HOW BIOEROSION AND SEAWATER GEOCHEMICAL CYCLES HAVE MADE AND DESTROYED MARINE SCLEROBIONT NICHES

Marine sclerobionts (organisms which live on hard substrates) include species (such as corals and oysters) which with their skeletons build additional hard substrate, and other species (such as boring bivalves and sponges) which remove hard substrate. The relationship between these bioconstructors and bioeroders has fluctuated over time as a function of seawater chemistry and community evolution. The ecological effects of bioconstruction are well known, as in the diversification associated with coral reefs or oyster banks, but the historical role of bioerosion in sclerobiont ecology has not yet been fully explored. Marine hard substrates have varied through the Phanerozoic with the broad Calcite-Aragonite Sea cycle. Calcite Seas were characterized by extensive carbonate hardgrounds and thick calcitic skeletons. These substrates provided deep resources for a variety of boring organisms. During Calcite Sea times in the Early Paleozoic, the primary bioeroders produced simple cylindrical tunnels (Trypanites and Palaeosabella). These became especially abundant during the Middle Ordovician into the Silurian (the Ordovician Bioerosion Revolution), producing cryptic niches for nestling organisms such as lingulid brachiopods and at the same time undermining hard substrates occupied by bryozoans and echinoderms. A much greater effect occurred in the Calcite Seas of the Middle to Late Mesozoic (part of the Mesozoic Marine Revolution) when bivalve (Gastrochaenolites) and sponge (Entobia) borings deeply eroded carbonate hardgrounds and carbonate skeletal substrates. The large size of the cavities and their common intersections provided numerous cryptic habitats for nestling bivalves and encrusting bryozoans, serpulids, brachiopods, foraminiferans and sponges. Marine hard substrate communities are thus in large part controlled by the competing forces of bioconstruction and bioerosion, mediated in turn by the availability of hard substrates facilitated by first-order cycling of carbonate geochemistry.