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Mikhail Fedonkin, Paleontological Institute RAS (Russian Federation)
Andrey Ivantsov, Paleontological Institute RAS (Russian Federation)
Maxim Leonov, Paleontological Institute RAS (Russian Federation)
Ekaterina Serezhnikova, Paleontological Institute RAS (Russian Federation)
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The Vendian shallow-water siliciclastic succession (over 600 meter thick) exposed in the White Sea Region of Russia has preserved a uniquely rich fossil record of the soft-bodied invertebrates and bioturbations as well as the megascopic algae, organic-walled microorganisms, and bacterial mats (see abstracts by M.V.Leonov, this volume). Time range of the documented fossil record goes well beyond the radiometrically dated interval 558-555 Ma. The stratigraphic distribution of the fossil taxa is controlled by the taphonomic, paleoecological and paleoclimatic factors that is reflected in the step-wise mode of the biodiversity change, punctuated occurrence ranges of the taxa, and severe alterations in the structure of the successive faunal communities preserved in situ. Distribution of the cosmopolitan taxa may be also related to the changes in the paleobiographic connections of the Vendian paleobasin. The taxonomic and statistical study of the Vendian Fossils Collection (over 10000 specimens) accumulated by the staff of the Laboratory of the Precambrian Organisms (PIN RAS) after thirty years of paleontological and stratigraphic research in the White Sea region reveals distinct trends in the metazoan diversity change during the late Vendian.
The abundance of taxa shows gradual growth followed by a leap, then by a prolonged period of stability at the high level of diversity, and then, by some decline by the end of the fossiliferous Vendian succession in the region. On this background we observe an increasing portion of the bilaterians represented by the body fossils and bioturbations. The overall decline of the metazoan diversity by the end of the Vendian succession may in part be related to the regressive trends and to the change from the marine to the brackish environments. However the growing diversity of the bilaterian body fossils and evidences of grazing over bottom surface and bacterial mats as well as the increasing variety of the bioturbations are indicative of the fast radiation in some bilaterian clades prior the Cambrian Period. Taking into account a high proportion of the cosmopolitan taxa in the Vendian metazoan assemblages we assume that the revealed biotic trends reflect the global evolutionary process, and thus can serve as a solid basis for the biostratigraphical division and global correlation of the Vendian deposits. The cosmopolitans/endemics ratio of the Vendian metazoan record can be also interpreted in terms of the late Neoproterozoic paleogeography and climate change. The study is supported by the Program 18 of the RAS Presidium and by the Russian Fund for Basic Research.
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