Clypeasteroid echinoid tests as benthic islands for gastrochaenid bivalve colonization: evidence from the Middle Miocene of Tarragona, north-east Spain

Middle Miocene tests of Clypeaster from L'Arrabassada (Tarragona, north-east Spain) show evidence of intense endoskeletozoan colonization, preserved as borings and associated carbonate secretions that allow gastrochaenid bivalves to be identified as the colonizers. Two modes of occurrence have been recognized for these bivalve dwelling cavities; intrastereom clavate borings' which are restricted to the echinoid stereom, and semi-endoskeletal dwellings', which penetrate across the test wall and extend as carbonate crypts into the sediment fill of the internal test cavity. Their size, density and position rule out a syn-vivo relationship with the echinoids and demonstrate that colonization was post mortem. Because of the endurance of clypeasteroid skeleton and the pronounced bell-shaped morphology of Clypeaster, the tests of these echinoids provided the most suitable substrates for hard-bottom colonizers on an otherwise sandy seafloor. The scenario described from Tarragona can be extended to other Neogene and Quaternary strata elsewhere; there is ample evidence for the long-term utilization of tests of Clypeaster by gastrochaenid bivalves in shoreface palaeoenvironments.