Neoproterozoic-Early Cambrian organic-walled microfossils : Ultrastructural and biogeochemical search for their affinities

Abstract: The Neoproterozoic-Cambrian transition is marked by conspicuous radiation of skeletal fauna comparable to modern invertebrates. Phylogenetic relationships of living organisms, based on their genetic and ultrastructural complexity, suggest that most prokaryotic and protistan organisms should have evolved by the time of origination of coelomate organisms. Identified remains of only chrysophytes and green algae have been reported fromPrecambrian, and foraminifera and radiolaria from Cambrian successions. However, it is obvious that the development of complex multicellular organisms would not be possible without a substantial radiation of the primary food producers.Numerous organic-walled microfossils of probable protistan, but uncertain biological affinity (acritarchs) are known in Neoproterozoic-Early Cambrian rocks. The search for their possible links with living microbiota is at the foundation of understanding of Earth' early ecosystems.The thesis presents a study of acritarchs from the Lower Cambrian Lükati Formation, Estonia, and a Precambrian species Chuaria circularis from the Visingsö Group, Sweden. Additional samples from the Lower Cambrian Buen Formation of Greenland were analysed for biogeochemical correlations.Apart from morphological and ultrastructural methods commonly applyied in palynology, a study of fossilizable molecules (biomarkers) of selected acritarch fractions is performed A fluorescence-activated cell sorting (FACS) instrument earlier used in cell and molecular biology is introduced to the study of palaeontological objects. It was used during preparation procedures for the additional chemical analysis.The summarised data confirm that microfossils grouped by Evitt (1963) into the informal category of Acritarcha include a polyphyletic assemblage of microorganisms. Dinoflagellate and chlorophycean green algae affinities are proposed for some of the Early Cambrian acritarchs studied. Microfossils referred to the informal genus Leiosphaeridia are considered to comprise taxa which belong to various systematic groups. Among other possible affinities, at least two classes of green algae, Prasinophyceae and Chlorophyceae, can be identified among these microfossils. The proposed dinoflagellate affinity of some acritarchs from the Lükati and Buen Formations, together with the presence of dinoflagellate molecules in kerogen and rock samples from the Visingsö Group and previously reported record of dinosteranes from petroleum and rock samples, support phylogenetic assumptions of the Precambrian origin of dinoflagellate-related phyla.