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Mitta V.V. (2005) New data on the Age of the Ryazanian Stage basal leyers. Stratigraphy and geol. Correlation. V. 13. No. 5. P. 503-511.
The Riasanites rjasanensis Zone, the basal one in the Ryazanian Stage, is characterized by the Boreal Craspeditidae and Tethyan Neocomitidae (Ammonoidea forms). The family Craspeditidae is represented by genera Hectoroceras, Praesurites, and Pseudocraspedites. The assemblage of the rjasanensis Zone is correlated with that of the kochi Zone and upper subzone of the sibiricus Zone of Siberia. The Praetollia and Chetaites Beds uppermost in the Volgian Stage of the Russian platform are probably equivalent to the maynci Subzone of the last zone coupled with the chetae Zone of Siberia. In the Russian platform, Neocomitidae s.l. are represented by genera Riasanites, Subalpinites, Transcaspiites, and others. The finds of Dalmasicearas ex gr. Djanelidzei and Malbosiceras nikolovi suggest correlation of the rjasanensis Zone lower boundary with that of the jacobi Zone in West Europe. The rjasanensis Zone corresponds probably to the greater part of the Berriasian Stage to the interval spanning the jacobi and occitanica zones and the paramimounum Subzone of the boissieri Zone. Accordingly, the Volgian Stage, all three substages included, should be referred to the Jurassic System and regarded in the general scale of Boreal realm as an autonomous stage concurrent to the Tithonian Stage of Submediterranean areas.