Stepwise growth changes in early post-embryonic stages among Cretaceous tetragonitid ammonoids

Abstract

In this study, we analyzed the ontogenetic trajectories of shell morphology in some Cretaceous tetragonitid ammonoid specimens (Tetragonitidae) collected from the Tomamae, Mikasa, and Hamanaka areas of Hokkaido, Japan. In all examined species, the ontogenetic trajectories of septal spacing between successive chambers had similar characteristics during their early ontogeny: two cycles, each comprising an increase and subsequent decrease in septal spacing until ~ 30th septum. The trends of whorl expansion rate changed at 5–7 or ~ 10 mm in the Gaudryceratinae and ~ 3 mm shell diameter in the Tetragonitinae. Based on these observations, we propose that the planktic phases of Gaudryceratinae and Tetragonitinae ended at those shell diameters. These different shell diameters at the end of the planktic phase suggest slightly differing strategies within the family Tetragonitidae.