ABSTRACT
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 Archaeoellpsoides 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.
The principal biological event occuring at the Meso-Neoproterozoic transition was the explosive radiation of eukaryote microorganisms and the consequent appearance of morphologically complex microfossils in the Neoproterozoic. However, this transition was not instantaneous, and many taxa widely distributed in the Neoproterozoic are known in the latest Mesoproterozoic. Nonetheless, the composition of conservative communities in the restricted peritidal environments has also changed. This perhaps reflects the “hidden” expansion of the eukaryote microorganisms and the changing substrate conditions.
