Relationships between the morphologic rates of diatom evolution and north Pacific Miocene circulation

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Several centric diatoms having plicate valve face have been reported from Pacific Neogene to Quaternary ranging in age from approximately 0 to 17 Ma ; Thalassiosira yabei group (=Thalassiosira brunii, T.flexosa, T.grunowii, T. mioplicata, T.moholensis, T.praeyabei, T.temperei, T.yabei, and others). These forms share a quite alike plication, one side convex and other side concave, of valve and intermediate forms occur fairly commonly among forms. Hence it appears that these plicated forms seem to be on the same evolutionary line.To provide information for the relationship between the morphologic rates of evolution of T.yabei group and mid Miocene high diatom production associates with Miocene North Pacific paleoclimatic and paleoceanographic changes, measurements of valve dimensions were made on 11 morphotypes, classified in this study, from Middle to Late Miocene part of their evolutionary line.The biometric evidence suggests that 1) the stock of this evolutionary line is T. praeyabei, 2) morphologic rates of evolution increases in Middle Miocene (15-13Ma), and 3) several descendants evolve from T.praeyabei at and around the interval. This increases in morphologic rates of evolution might have close relationship to auxospone formation which may be partly caused by intensified upwelling and high diatom production associated with mid Miocene cold events.This lineage also will define new biozones in a diatom zonal scheme for the Pacific Neogene, and a new genus, Plicatodiscus will be proposed in a following work.