The terminal Neoproterozoic radiation of planktonic photosynthetic microbiota (acritarchs) is one of the most significant evolutionary events of the time, including diversification of prokaryotic cyanobacteria and eukaryotic green and brown algae, the appearance of thecoamoebaens and subsequently metazoans (the Ediacara fauna). The Ediacaran radiation of phytoplankton is recognizable by the first appearance of more than fifty new species of large ornamented acritarchs in a short interval of time at ca. 570 Ma. This radiation event occurred after the Snowball Earth conditions returned to a kind of "normal" environmental stasis, and it may be interpreted as a recovery diversification of phytoplankton after a major biotic extinction caused by the global glaciation.
The appearance of numerous, morphologically innovative and large acritarch taxa may also be connected with the Acraman impact event in South Australia, suggested recently by Grey et al. (2003), as a biotic recovery after the catastrophic environmental disturbance caused by the giant bolide. The latter hypothesis has to be tested, however, because a few individual taxa of ornamented acritarchs may have actually appeared below the ejecta layer, which is difficult to recognize with certainty in some borehole successions.
The Ediacaran acritarch records are from Australia (the Officer and Amadeus Basins), China and Siberia, showing a worldwide distribution in a relatively short interval of time (ca. 20 Ma; Grey, 2004, in press). The greatest taxonomic diversity is known from Australia (ibidem), and the present study is focused on the investigation of Ediacaran microbiota in greater detail and from different stratigraphic levels, their palaeobiology and affinities, mode of life and reproduction cycle. The new assemblage of organic-walled microfossils from the Murnaroo 1 borehole comprises filamentous cyanobacteria, and ornamented and spheroidal acritarchs.
The Ediacaran successions in Australia have been well documented in terms of lithostratigraphy, depositional settings and structural geology. The sedimentation proceeded in two different depositional regimes, recognized today in a series of sub-basins. One of them is the Officer Basin, comprised of complex intracratonic, east-west trending troughs and sub-basins extending from Western Australia to South Australia. The studied Murnaroo 1 borehole is also located there. The sediments accumulated in tidal, sub- and intertidal shelf conditions, and the predominantly mudstone lithology from which the samples were collected, is ideal for palynological processing and preservation of microfossils. The lack of macrofossils in the successions rendered efforts and advances in acritarch biostratigraphy since the 1980's, which helped to reveal a complex history of the Officer Basin. The discovery of two distinct palynofloras, an older leiosphere-dominated flora (ELP) and a younger acanthomorph-dominated flora (ECAP), is suggested to be largely environmentally independent (Grey, 2004, in press) in terms of the observed lithology and sedimentological sequences. However, the possible coupling between the Marinoan glaciation, the Acraman impact and the radical change in the palynofloras was inferred (ibidem) and this will be examined with the new data available from the Murnaroo 1 borehole. Previously, the Murnaroo succession was only studied preliminarily.
In my communication, I will discuss the stratigraphic sequence of appearances of various species and their relationship to the environmental conditions, the Acraman impact event and the changes associated with the global glaciations.
GREY, K., WALTER, M.R. and CALVER, C.R. (2003) Neoproterozoic biotic diversification: "Snowball Earth" or aftermath of the Acraman impact? Geology 31, p. 459-462.
GREY, K., (2004, in press) Ediacarian Palynology of Australia. Australasian Association of Palaeontologists, Memoirs.
2005. 64- p.