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Otoscaphites puerculus (JIMBO) and Scaphites planus (YABE) occur abundantly in the Turonian of Hokkaido and south Sakhalin. The two species are nearly satisfied with an indispensable condition of the ...fossil population concept by reason of the characteristic in the mode of occurrence. Biometric and hydrostatic examinations on 42 samples for the two species from Naibuchi, Saku, Obira and Oyubari areas, have been made to obtain the quantitative data on their functional evolution. The analyses of individual relative growth of radius length to total rotation angle, septal and ventral wall thickness to radius length and variation of phragmocone size of adult shells indicate that the two species evolved in parallel in the direction of a buoyancy excess. Approximate calculation of the buoyancy of several well-preserved adult shells suggests that the two species changed with time in the mode of life of adult stage from benthic to planktic. The suture complexity of the two species increases with time in inverse proportion to the growth of septal and ventral wall thickness. Their inhabitable depth range may be shallower than that of modern Nautilus, because of their smaller value of the relative siphuncular strength than in Nautilus. Judging from the coexistent and sympatric mode of occurrences, similarity of ontogeny until 8 or 9π stage and parallel historical change of certain shell characters, it may be possible that the relationship between the two nominal species is a sexual dimorphism.続きを見る
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Mendeley出力
23_03_p367