Mesoproterozoic Microbiotas of the Northern Hemisphere and the Meso-Neoproterozoic Transition

摘要

The silicified peritidal carbonates of the Mesoproterozoic (Lower to Middle Riphean) Kotuikan and Yusmastakh formations of the Billyakh Group, Anabar Uplift, Siberia, contain diverse microfossil assemblages dominated by fossilized akinetes of the Genus Archaeoellipsoides and associated short trichomes and enthophysalidacean cyanobacteria. All these fossils bear close counterparts with living cyanobacteria. However, similar Archaeoellipsoides-dominated silicified microbiotas were found only in Mesoproterozoic formations in Canada, China and elsewhere, but are missing in the Neoproterozoic deposits. This biostratigraphic and evolutionary paradox seems to reflect ecological effects of the newly evolved eukaryotes and the changing chemical composition of sea water and sea floor substrate. The morphologically complex unicellular eukaryote microorganisms are missing in the Mesoproterozoic microbiotas.