Shell Ontogeny of the Type Species of the Genus Neouddenites (Ammonoidea, Medlicottiidae) from the Kungurian of Western Verkhoyanie

Reconstructions of the cross sections of two specimens of the type species of Neouddenites , N. andrianovi Ruzhencev from the Kungurian of Western Verkhoyanie, revealed patterns in its ontogenetic transformation. The species is characterized by slow expansions of the medial spiral, which only stops at the last whorl. The shell shape continuously changes from very wide in the first whorl to very narrow in the last whorl. The umbilicus expands slightly at the beginning of ontogeny, making the shell evolute. The umbilicus then strongly narrows, and at the last whorl the shell becomes completely involute. Significant changes in the shape of the whorl cross-section are observed throughout ontogeny allowing recognition of four successive stages, designated here as “ Eocanites ”, “ Pronorites ”, “ Prouddenites ”, and “ Neouddenites ”. Neouddenites andrianovi was found to lack lateral grooves and ventrolateral ridges, previously considered to be diagnostic characters of the genus Neouddenites .