Ammonites and stratigraphy of the terminal part of the Middle Volgian Substage (Upper Jurassic; Epivirgatites nikitini zone and its equivalents) of the Panboreal Realm: 1. Late morphogenesis and systematics of Dorsoplanitinae

Resumo

Problems of correlation of the upper part of the middle Volgian beds of European Russia and Arctic regions with the Portlandian of England and northern France largely result from existing uncertainties in the taxonomy of the ammonite subfamily Dorsoplanitinae. The poor knowledge of the morphogenesis of this group precludes effective and stable taxonomy and recognition of parallelisms, thereby decreasing the group’s correlative-biostratigraphic potential. In Dorsoplanitinae, dimorphism is revealed in the differences in the terminal shell diameter and, to a lesser extent, in the sculpture, whereas apertural modifications characteristic of many Perisphinctoidea are not observed in members of this group. The middle Volgian Dorsoplanitinae are represented by minimorphs and macromorphs, taxonomically different at the species or genus level. The Volgian minimorphs are mainly represented by the genera Epivirgatites and Lomonossovella, whereas macromorphs are represented by Titanites s.l.; in the genus Taimyrosphinctes, micro- and macromorphs belong to different subgenera. All morphological types of Volgian Dorsoplanitinae have vicariate equivalents in the Portlandian, which can be used for the interregional correlations.