Two ammonite species of Mortoniceras from the Yubari Mountains (Hokkaido) and their geological implications : Studies of the Cretaceous ammonites from Hokkaido and Sakhalin-LXXXII

Abstract

Two ammonite species of the genus Mortoniceras have been recently obtained from two stratigraphic units, Member Ld of the Lower Yezo Subgroup and Member Mb of the Middle Yezo Subgroup on the Tengu-zawa route of the Yubari Mountains, central Hokkaido. They are identified respectively with Mortoniceras (Mortoniceras) cf. geometricum Spath and Mortoniceras (Mortoniceras) rostratum (J. Sowerby). M. (M.) geometricum, which is taken here as allied to M. (M.) pricei (Spath), probably includes some specimens described as Pervinguieria arietiformis by Haas (1942b) from Angola and as M. (M.) arietiforme by Renz (1971) from Venezuela. Our study of Mortoniceras (Mortoniceras) rostratum suggests that Ammonites rostratus J. Sowerby should be systematically assigned to Mortoniceras (Mortoniceras) rather than to M. (Subschloenbachia). On the evidence of the two ammonite species, Member Ld is correlated with the middle part (probably the Hysteroceras varicosum Subzone) of the Upper Albian and Member Mb with the upper part (probably the M. (M.) rostratum Subzone) of the same substage. Therefore, no significant time gap exists at the boundary of the Lower and Middle Yezo Subgroups.