Ordovician island biotas: New South Wales record and global implications

The Ordovician period was characterized by significant phases of subduction-related volcanic activity and major evolutinary radiations of marine invertebrate taxa. Two important associations of Ordovician volcanics and sediments are preserved in fault-bounded remnants of the Lachlan Fold Belt in central New South Wales. A uniquely well-preserved record of low-latitude, offshore island shelf to slope biofacies assemblages of Gisbornian-Eastonian age has been documented. The major Ordovician radiations have been attributed mainly to the global expansion of major invertebrate taxa, especially elements of the "Paleozoic' fauna of Sepkiski. Sepkiski proposed that elements of the "Palaeozoic' fauna, after originating onshore, were displaced progressively offshore through time. Inspection of the New South Wales onshore-offshore record of island biotas shows, on the contrary, the highest levels of productivity of new higher taxa and community types in the mid-outer shelf and slope habitats, not onshore. Moreover, there is no evidence of offshore displacement of stocks through the 10-15 Ma history of the island complex. The global implications of this are discussed.