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Uppsala University, Palaeontological Institute, Micropalaeontological Laboratory, Box 558, S-751 22 Uppsala, Sweden
Late Proterozoic and early Phanerozoic phytoplankton comprise a wide array of microfossils of possibly bacterial and algal affinity. Their radiation patterns around the Proterozoic-Cambrian boundary interval are becoming increasingly well-known for numerous sequences in the East European and Siberian Platforms, China, Baltoscandia and North America. Transgressive sequences in the East European Platform and northern Baltoscandia comprise alternating sandstones, mudstones and claystones. Their sedimentology is well documented and microfossils (acritarchs), ‘chitinous’ fossils (sabelliditids), shelly fossils and trace fossils are well represented. Rocks immediately overlying the transitional beds have yielded lower Cambrian trilobite faunas that are utilized for regional correlations. Continuous successions of planktic microorganisms, consisting of prokaryotic photoautotrophic cyanobacteria and algal protists, are known from rocks comprising the transitional Vendian-Cambrian sequence. In the East European Platform the rock sequence attributed to the lower Cambrian Platysolenites antiquissimus Zone has yielded acritarchs of 'Cambrian type', abruptly replacing a morphologically simple ‘Proterozoic type’ microbiota recorded in beds of the upper Proterozoic Sabellidites-Vendotaenia Zone. The radiation of novel forms of phytoplankton appears to have been synchronous over a wide part of the late Proterozoicarly Cambrian shelf, and is parallelled by the nearly contemporaneous emergence of Cambrian shelly fossils and trace fossils. Along with new acritarch taxa, the diagnostic acritarch assemblage of the Platysolenites antiquissimus Zone consists of widely distributed species, occurring also in the lowermost Cambrian deposits of the Siberian Platform, Baltoscandia, Greenland, North America, Svalbard and Scotland. It is suggested that along with shelly faunas, trace fossils and geochemical data, the general radiation patterns of phytoplankton shed light on the events shaping the important biotic events around the Precambrian-Cambrian boundary. It is also believed that a better understanding of the regulatory mechanisms affecting the base of trophic chains may in turn be of significant importance in defining the Precambrian-Cambrian boundary.
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