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1 July 2012 Different Modes of Migration Among Late Cretaceous Ammonoids in Northwestern Hokkaido, Japan: Evidence from the Analyses of Shell Whorls
Yasufumi Ikeda, Ryoji Wani
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Abstract

The thickness ratios of shell whorls ( = whorl breadth/shell diameter) in the ammonoids Damesites sugata (Forbes, 1845), Hypophylloceras subramosum (Shimizu, 1934), and Gaudryceras tenuiliratum Yabe, 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 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 or for lateral direction at a similar depth. This suggests that H. subramosum and G. tenuiliratum frequently 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.

The Paleontological Society
Yasufumi Ikeda and Ryoji Wani "Different Modes of Migration Among Late Cretaceous Ammonoids in Northwestern Hokkaido, Japan: Evidence from the Analyses of Shell Whorls," Journal of Paleontology 86(4), 605-615, (1 July 2012). https://doi.org/10.1666/11-089R.1
Accepted: 1 February 2012; Published: 1 July 2012
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KEYWORDS
ammonoids
migration
morphology
shell whorl
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