Late Bathonian Cardioceratidae (Ammonoidea) from the middle reaches of the Volga River

Three new cardioceratid species with an abnormally short (0.5-0.6 whorls) body chamber were found while studying the coevolution of early Cardioceratidae and Kosmoceratidae (Paracadoceras keuppi, P. nageli, and P. efimovi). The interval marked by the occurrence of these ammonites is proposed as a new keuppi Zone, subterminal in the Upper Bathonian in the Russian Platform. This zone is apparently equivalent to the variabile and calyx zones of the Boreal (Eastern Greenland) scale and is represented in the basin of the Sura River by a series of quartz sands referred to the upper part of the Prialatyrskaya (=Lukoyanovskaya) Group. The Late Bathonian Cadoceras apertum Callomon et Birkelund and C. calyx Spath are described. The stratigraphic scheme of the Bathonian of the Russian Platforms and its correlation with the Boreal standard are proposed.