Reconstruction of the Shell Shape Ontogeny of the Early Permian Ammonoids Uraloceras Omolonense

Abstract

The shell shape modeling features of fragmentarily preserved goniatites of the species Uraloceras omolonense Bogoslovskaya et Boiko, 2002 from the Lower Permian of the Omolon massif are considered. The basic trends of the ontogenetic development of the shell shape of this species by the respective charts are revealed. The cross shell shape and ontogeny of U. omolonense models are created on the basis of a series of the shell cross-sections and measurements of fragmentarily preserved spirals. A cyclical development of the spiral is revealed: initially a medial spiral intensely stretches, at medium speed it stabilizes, and in the final ontogenesis it shrinks. During the stabilization period the shell wound spirally as close as possible to the logarithmic form. With respect to the external shell shape the following ontogenetic sequence is established: on the internal whorls the intense contraction of the shell and reducing of the umbo take place; at the medium whorls the shell narrowing increases but umbo narrowing weakens; in the terminal phase the shell shape stabilizes. According to the morphogenetic features the species U. omolonense is classified as the moderately involute subgroup of the U. simense group which dominated in the community of Sakmarian Uraloceras.