Diversity and evolution of late Eocene to late Oligocene silicoflagellates from IODP Expedition 378 Holes U1553A and U1553B, southwest Pacific Ocean

Study of the South Pacific Ocean sediments from IODP Expedition 378, Holes U1553A and U1553B, shows 19 late Eocene to late Oligocene silicoflagellate species. The late Eocene includes Corbisema tajmahalii n. sp., observed over a ∼ 8.4 m interval and proposed as a new subzone within the Corbisema apiculata Zone. The new species is characterized by the presence of basal ring that has three sides and an arrowhead-shaped outline with the two minor-axis corners more closely spaced. This study has found the first known Bachmannocena double skeleton, in corner-aligned configuration that suggests evolutionary relationship with the Corbisema triacantha group of Cenozoic silicoflagellates. Also observed is the apparent early evolution of Stephanocha raupii at ∼29.6 Ma, derived from a skeletal diversity related to Distephanopsis crux, thus the species is here transferred to Distephanopsis (abbreviated as Ds.). Silicoflagellate skeletal abundance declines after the Eocene-Oligocene boundary, with generally low abundance except for two sediment samples immediately below the Ds. raupii n. comb. first appearance.