Island benthic assemblages: with examples from the late Ordovician of Eastern Australia

The Benthic Assemblage (B.A.) concept, developed by A.J. Boucot two decades ago principally for continental marine margins, is extended to offshore island settings. Characteristics of modem island biotas, including ecological displacement, and the effects of r and K selection, can be identified in Late Ordovician volcanic islands of central New South Wales. Two variants of B.A.1 are represented, a quiet-water lingulide biofacies, and a rough-water rhynchonellide biofacies. The quiet-water Eodinobolus biofacies occupied a B.A.1-2 position, onshore of and sheltered by a Tetradium wave-baffle (B.A.2). The offshore shelfal high diversity strophomenide biofacies is equivalent to B.A.3. Remnants of periplatformal B.A.4-5 communities are recognised in allochthonous limestone breccias which were displaced downslope into graptolitic B.A.6 sediments. Steeper offshore gradients, typical of islands, laterally compress the B.A. profiles, and also contribute to downslope slumping. Ecological displacement in island environments results in extension of the habitat range of species into adjacent B.A.s. Reefs complicate the usual B.A. profile by introducing distinct sheltered and turbulent water environments. These characteristics may have applicability in interpretation of islands throughout the Palaeozoic record.