Biospheric perturbations during Gondwanan times: From theNeoproterozoic-Cambrian radiation to the end-Permian crisis

Abstract

The history of marine life during the Palaeozoic is structured by the NeoproterozoicEarly Cambrian radiation, the Ordovician radiation and a diversity plateau from the Late Ordovician into the Permian. This plateau is punctuated by mass extinctions in the end-Ordovician, Late Devonian, and finally the catastrophic end-Permian mass extinction, which brings the Palaeozoic to a close. Underlying these radiations and extinctions is a constant pattern of evolutionary change, as species respond to environmental changes, biological innovations, shifting biogeography and other factors. Are these patterns structured by global processes, or are they the summation of a host of local and regional events? One school of palaeontologists has argued that these patterns are driven by biotic interactions on a global scale, with mass extinctions as perturbations to longer-term processes, but with little lasting effect. Others have accorded the primary influence to radiations and mass extinctions. New studies from of the Ordovician radiation support a third possibility, that global signals are the summation of events at scales from local to global mass extinctions. Variations in biotic diversity, at scales ranging from global evolutionary radiations and mass extinctions to regional fluctuations or the changing fortunes of individual community assemblages, reflects the vicissitudes of environmental circumstance. Determining the appropriate level of analysis (global, regional or local) will require greater biogeographic data, particularly from Gondwanan continents. This will allow the development of explicit, testable models linking geological, geochemical and palaeontological patterns to common processes.