Data from: Ontogenetic trajectories of septal spacing and shell shape in the Late Cretaceous gaudryceratid ammonoids: implications for their post-embryonic palaeoecology

In this study, we analysed the ontogenetic trajectories of septal spacing
between succeeding chambers of the gaudryceratid ammonoid, Gaudryceras
tenuiliratum, which were excavated in the Tomamae and Mikasa areas of
Hokkaido, Japan. The ontogenetic trajectories of septal spacing in G.
tenuiliratum demonstrate a general trend: two cycles of increasing to
decreasing septal spacing until the ~30th septum, gradually decreasing
septal spacing until the ~70th septum, and then gradually increasing
septal spacing at least until the ~110th septum. The ontogenetic
trajectories of the whorl expansion rate (WER) in our specimens also
demonstrated that a decreasing trend changed into an increasing trend at a
shell diameter of ~5–7 mm. This shell diameter corresponds to the end of
the second cycle of increasing to decreasing septal spacing occurring
before the ~30th septum, at which point G. tenuiliratum possibly
transitioned from planktic to nektobenthic habits. No significant
differences were detected in the ontogenetic trajectories of septal
spacing and conch shape between the two areas, which suggests that the
ontogenetic trajectory patterns of septal spacing in the Late Cretaceous
ammonoids were taxonomy-dependent rather than environment-dependent.