Oligocene ostracode assemblages from the Itanoura Formation, Nishisonogi Group, Nagasaki Prefecture, southwestern Japan

Abstract

At least thirty-three ostracode species are reported for the first time from the Oligocene Itanoura Formation, Nishisonogi Group. This is also the first report of Oligocene ostracodes from the Japanese Islands. Fifteen samples bearing fossil ostracodes were grouped into three biofacies (A to C). Three biofacies have characteristics as follows: Biofacies A is dominated by Acanthocythereis volubilis (Liu) and Eopaijenborchella sinensis (Liu); Biofacies B is dominated by Krithe sp. associated with Palmoconcha oujiangensis (Liu) and A. volubilis; Biofacies C dominantly includes P. oujiangensis. The lithofacies, fossil molluscan assemblages and planktonic/total foraminifers ratios suggest the following habitats for the ostracodes: all ostracode species from the formation inhabited a shelf influenced by coastal water; dominant species of Biofacies A flourished in environments shallower than those of Biofacies B. Fossil ostracode assemblages from the Itanoura Formation have lower numbers of species and lower species diversity indices than other such assemblages from shelf habitats through the Neogene to Recent. These results also suggest that some genera such as Cytherella, Eopaijenborchella and Palmoconcha may differ from their Recent countertparts in their habitats, and that shallow-marine ostracodes in the northwestern Pacific might have diversified after the early Oligocene.