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.

Each specimen was polished along its median plane (plane of symmetry) using a silicon carbide powder. The septal spacing between successive septa was defined as the rotational angle between two consecutive septa (i.e., N and N-1 septal numbers) at the positions where the septum met the siphuncle and was measured using a digital optical microscope (Keyence VHX-900; magnification × 25–175; error < 0.01°). The center of rotation was defined as the center of the initial chamber’s maximum diameter through the base of the caecum (Fig. 3.1). The measured septal spacings are shown as graphs of the septal spacing between two successive septa (N and N-1) against the phragmocone diameter through ontogeny. This is because the septal numbers could not be accurately determined in most specimens owing to partial dissolution, especially of the earliest whorl.

To measure the conch shape, we measured the aperture height (Klug et al., 2015) on the median plane, every 180° in Cleoniceras sp., Perisphinctes sp., and Yezoites puerculus or 45° in the other examined species (regarding differences in the accuracy of the resultant growth trajectories, refer to Tajika and Klug, 2020). Based on these measurements, scatter diagrams of the aperture height and conch diameter were constructed. We discerned the critical point(s) at which the slopes of the regression lines (calculated by the reduced major axis) changed (statistically significant, p < 0.05; Kermack and Haldane, 1950; Hayami and Matsukuma, 1970).

Furthermore, the whorl expansion rate (WER), one of the major parameters of ammonoid conchs, was measured on the median plane, as a representative parameter of conch shape (Klug et al., 2015). In each specimen, the WER on the median plane was measured (the measurement intervals were the same as those of the aperture heights), and the ontogenetic trajectories of each WER were recorded.

Ammonitella diameters were measured using an optical microscope with a digital measurement tool (Keyence VHX-900; magnification × 25–175; error < 0.01 mm). In this study, the ammonitella diameter was defined as the maximum diameter of the ammonitella from the primary constriction (Landman et al., 1996; De Baets et al., 2015a).