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

Abstract

The thickness ratios of shell whorls (=whorl breadth/shell diameter) in the ammonoidsDamesites sugata(Forbes, 1845),Hypophylloceras subramosum(Shimizu, 1934), andGaudryceras tenuiliratumYabe, 1903 from 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 ofD. sugatadiffer significantly with depth on the outer shelf environment but show no significant difference laterally at a similar depth. These results suggest thatD. sugatadid 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 ofH. subramosumandG. tenuiliratumshow no significant difference between different depths on the outer shelf environment or for lateral direction at a similar depth. This suggests thatH. subramosumandG. tenuiliratumfrequently migrated at different depths on the outer 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 diversified paleoecology of Late Cretaceous ammonoids.