Khetoceras (Craspeditidae, ammonoidea)—A new genus from the Volgian stage of northern Middle Siberia, and parallel evolution of Late Volgian boreal ammonites

Abstract

A new ammonite genus, Khetoceras (Craspeditinae), is proposed, showing an atypical combination of features for the subfamily (arrowlike cross-section and prominent constrictions). A new subfamily Subcraspeditinae is proposed within the family Craspeditidae. An increase in whorl overlap was the major trend in the evolution of the Late Volgian subfamilies belonging to Craspeditidae (Craspeditinae, Garniericeratinae, and Subcraspeditinae) leading to the appearance of discoid morphotypes with a narrow (or acute) cross-section in all these subfamilies. However, while this evolutionary trend dominated the evolution of Garniericeratinae and less so Subcraspeditinae, it was not so prominent in the Volgian Craspeditinae, in which this morphotype only developed in northern Siberia (genus Khetoceras), where at that time the other two subfamilies were absent.