence from the Late Carboniferous. This trend is explained as the probable result of corresponding increases in productivity (richness, biomass, energetics, ecospace utilisation) and resources in the marine environment, including epibionts and their hosts. This conclusion matches other indicators, including the number and thickness of shell beds, bioerosion and predation intensity or biodiversity. The trajectory might have been altered to some degree by biases (e.g. selective recording, sampling effort, outcrop area, rock volume) in the same way as palaeobiodiversity estimates. Two recognised long-term gaps in echinoid encrustation (Upper Ordovician–Lower Carboniferous and Permian–Lower Cretaceous) are explained in part as bias and as biological and taphonomic signals. These gaps are caused mostly by the rapid disarticulation of Palaeozoic-type echinoids, the methodology applied here, and a lack of interest in the encrustation of Jurassic echinoids. Conversely, three short-term gaps in the Cenozoic are interpreted exclusively as bias. If correct, the present study demonstrates quantitatively the step-wise increase of productivity through time. It also suggests potential focus on further study, including the collection of new data from the field and pre-existing collections, as best for other encrustation proxies (e.g., percent of coverage by epibionts, ratio of encrusted to nonencrusted shells, taxa richness or numerical abundance of sclerobionts) in cases of large-scale analyses.