RECONSTRUCTION OF THE ONTOGENETIC DEVELOPMENT OF EARLY OTOCERAS (AMMONOIDEA) FROM CROSS-SECTIONS OF THEIR SHELLS

The ontogeny of the Late Permian Otoceras concavum Tozer has been reconstructed from cross-sections of shells. A moderately wide shell, with a moderately narrow umbilicus, narrows intensely at tiny sizes, becoming a narrow shell. At the stage of very small sizes, the shell expands, again becoming moderately wide, and the umbilicus narrows slightly, still remaining moderately narrow. At small sizes, the change in these characteristics occurs in the same direction, but with increased intensity. Important changes are observed at the medium-sized stage, when the expansion of the shell stops and the umbilicus becomes narrow. At the end of the studied ontogenesis, the morphological development of the mollusk is aimed at the formation of a moderately narrow shape with a very narrow umbilicus. The constructed ontogenetic model clearly demonstrates the features of the development of the cross-sectional shape of O. concavum shells during their growth, emphasizing the considered ontogenetic trends in changes in the most important parameters of the shell. The identified transformations of the shell shape contribute to the diagnosis of small-sized Otoceras and can serve as the basis for subsequent reconstruction of the morphogenetic development of the family Otoceratidae.