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

A detailed analysis of the dynamics of the taxonomic diversity and structure (age stages of colony evolution, rhabdosome morphology) of the Ordovician-Silurian graptolite associations of the Altai-Sayan folded region and Siberian Platform was carried out. It was shown that the evolution of the graptolite paleoassociations passed through the following stages: the beginning of a crisis (lower supernus subzone of the supernus zone), the maximum of the crisis (upper part of the supernus zone to middle part of the ornatus subzone), diapause (“survival”) (upper part of the ornatus subzone of the supernus zone, and the lower two thirds of the persculptus zone), the beginning of radiation (upper part of the persculptus zone to lower part of the acuminatus zone), and the final stage of radiation (upper part of the acuminatus zone-lower part of the sibiricus, extenuatus zone). Specific indices of taxonomic diversity, population density, colony sizes, and proximal evolution of rhabdosomes and theca shapes were established for each of these states of graptolite associations. Analysis of the lithological and geochemical features of rocks and reconstructed environmental parameters for Central Siberia reveals the causes of the change of facies and paleogeographic settings that existed about the time of the Ordovician-Silurian transition. It is shown that the changes in the structure of associations of one of the groups of marine pelagic organisms – graptolites – are related to the geochemical parameters of the paleobasin setting. To describe the global Ordovician-Silurian event, double nomenclature is proposed: Chineta (persculptus) event.