Ediacaran acritarch biota from the Giles 1 drillhole, Officer Basin, Australia, and its potential for biostratigraphic correlation

The remarkable diversification of single-celled photosynthesising biota of algal and other as yet unknown affinities (acritarchs), followed by the diversification of metazoans, occurred during the Ediacaran Period, which is marked by extreme climatic and environmental changes. Here we describe a taxonomically diverse acritarch association from the Ediacaran part of the Giles 1 drillcore in the Officer Basin, South Australia, which documents further the Ediacaran phytoplankton radiation. The studied palynoflora comprises 21 known acritarch species belonging to 15 genera. One new monospecific genus is described (Calyxia xandaros sp. nov.) as well as one new species of Tanarium (Tanarium anozos sp. nov.). We also propose the genus Knollisphaeridium to replace the preoccupied genus Echinosphaeridium. Three stratigraphically successive assemblages that match previously observed patterns of acritarch replacement are distinguished. The present record is from slightly older strata than in previous records, thus extending the ranges of certain acanthomorphic species. The lower boundaries of three stratigraphically higher assemblage zones among the four formerly established zones, are identified by the occurrence of the index species Tanarium conoideum, Tanarium irregulare and Apodastoides verobturatus, respectively. The substantial morphological disparity of acritarchs in the Giles 1 succession suggests that they may represent a great diversity of microorganisms, not only as biological species but also representing perhaps vegetative and encysted stages in their life cycle. The reconstructed palaeogeographic distribution of several species between Australia, Siberia, Baltica (the East European Platform), and to South China, shows that acritarchs are suitable for both intra- and inter-regional correlation.