A rare coleoid mollusc from the Upper Jurassic of Central Russia

The shell of the coleoid cephalopod mollusc Kostromateuthis roemeri gen. et sp. n. from the lower Kimmeridgian of Central Russia consists of the slowly expanding orthoconic phragmocone and aragonitic sheath with a rugged surface, a weakly developed post-alveolar part and a long, strong, probably dorsal groove. The sheath lacks concentric structure common for belemnoid rostra. It is formed by spherulites consisting of the needle-like crystallites, and is characterized by strong porosity and high content of originally organic matter. Each spherulite has a porous central part, a solid periphery and an organic cover. Tubular structures with a wall formed by the needle-like crystallites are present in the sheath. For comparison the shell ultrastructure in Recent Spirula and Sepia, as well as in the Eocene Belemnosis were studied with SEM. Based on gross morphology and sheath ultrastructure K. roemeri is tentatively assigned to Spirulida and a monotypic family Kostromateuthidae nov. is erected for it. The Mesozoic evolution of spirulids is discussed.