Morphologic Variation and Microspheric Forms ofParafusulina japonicafrom Tamanouchi, Itsukaichi-Ome Area, West Tokyo, Japan

Abstract

Specimens of Parafusulina japonica (Gumbel), middle Permian (late Wordian) Fusulinoidea from a small, exotic limestone block within the Middle Jurassic Kamiyozawa Formation exposed at Tamanouchi, west Tokyo, Japan, show considerable morphologic variation and include microspheric forms. The megalospheric specimens vary greatly in the shape and size of their tests, expansion of their tests, and in the shape and size of their proloculi. Two microspheric specimens are about twice the size of the megalospheric specimens and show early whorls to be schubertellid-like. The presence or absence of an initial one or two endothyroid whorl is not established. The shape of septal folds seems distinctive as they have slightly wider, flattened or slightly rounded dome-shaped tops both in megalospheric and microspheric forms. Cuniculi are absent in early whorls and quite poorly developed in later whorls where they are low inconspicuous features, suggesting a relatively early evolutionary stage in the Parafusulina lineage. The features of the megalospheric specimens have led some authors to place this species in the genus Parafusulina (Skinnerella) or even as a separate genus, Skinnerella. However, the microspheric specimens are much more typical of the genus Parafusulina (Parafusulina), and so P. japonica will be placed in the unsubdivided genus Parafusulina.