Ontogenetic Study of Late Cretaceous Gaudryceras tenuiliratum

Hirano Hiromichi

doi:10.5109/1544170

The ontogeny and variation of Gaudryceras tenuiliratum were studied on seven samples from the Santonian of Kawakami and Naibuchi of Saghalien and Haboro, Oyubari and Urakawa of Hokkaido. The growth pattern of the radius length is one of the most important factors which decide the shape of ammonites. The ontogenetic patterns were allometrically analysed on seven samples with reference to the individual and the interpopulational variation. The growth ratio is fairly stable not only intrapopulationally but also interpopulationally. The position of the critical point is also highly stable in each population but geographically shows some variability. The radius length is controlled by the growth ratio and the initial growth index and mostly is not different significantly in three defined growth stages. The growth pattern of the whorl width was analysed in one sample but the possibility of the sexual dimorphism was not detected. The pattern of the umbilical radius length shows the ontogenetic change less delicately in comparison with that of the radius length. The ontogenetic change and the variation of the septal thickness and the complexity of suture were analysed, and the role of these characters is discussed. The periodicity of the appearance of constriction was clarified and is compared with that of some recent mollusks. The quantitative data of the surface ornamentation is shown and its ontogenetic change is described. The individual and the interpopulational variation of the protoconch size was analysed and compared with that of other ammonites. The comprehensive description of G. tenuiliratum is given based upon the analyses mentioned above.

Ontogenetic Study of Late Cretaceous Gaudryceras tenuiliratum

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