Main changes in microfossil communities throughout the Upper Proterozoic of Russia. (Changements majeurs dans les assemblages de microfossiles au cours du Protérozoïque supérieur de Russie)

More than 50 years of study have resulted in the description of hundreds of taxa of Precambrian microfossils from the different regions of Russia. These forms are usually preserved either as silicified or organic-walled remains and generally are morphologically simple and stratigraphically long-ranging. The systematics of Precambrian microorganisms still needs a serious revision, although efforts by specialists from all over the world to solve these problems have helped to clarify the biological assignment of some taxa. Distinctive, exceptionally diverse and well-preserved microbiotas were found at several locations on the East-European and Siberian Platforms. They occur at discrete stratigraphical levels and are geographically sporadic. However, a comprehensive analysis based on the published data and original material (Fig. 1) permitted the drawing of an evolutionary trend in the main taxonomical changes that occurred in the organic world throughout the Upper Proterozoic of Russia. For the present study the only records we used were from strata controlled either by isotopic data or other evidence of age. Both silicified and organic-walled microfossils were analyzed taxonomically. Only species with real biological meaning or biostratigraphical significance were used to define noteworthy stages in the development of Upper Proterozoic microorganisms. Five main stratigraphical levels, characterized by the FAD of distinctive species or by the occurrence of new morphological forms have been distinguished (Fig. 2).