Abstract
The ornamentation and dorsal wall structure of Vidrioceras (Cycloloboidea, Goniatitina) in the early ontogenetic stage are described on the basis of specimens from the Upper Pennsylvanian in Kansas, USA. The exposed surface of the embryonic shell is smooth, without any trace of ornamentation or growth lines. Regularly spaced lirae abruptly appear on the early postembryonic shell just adoral of the primary constriction. The inner surface of the dorsal wall in the embryonic and early postembryonic stages exhibits a distinct ornament consisting of evenly spaced, longitudinal ridges, which are replaced adorally by the typical wrinkled ornament in the subsequent stage. Our observations are in accord with those of goniatites from the Upper Carboniferous Buckhorn Asphalt of Oklahoma, suggesting that in the Goniatitina, the outer surface of the embryonic shell is smooth. Comparison with the embryonic shell formation of extant Nautilus suggests that in the Goniatitina, the embryonic shell was uniformly secreted by the shell gland on the posterior side of the embryo.
