Current reestimation of the stratigraphic positions of some zonal ammonites has resulted in revision of the sequence of ammonite zones, particularly in the Lower substage, and in more detailed stage division. The work has been mainly finished for Central Russia. But it is still in the initial stage in the Volga Region. New data is presented in the paper. Possibility is discussed of compiling an updated biostratigraphic base for dividing the Callovian not only from ammonites, but from foraminifers, as well.
