Shell Ontogeny of Otoceras concavum Tozer (Ceratitida) and the Problem of the Origin of Otoceratids

As a result of a detailed study of cross sections of the most ancient (Late Permian) representatives of Otoceras Griesbach (O. concavum Tozer) from the base of the Nekuchan Formation in the upper reaches of the Vostochnaya Khandyga River in Southern Verkhoyanie, the features of ontogenetic shape changes were clarified. Trends in changes in the main parameters of the shell during its growth have been determined, the cyclical nature of the narrowing and expansion of the shell has been identified, and the sequence of changes in the shape of the whorl cross-section has been established. The formation of carinae and the emergence of a tricarinate form on the ventral side were traced. Identification of shell shape transformations in the ontogenesis of O. concavum contributes to the diagnosis of small-sized Otoceras and can serve as the basis for subsequent reconstruction of the morphogenetic development of Otoceratidae. The pentacarinate form of the outer whorl, established at the middle stage of ontogenesis, may be a character inherited from the ancestor, the genus Avushoceras Ruzhencev.