FOSSIL EVIDENCE FOR A CRYOGENIAN DIVERSITY CRISIS

At least two low-latitude glaciations occurred during the mid-Neoproterozoic Cryogenian period (850 to ~635 Ma). The older, Sturtian glaciation is constrained to have begun by ~717 Ma and the younger, Marinoan glaciation to have ended ~635 Ma. Little is known about the biosphere during and between these glaciations, but body fossil and biomarker evidence indicate that several modern eukaryotic lineages originated before Cryogenian glaciation and thus must have survived the climatic crises. The record of organic-walled microfossils, the primary source of information about Precambrian life, is restricted to a few reports that indicate limited diversity. The Cryogenian glaciations are hypothesized to have caused eukaryotic mass extinctions, consistent with Proterozoic compilations of eukaryotic diversity that indicate a decline coincident with the onset of the Sturtian. However, due to the dearth of palaeontological data this apparent decline in diversity could be equally interpreted as an artefact of sampling bias. To test the apparent link between Cryogenian glaciation and low biotic diversity, we have examined organic-walled microfossil assemblages from five successions in Australia and Svalbard that collectively span the Sturtian glaciation and the interglacial interval. Assemblages of both intervals are depauperate, consisting of smooth-walled sphaeroids, colonial aggregates of cells and simple filaments; aside from a higher proportion of colonial aggregates in the interglacial units, these assemblages are indistinguishable. Together with evidence from pre-Sturtian and early Ediacaran assemblages, our data suggest a protracted interval of biotic crisis began before the onset of Sturtian glaciation and persisted long after termination of the Marinoan glaciation. This temporal decoupling of low-latitude glaciations and low-diversity biotas suggests that, while probably related, low-latitude glaciation itself was not the cause of the Cryogenian diversity crisis.