Turonian collignoniceratid ammonites from Hokkaido, Japan : Stratigraphy and Paleontology of the Cretaceous in the Ishikari province, central Hokkaido, Part 3 (Department of general Studies)

In this paper five species belonging to the two genera of the Turonian collignoniceratid ammonites from Hokkaido are described, viz. Collignoniceras woollgari (Mantell), C. bravaisianum (d\u27Orbigny), Subprionocyclus neptuni (Geinitz), S. minimus (Hayasaka et Fukada) and S. (?) sp. Subprionocyclus bravaisianus is reasonably referred to the genus Collignoniceras and independent of S, neptuni. "Reesidrtes minimus" and "Subprionocyclus normalis" cannot be maintained as separate species. Furthermore, the relation between the Japanese "R. minimus" and "S. normalis" and the North American "S. normalis" [=Oregoniceras condoni, O. jacksonense and O. argonautarum] is examined, with a suggestion that they may represent geographical subspecies. A dimorphism represented by macroconchs and microconchs probably existed in the Turonian collignoniceratids. An evolutional change of the Turonian collignoniceratids as shown by a series of C. woollgari, S, neptuni and S. minimus shows a paedomorphosis, and probably is a case of the neoteny. This change is interpreted as an evolutional adaptation for the decrease in the resistance of water. In other words, the phylogenetic evolution of these Turonian collignoniceratids seems to suggest a trend of shell adaptation for more successful swimmer.In this paper five species belonging to the two genera of the Turonian collignoniceratid ammonites from Hokkaido are described, viz. Collignoniceras woollgari (Mantell), C. bravaisianum (d\u27Orbigny), Subprionocyclus neptuni (Geinitz), S. minimus (Hayasaka et Fukada) and S. (?) sp. Subprionocyclus bravaisianus is reasonably referred to the genus Collignoniceras and independent of S, neptuni. "Reesidrtes minimus" and "Subprionocyclus normalis" cannot be maintained as separate species. Furthermore, the relation between the Japanese "R. minimus" and "S. normalis" and the North American "S. normalis" [=Oregoniceras condoni, O. jacksonense and O. argonautarum] is examined, with a suggestion that they may represent geographical subspecies. A dimorphism represented by macroconchs and microconchs probably existed in the Turonian collignoniceratids. An evolutional change of the Turonian collignoniceratids as shown by a series of C. woollgari, S, neptuni and S. minimus shows a paedomorphosis, and probably is a case of the neoteny. This change is interpreted as an evolutional adaptation for the decrease in the resistance of water. In other words, the phylogenetic evolution of these Turonian collignoniceratids seems to suggest a trend of shell adaptation for more successful swimmer