茨城県常陸大宮市の瓜連丘陵に分布する更新統引田層から産出した2分類群のLindavia 属珪藻化石

抄録

We report on the occurrence and valve morphology of two fossil diatom taxa assigned to the genus Lindavia from the Pleistocene Hikita Formation in the Urizura Hills, Hitachiomiya City, Ibaraki Prefecture, central Japan. Lindavia omarensis (=Pliocaenicus omarensis), which was previously reported from the Upper Pliocene to the Lower Pleistocene in Japan, was identified from a wide stratigraphic interval in the Hikita Formation. Lindavia rhomboideoelliptica var. rounda, which was previously recognized from the Middle Pleistocene in Japan, occurred exclusively from the upper part of the Hikita Formation. Co-occurrence of L. omarensis and L. rhomboideoellitica var. rouda in the upper part of the Hikita Formation is discordant with the previously known occurrence range of the two taxa, and this suggests that both or either of the two taxa possibly reworked fossil. However, the depositional age of the Hkita Formation is not well determined. It would be necessary to examine the age of the Hikita Formation in order to clarify the biostratigraphy of both taxa in this formation.