Выпуск: 6 , Том: 22 , Год издания: 2014

The revision of the geochronological ranges of the Boreal genera Arctocephalites and Arcticoceras found in the Middle Volga region (Middle Povolzhye) together with Peri-Tethyan ammonites resulted in the revision of the stratigraphic position of several zones in the Middle Jurassic ammonite scale of Siberia. It is shown that the Arctic genus Arcticoceras appeared in the early Bathonian, rather than in the middle Bathonian, as is cited in the current Siberian zonal scale. The geochronology of zones preceding the zones with Arcticoceras also needs revision. The lower jugatus Subzone in the arcticus Zone (equivalent to the upper Bajocian niortense Zone in the northwestern European zonal standard) can be used as a correlation marker level. The upper arcticus Subzone is correlated to the two upper Bajocian zones, greenlandicus and harlandi zones, whereas the ishmae Zone correlates with the lower Bathonian zigzag Zone. The gracilis and borealis zones, considered as equivalents of the humphriesianum Zone, should be transferred to the lower Bajocian. No ammonites were found in the middle Bathonian