The Toarcian zonal scale in northern Siberia

In connection with forthcoming regional meetings on Mesozoic biostratigraph y, data on the Toarcian Stage in northern Siberia and its zonation are revi ewed. Emphasis is placed on the zonation of the Upper Toarcian, for which t wo versions were proposed. The author supports the opinion of Yu. S. Repin and I. V. Polubotko that the unified Upper Toarcian scale on ammonites shou ld be different for northern Siberia and northeastern Russia and that the z onal subdivision of the Upper Toarcian in northern Siberia, containing a Ps eudolioceras alienum Zone, is premature. Different zonations are proposed f or the upper Lower Toarcian: Zugodactylites braunianus Zone in northern Sib eria and local Z. monestieri and Peronoceras spinatum Zones in northeastern Russia. In the standard scale, they correspond to two upper subzones of th e Hildoceras bifrons Zone.