FOSSIL MARINE PLICATED THALASSIOSIRA: TAXONOMY AND AN IDEA ON PHYLOGENY

Abstract

Thalassiosira yabei and its allied forms, which are important constituents of North Pacific high-resolution biostratigraphic framework, are taxonomically re-examined. Eleven morphotypes within the morphological variation of the diatoms previously reported as fossil marine plicated Thalassiosira have been distinguished in a single Miocene section of northeast Pacific DSDP Site 173. Of these, five new forms, Thalassiosira californica, T. flexuosa var. tenella, T. kanayae, T. perispinosa and T. transitoria are named and described, and six previously described taxa are also examined. The eleven plicated Thalassiosira from the Miocene of Site 173 can be classified into three morphological groups by their internal rimoportula and external fultoportula shapes. Members of the first group, T mioplicata, T praeyabei and T. perispinosa, have a simple rimoportula and tubular fultoportula openings on the valve mantle. A flattened rimoportula stalk and a ring of marginal fultoportula spines distinguish the second group: T. californica, T. flexuosa, T. flexuosa var. tenella, and T. grunowii. A thick lipped rimoportula and asymmetrically developed fultoportula openings are characteristics of the third group: T. brunii, T. kanayae, T. temperei and T. transitoria. The phylogeny of fossil marine plicated Thalassiosira in the context of the high-resolution biostratigraphic framework in the DSDP core at Site 173 has also been considered. The first group evolved upward into both the second and third groups. Stratigraphic changes in marginal fultoportulae structure characterize trends in morphology from the first group to the second; a marginal ring of external fultoportula tubes in the ancestors changed to a marginal ring of external openings with spines located just above the openings in the descendants. Both valve outlines, from circular to elliptical, and reduction of the external fultoportula tubes on the concave side of valve mantle are obvious as trends from the first group to the third. The lineage clarified here also will define new biozones in a diatom zonal scheme for the Pacific Neogene.