The Problematic Fossil Mobergella radiolata as an Index Species of the Lower Cambrian Stages

The biostratigraphy of the lower boundaries of the Atdabanian Stage of the General Stratigraphic Scale of Russia and Stage 3 of the International Chronostratigraphic Chart is discussed. It is generally accepted that the first appearance of trilobites is the main characteristic of these stages. The difficulties in selecting the correlation level for the lower boundary of Stage 3 on the basis of trilobites are explained by the different taxonomic compositions of the assemblages of the most ancient representatives of this arthropod group on different paleocontinents and evident diachronism of the levels of their first appearance. For these purposes, it is proposed to use the species Mobergella radiolata Bengtson, 1968, the geographical distribution of which is wider than that of any species of ancient trilobites. It was shown that in the stratotype region of the Lower Cambrian stages in the Siberian Platform (the interfluve between the Lena and Aldan rivers), M. radiolata appears in sections at the same level as the first Atdabanian archaeocyaths of the Retecoscinus zegebarti Zone. This allows M. radiolata to be used as an index species for the base of the Atdabanian. Finds of M. radiolata in other regions of the Siberian Platform (west, north, and southeast, interior areas of the platform) make this microfossil an extremely valuable tool for biostratigraphy of the Lower Cambrian, with a much higher correlation potential than locally common early Atdabanian archaeocyaths or trilobites.