977. KOIZUMIA YANAGISAWA GEN. NOV., A NEW MARINE FOSSIL ARAPHID DIATOM GENUS

Abstract

A new marine fossil araphid diatom genus Koizumia is established to receive three species, Koizumia tatsunokuchiensis (Koizumi) Yanagisawa comb. nov. (originally described as Rhaphoneis tatsunokuchiensis by Koizumi, 1972), K. adaroi (Azpeitia) Yanagisawa comb. nov. (originally described as Cymatosira adaroi by Azpeitia, 1911), and K. akibae Yanagisawa sp. nov. Detailed morphologies of the genus are presented with its precise stratigraphic and geographic distributions. The genus is characterized by cribrum-like siliceous layers across the pores of marginal ridge, the suture line on the marginal ridge and the presence of transapical and longitudinal rows of areolae on the valve face. The three features are unique to the genus and have never been observed in its related genera Rossiella Desikachary et Maheshwari and Bogorovia Jouse. Koizumia is typically neritic in distribution mainly in near-shore or upwelling regions of the middle latitudes, and is significantly different from Rossiella and Bogorovia, both of which have essentially an oceanic and tropical to subtropical distribution. Both the morphological uniqueness and the differing geographic distribution pattern are considered to be distinctive enough to form the basis of a new generic erection of Koizumia. The genus has a rimoportula in the pattern of "one per cell" with an apical pore field at each apex, and forms a long colony chain connected by marginal ridges. These features suggest that the genus can be placed in the family Cymatosiraceae Hasle, von Stosch et Syvertsen.