ECOLOGY OF THE EDIACARAN EXPLOSION

Fossils of the Ediacaran biota are the oldest diverse and abundant assemblages of macroscopic life on earth containing an impressive array of body fossils, including macrophytes and the oldest candidate bilaterian metazoans. However, only rarely do outcrops permit assemblage level ecosystem reconstructions. Fourteen excavated fossiliferous beds in the Ediacara and nearby hills of South Australia provide over 100 square meters of bedding plane surfaces. The 14 beds display a considerable degree of heterogeneity between beds with overall levels of low evenness and high dominance. While some of this heterogeneity is demonstrably taphonomic, much of it can be interpreted as due to ecologic processes. Many of the beds show a near continuous cover of structures likely to be of microbial origin, indicating their ecological significance. These structures show a broad diversity of form with many similarities to structures present in the White Sea and Newfoundland sections. Though previously undescribed, the exquisite preservation, abundance, and obvious ecological role, makes their presence of first order importance. While some problematic structures can be interpreted as tapho-fossils, the majority of other structures and surfaces are most conservatively interpreted as microbial or algal in origin. This diversity of organic structures and surfaces are not known in rocks older than 600 mya although there is no reason to suggest that the taphonomic window allowing preservation in Ediacaran aged rocks should not have been present in older rocks. The concomitant Ediacaran diversification of animals and macrophytes (Knoll and Xiao, 2003) as well as organisms producing widespread organic surfaces further supports the rise of oxygen as the environmental trigger to the radiation of this broad range of unrelated organisms and importantly constrains the timing of this event (at least the attainment of a critical level) to post Marinoan (~620 ma).