Proterozoic and Cambrian bioevents

The modern ecosystems began to be formed in the Proterozoic Eon. The Proterozoic fossil record includes bacteria and cyanobacteria, heterotrophic and autotrophic protists, multicellular thallophytes, and metazoans. The gradual increase of the oxygen pressure in the atmosphere took place during the Proterozoic causing a great diversification of eukaryotic life and the origin of metazoans. The first autotrophic protists (acritarchs) originated in the Palaeoproterozoic and reached a high level of diversification in the early Neoproterozoic, when the first heterotrophic protists and biomineralized forms appeared. Several thallophytic groups may have originated during the Palaeoproterozoic, while the Rhodophyta probably radiated during the Mesoproterozoic, and Chlorophyta during the Neoproterozoic. The glacial episodes in the Neoproterozoic led to a mass extinction among protists. The Late Vendian ecosystems reveal deeply renewed assemblages of planktonic protists, problematic metazoan fauna (Ediacara Fauna), ichnofossils and slightly mineralized metazoans (Cloudina). The bioturbation characteristic for these ecosystems appears in form of small, simple and shallow burrows. The biotic turnover at the Neoproterozoic/Cambrian boundary is expressed by a mass extinction and the origin of new primary producers and consumers. The Cambrian rocks show a higher degree of bioturbation, suggesting a drastic change in the techniques of exploitation of the nutrients accumulated in the sediments.