A new Pseudobigaleaid rostroconch with unique morphology from the Pennsylvanian of the Russian Platform

Mokshatuba okaensis gen. and sp. nov. (Pseudobigaleaidae) is described from the Pennsylvanian of the Russian Platform. Despite its unique morphology, Mokshatuba okaensis gen. and sp. nov. is closely related to Hadropipetta ancora (Hoare et al. 1988). A detailed morphological analysis of the external and internal shell layers of both genera is provided, enabling a revised diagnosis ofHadropipetta Hoare et al. 2002. New approaches to the shell zonation of representatives of the subfamily Pseudobigaleainae are proposed. The combination of morphological characters of the zones on the external shell surface and the surfaces of the inner shell layer is unique to this subfamily, ensuring its clear diagnosis. The revised descriptive terminology and new approaches to the morphology of shells may facilitate further research into the systematics of the order Conocardiida. The unique shell of Mokshatuba gen. nov. represents a specialised morphotype. This and similar conocardioid rostroconch morphotypes are interpreted as mobile ecoforms adapted to both semi-infaunal and epifaunal lifestyles. These forms emerged iteratively and independently within the order Conocardiida. This paper discusses the evolutionary constraints and eventual decline of these morphotypes, alongside those represented by Euchasma Billings, 1865 and Anetshella Mazaev, 2012.http://urn:lsid:zoobank.org:pub:4BF8B953-42E9-4858-807A-23ED4A641E63