Abstract

The thickness ratios of shells (=whorl breadth/shell diameter) in the heteromorphic scaphitid ammonoid Scaphites planus (Yabe, 1910) from the lower middle Turonian in the Oyubari and Kotanbetsu areas of Hokkaido, Japan were examined in order to determine their mode of migration. The thickness ratios of S. planus differ significantly between the two localities, which suggest that these different populations did not frequently migrate between the two areas (currently ∼130 km apart, although the actual distance during Turonian is uncertain due to the presence of faults and folds between the two areas). There is no difference in hatchling diameters between the two areas, suggesting that the thickness ratios became manifested after the post-hatchling stage due to limited migration with in a nektobenthic habitat. This study suggests that scaphitid ammonoids became nektobenthic with limited migration at a stage earlier than previously thought (not during the transition from normal to abnormal coiling). The limited migration in scaphitid ammonoids might relate to their higher evolutionary volatility.

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