Ammonoids are an extinct group of cephalopods that lived from the Devonian until the end of the Cretaceous periods. In the Jurassic and Cretaceous periods, there were four suborders, Ancyloceratina, Perisphinctina, Lytoceratina, and Phylloceratina. Ancyloceratina formed a conch with detached whorls (open coiling) or non-planispiral coiling. The origin of Ancyloceratina remains unclear. In this study, we analyzed conch morphology in detail using specimens collected from southern India, Madagascar, and Japan. As a result, we found a common trend in conch morphology in early ontogeny of Ancyloceratina and Perisphinctina. We think that the similarity of conch morphology suggests a closer relationship between them, relative to Lytoceratina or Phylloceratina. Our findings are meaningful to consider the phylogenetic relationship and evolution of Jurassic–Cretaceous ammonoids.

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.