968. MEDIARIA MAGNA YANAGISAWA, SP. NOV., A NEW FOSSIL RAPHID DIATOM SPECIES USEFUL FOR MIDDLE MIOCENE DIATOM BIOSTRATIGRAPHY

Abstract

A new fossil diatom species Mediaria magna Yanagisawa sp. nov. is described from the lower Middle Miocene Oidawara Formation in Mizunami area, Gifu Prefecture, Japan. Morphologic investigation by light and scanning electron microscopes has revealed that Mediaria magna is distinct from Mediaria splendida f. splendida and M. splendida f. tenera by its unique characteristics. M. magna is of great value in Neogene diatom biostratigraphy because it is easily identifiable even when fragmented and also because it is very short-lived, being restricted to the lower part of the early Middle Miocene Denticulopsis lauta Zone (NPD 4A) of the Neogene North Pacific diatom zone (Akiba, 1986). Scanning electron microscopy reveals the presence of a raphe in the genus Mediaria for the first time. The genus has a raphe of simple slit without fibulae and therefore it cannot be placed in the order Bacillariales Hendey. It is morphologically similar to some genera in the family Catenulaceae Mereschkowsky in the order Thalassiophysales Mann.