United States
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The new geological survey of the Ediacaran Doushantuo Formation and micropalaeontological studies have yielded a much larger and more diverse association of organic-walled microfossils than previously reported. Over 100 species recovered from chert nodules have now been identified, including 24 new species. The microfossil taxonomy has been revised accordingly, adding to the appreciation of biodiversity, global distribution and biostratigraphic significance. The species ranges have been scrutinized and the first and last appearance datum have been established for several worldwide distributed species that are used for proposed new assemblage zones in the Yangtze Gorges area, and potentially for global subdivision of the Ediacaran System. These are, in ascending stratigraphic order, the Appendisphaera grandis–Weissiella grandistella–Tianzhushania spinosa, Tanarium tuberosum–Schizofusa zangwenlongii, Tanarium conoideum–Cavaspina basiconica and Tanarium pycnacanthum–Ceratosphaeridium glaberosum Assemblage Zones. The successive zones’ lower boundaries are estimated at 633, 620, 610 and shortly after 580 Ma time horizons. The geological review of the Doushantuo Formation and new observations of eleven successions in an attempt to discern unconformities and stratigraphic hiati have recognized the relative time relationships between dismembered successions and have updated the species ranges. The Doushantuo Formation is sedimentologically discontinuous, partially condensed and comprises several un- and paraconformities involving hiati of unknown duration. Therefore, the species and zone ranges, with the exception of the two basal zones that are bracketed by isotopic datings, are approximate. The biotic radiations documented by microfossil appearance and the origin of new species at the very beginning of the Ediacaran Period at 633 Ma, followed exponentially through the interval up to 565 Ma (ca. 68 Ma) are innovative evolutionary events. Ediacaran microbiota phylogenetic affiliations are not fully resolved and assumed to be among auto- and heterotrophic protists or even metazoans. Many taxa are algal in origin, including those studied here. Biological affinities of microfossils remain among the most intriguing problems in the evolution of life at that time.
