Kepplerites keppleri (Ammonoidea: Kosmoceratidae), an Index Species of the Lowermost Subzone of the Callovian Standard Scale, in Localities of the Russian Platform

The species Kepplerites keppleri (Oppel), first described from southern Germany, is an index species of the lowermost subzone of the Callovian standard scale. This and the wide geographical distribution of this taxon determine its importance for interregional correlation. At the same time, K. keppleri is the ancestor of all the Callovian representatives of the genus Kepplerites (and the family Kosmoceratidae) in a vast territory of Boreal, Subboreal and Peri-Tethyan marine basins. The paper discusses the finds of this species in Central Russia (Middle Volga area) and their significance for systematics and biostratigraphy. The Bathonian-Callovian boundary on the Russian Platform is proposed to be drawn based on the first appearance of Kepplerites keppleri, as is customary in southern Germany.