Neoproterozoic (Ediacaran) diversification of acritarchs: a new record from the Murnaroo 1 drillcore, Eastern Officer Basin, Australia

The Neoproterozoic is characterised by global environmental change and the diversification of many groups in the fossil record including animals and photosynthesising microflora. The cause-effect relationships between environmental and biotic changes are poorly understood but drastic events such as low-latitude glaciations, ocean oxidation and perhaps a bolide impact (on a more regional scale) may have had a decisive influence on the biota at the time. A new study on organic-walled microfossils (acritarchs) from the Murnaroo 1 drillcore, eastern Officer Basin, Australia, reveals the presence of assemblages with great morphological disparity, taxonomic diversity and abundance. This microbiota represents a part of the Ediacaran diversification event recognised previously from the Officer Basin, and other sedimentary basins in south-central Australia. Earlier studies included only a cursory analysis of Murnaroo 1. Our more extensive study of this drillhole supports the acritarch-based zonation established by Grey for the middle Ediacaran System and both the uppermost and the lowermost acanthomorph zones in the Ediacaran biostratigraphic scheme have now been recorded in Murnaroo 1. Some species appear cosmopolitan, being known from Siberia and China, providing good potential for interregional correlation and palaeobiogeographic reconstructions. The significance of the present acritarch palynoflora, in addition to being more abundant, diverse and covering a wider stratigraphic interval, lies in the fact that (in Australia at least) it tightens the constraints on the first appearance of acanthomorphic acritarch species, and indicates that it is much closer to the Acraman impact ejecta layer than previously inferred. In the aftermath of the global glaciations and the Acraman impact, the Ediacaran acritarch diversification may have had a profound influence on the evolution of other life forms, such as the Ediacara fauna.