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The oldest of recently known diverse Vendian (Ediacaran) floras is Doushantuo flora. The Doushantuo assemblage includes algae with compact, laminated thalli of tissue-grade organisation. Many Doushantuo taxa are characterized by regular dichotomous branching and the presence of holdfasts or rhizoids for attachment to the substrate. The Laymtsa assemblage (Archyfasma flora) of the Eastern European Platform is contemporary to or slightly younger than the Doushantuo assemblage. Laymtsa taxa demonstrate thalli with tissue organization and differentiation into layers. The younger Redkino assemblage (Eoholynian flora) includes exclusively simpler, crust-like and bush-shaped thalli. A significant part in Redkino assemblage is represented by prokaryote taxa, such as oscillatorian cyanobacteria, which form biofilms. The uppermost Vendian Kotlin assemblage is typified by its Vendotaenian flora and cyanobacterial biofilms. The dominant eukaryotic taxa - members of family Vendotaeniceae - is more primitive when compared with older forms. Biodiversity is low - 1-4 taxa versus around 20 in the Doushantuo assemblage. The highly organised algae and diverse algal assemblages appear in geological record again only in Cambrian (Botomian Sinsk assemblage and Middle Cambrian Burgess Shale assemblage).
Thus there is a stage that falls outside the gradual trend for the overall evolutionary "progress" of eukaryotic algae. It occur in the gradual reduction of the representation in assemblages of forms that demonstrate morphological and histological features of regularly, complex organized growth. This stage corresponds to the second part of the Upper Vendian (558-543 ma).
The analysis of the time and environmental constraints for the recently known fossil algal assemblages tends us to assume that this phenomenon is artificial only in some extent. It expressed by the supplanting of the evolutionary complex algae from the biotopes of lower storm base shelf in epicontinental seas with dominated siliclastic type of the sedimentation - the best environments for the preservation of organic matter. This processes is caused by periodic changes in global tectonic regime and by climate fluctuations. Research was supported by RFBR grant N 05-05-64825 and grant for leading science schools NSH-1790.2003.5.
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