Different modes of migration among Late Cretaceous ammonoids in northwestern Hokkaido, Japan: evidence from the analyses of shell whorls

The thickness ratios of shell whorls (= whorl breadth / shell diameter) in the ammonoids Damesites sugata, Hypophylloceras subramosum, and Gaudryceras tenuiliratum from the Late Cretaceous outer shelf deposits in the Kotanbetsu area of northwestern Hokkaido, Japan, were examined in order to determine their mode of migration. The thickness ratios of D. sugata differ significantly with depth on the outer shelf environment, but show no significant difference laterally at a similar depth. These results suggest that D. sugata did not frequently migrate to different depths on the outer shelf environment, but tended to live and migrate laterally at a similar depth on the outer shelf. The thickness ratios of H. subramosum and G. tenuiliratum show no significant difference between different depths on the outer shelf environment nor for lateral direction at a similar depth. This suggests that H. subramosum and G. tenuiliratum frequently migrated at different depths on the out er shelf environment and laterally at a similar depth. There is no difference in hatchling diameters in all the examined species at different depths on the outer shelf environment, suggesting that the thickness ratios became manifest after the post-hatchling stage due to limited migration, which would have been in a nektobenthic habitat. The various modes of migration in the study area are reconstructed in this study, suggesting a diversified paleoecology of Late Cretaceous ammonoids.