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

A detailed study of cross-sections of the earliest (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 allowed features of ontogenetic shape changes to be 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 development of the tricarinate shape of the venter were recognized. Identification of shell shape transformations in the ontogeny of O. concavum contributes to the diagnosis of small-sized Otoceras and can serve as the basis for subsequent reconstruction of the evolution of morphogenesis in Otoceratidae. The pentacarinate form of the external part of the whorl observed at the middle stage of ontogeny may be a character inherited from the ancestral genus Avushoceras Ruzhencev.