The well-known Cambrian explosion was an evolutionary event that lacks counterparts in Earth history. The appearance of shelly organisms easily preserved as fossils illustrates an exceptional record of diversifying organisms. It is established that this was a true biological diversification but was it the first one? The Cambrian was preceded by the Neoproterozoic, a period that was characterised by severe environmental turbulence during its terminal interval (Ediacaran). At least two global glaciations between ca. 700-580 Ma might have acted as evolutionary bottlenecks that led to a rapid diversification of several lineages of single-celled and, eventually, multicellular organisms. The appearance of more than 50 ornamented acritarchs (Grey et al., 2003) in the Ediacaran of present day southern Australia suggests a possible Precambrian evolutionary explosion. Acritarchs, being primary producers, may have had a profound effect on the evolution of metazoans resulting, among other factors, in the Cambrian explosion. The apparent diversification event requires studies of more material but will hopefully result in either recognition of a Precambrian explosion hypothesis or simply a normal, gradual development of phytoplankton.
The diversification of acritarchs provides an excellent potential for these organisms to be used in biostratigraphy. The Ediacaran System’s lower boundary is defined at the base of glacial deposits from the last major glaciation, the Marinoan glaciation, and the upper boundary at the base of the Cambrian System. The Ediacaran System is not very well known and in terms of fossil studies it is just in its infancy. Palynomorph assemblages containing mainly eukaryotic acritarchs and prokaryotic bacteria were recovered from numerous drillholes located in southern Australia. Preliminary studies of a large number of acritarch samples from more than 30 drillcores (Grey, 2005) have resulted in a subdivision of the middle Ediacaran into biozones based on the first appearance of index species and characteristic assemblages. Additional studies of drillcores from the Officer Basin in Australia will aid in the correlation between different basins in Australia and hopefully also in global correlation. The studies of the Murnaroo 1 succession indicate a consistency with the previously examined boreholes and allow more accurate recognition of the acritarch biozones.
Grey, K., Walter, M.R. & Calver C.R., 2003. Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact? Geology, v. 31.
Grey, K., 2005, in press. Ediacaran Palynology of Australia. Australasian Association of Palaeontologists, Memoirs, v. 31.
2005. 200- p.