TAXONOMY OF THE THALASSIOSIRA TRIFULTA GROUP IN LATE NEOGENE SEDIMENTS FROM THE NORTHWEST PACIFIC OCEAN

Abstract

Several lower Miocene, Pliocene, and Pleistocene samples from the northwest Pacific Ocean were used to study the taxonomy of the Thalassiosira trifulta group, which is characterized by the inward tubes of the fultoportulae and a rimoportula located away from the valve margin. T. ferelineata, T. frenguelliopsis, T. tetraoestrupii, and Thalassiosira sp. A (referred to as the T. frenguelliopsis subgroup) were found to have operculate fultoportulae with an operculate basal structure covering the 4 satellite pores in the basal siliceous membrane. A second subgroup, consisting of T. oestrupii, T. praeoestrupii, and T. trifulta (referred to as the T. oestrupii subgroup), have a trifultate fultoportula with 3 columnar supports and 3 satellite pores. In T. praeoestrupii sensu stricto the trifultate fultoportula is intermediate in structure with 3 columnar supports and an operculate basal structure covering the 3 satellite pores.