Covariable changes of septal spacing and conch shape during early ontogeny: a common characteristic between Perisphinctina and Ancyloceratina (Ammonoidea, Cephalopoda)

We analyzed the ontogenetic trajectories of conch morphology and septal spacing between successive chambers in Cretaceous ammonoids (suborders Perisphinctina and Ancyloceratina) collected from southern India, Madagascar, and Japan. All examined species, except for the family Collignoniceratidae, exhibited similar characteristics during early ontogeny. The common ontogenetic trajectories of septal spacing show a cycle comprising an increase and a subsequent decrease in septal spacing during early ontogeny. The conch diameters at the end of the cycle were estimated to be 1–4 mm. The conch shape (aperture height and whorl expansion rate) covariably changed at this conch diameter. Such covariable changes are commonly recognized in the suborders Perisphinctina and Ancyloceratina. The similarity in the ontogenetic trajectories of conch morphology implies a closer phylogenetic relationship between these suborders compared to Lytoceratina or Phylloceratina.