MANIFESTATIONS OF GLOBAL ORDOVICIAN-SILURIAN BIOTIC CRISIS IN THE CENTRAL-SIBERIAN GRAPTOLITE COMMUNITIES

A detailed analysis of the dynamics of the taxonomic variety and struc ture (age stages of colony evolution, rhabdosome morphology) of the Or dovician-Silurian graptolite communities of the Altai-Sayan folded reg ion and Siberian Platform has been carried out. It is shown that the e volution of the graptolite paleocommunities has gone through the follo wing stages: beginning of a crisis (lower subzone supernus, zones supe rnus), maximum of the crisis (upper par? of the subzone supernus-middl e part of the subzone ornatus), diapause (,,survival'') (upper part of the subzone ornatus, zones supernus, and the lower two thirds of the zone persculptus), beginning of radiation (upper part of the zone pers culptus - lower part of the zone acuminatus), and the final stage of r adiation (upper part of the zone acuminatus - lower part of the zone s ibiricus, extenuatus). For each of these states of graptolite communit ies, specific indicators of taxonomic variety, population density, col ony sizes, and proximal evolution of rhabdosomes and theca species hav e been established. Analysis of the lithologic and geochemical peculia rities of rocks and reconstructed environmental parameters for Central Siberia suggests the causes of the change of facies and paleogeograph ic settings at the Ordovician-Silurian border. It is shown that the ch anges in the structure of communities of one of the groups of marine p elagic organisms - graptolites - are related to the geochemical parame ters of the paleobasin environment. Double terminology to describe a g lobal Ordovician-Silurian event Chineta (persculptus) event - are prop osed.