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Abstract
Organically preserved, unicellular microfossils of Ediacaran and Cambrian age, which comprise single and multiple internal bodies within the vesicle, have been studied with a view towards explaining the origin and function of these structures. Assessed by body plan, ornamentation, excystment structure, cell wall resistance and ultrastructure (where available), and newly observed internal bodies defined by their own, robust wall, these microfossils are recognised as algal zygotic cysts and phycomata by comparison with extant green algae. Although rare, these internal bodies are a persistent feature of specific morphotypes regardless of geological age, and are reviewed. Internal bodies are a part of reproductive cysts, resembling those known in different clades of green algae: prasinophytes, chlorophytes and streptophytes, as well as alveolates (including dinoflagellates). They are inferred to be endocysts containing zygotes and/or sacs of swarmers, if single, or offspring cells (spores), if multiple, in sexual and asexual generations of ancient taxa of the classes Prasinophyceae and Chlorophyceae. It may not be excluded, however, that they represent the earliest, stem-group streptophytes or alveolates. The diagnoses of the Ediacaran taxa Ancorosphaeridium, A. magnum, Densisphaera, D. arista, D. fistulosa, Multifronsphaeridium ramosum and Tanarium tuberosum are emended to include the internal bodies as integral morphological structures of microfossils, and to abandon the erroneous identification of the double wall of the vesicle. Based on the earliest occurrence of microfossils with an internal body in the Dictyosphaera–Shuiyousphaeridium plexus, sexual reproduction among photosynthesising microbiota is interpreted at c. 1.6–1.4 Ga, a common phenomenon in the Ediacaran (Tanarium, Ancorosphaeridium, Densisphaera), and dominant in the Cambrian (Skiagia, Polygonium and many others) periods.
Acknowledgements
The study was supported by research grant no. 621-2012-1669 from the Swedish Research Council (VR). I greatly appreciated discussions on phytoplankton diversity and origins with Kath Grey (Perth), Konstantin Nagovitsin (Novosibirsk), Sebastian Willman and Heda Agić (Uppsala); however, the views expressed here are my own responsibility. Marien van Westen (Groningen), Pavel Škaloud (Praha) and Vera Pospelova (Victoria) kindly provided images of modern algae from their collections. I especially thank Reed Wicander for his comments and thorough reading of the manuscript, and the anonymous reviewer for useful suggestions. I acknowledge discussions with the Editor Jim Riding on various aspects of fossil algae, and his editorial work.
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Notes on contributors
Małgorzata Moczydłowska
MAŁGORZATA MOCZYDŁOWSKA-VIDAL is a professor of micropalaeontology at Uppsala University, Sweden. She graduated with an MSc degree in palaeontology from Warsaw University in Poland, and then worked as a Senior Research Fellow at UCLA with Professor J. William Schopf between 1986 and 1987. Malgorzata completed a PhD in palaeontology at Lund University, Sweden in 1989, then undertook a postdoctoral fellowship at Harvard University with Professor Andrew H. Knoll. Since 1991, Malgorzata has worked at Uppsala University. Her research interests are Proterozoic and Early Palaeozoic organic-walled microbiotas (i.e. acritarchs, green microalgae and cyanobacteria) and their biological affinities, the evolution of photosynthetic microbiota and geochronology. Specifically, Malgorzata works on reconstructing life cycles and the analysis of the reproduction of these microbiotas with reference to their respective phylogenetic affiliations. Malgorzata is a past Vice-Chair of the International Subcommission on Cambrian Stratigraphy and is a voting member of the Neoproterozoic Subcommisssion.