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 slowl y expanding orthoconic phragmocone and aragonitic sheath with a rugged surf ace, a weakly developed post alveolar part and a long, strong, probably dor sal groove. The sheath lacks concentric structure common for belemnoid rost ra. It is formed by spherulites consisting of the needle-like crystallites, and is characterized by strong porosity and high content of originally org anic matter. Each spherulite has a porous central part, a solid periphery a nd an organic cover. Tubular structures with a wall formed by the needle-li ke crystallites are present in the sheath. For comparison the shell ultrast ructure in Recent Spirula and Sepia, as well as in the Eocene Belemnosis we re studied with SEM. Based on gross morphology and sheath ultrastructure K. roemeri is tentatively assigned to Spirulida and a monotypic family Kostro mateuthidae nov. is erected for it. The Mesozoic evolution of spirulids is discussed.