Biostratigraphic study of Tertiary benthic foraminiferal Family Cassidulinidae.

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Benthic foraminifera has been recognized as one of the most useful microfossils in a interpretation of paleoenvironment. The application of benthic foraminifera to paleoenvironmen analyses has recently been much progressed in relation to palcoccanography. In most cases. however, such paleoenvironmental analyses are based on the faunal variation of benthi foraminifera and the species identified sense lato. In this study, I focused on the benthic foraminifcral family Cassidulinidae in order to know the stratigraphic and geographic distributioi of respective species in the Japanese Islands, and also to clarify the tempo and mode of evolutio of cassidulinid foraminiferal species during the Tertiary epoch. The main results are as followsBased on the stratigraphic ranges of 45 species distributed in 9 genus,1 Paleogend cassidulinid assemblage Zonc and 5 Neogene cassidulinid concurrent Zones are recognized Palcogenc cassidulinid zone is predominated by Globocassidulina globosa. In the 5 Neoge … More ne cassidulinid zones, the most dramatic change is found between Zones III and IV in the late Midd Miocene, where globocassidulinid species are drastically reduced in its composition in contrast the development of islandiellid spccics. Such cassidulinid change is intimately related to the paleoceanographic change such as the drop of marine water temperature resulted from the Antarc ice sheet expansion. Another study, suggesting the positive correlation between the number of islandiellid species and the organic carbon contents in the Recent sediments off San'in, Sea o Japan, supports the evolution of cassidulinid species caused by temperature drop and the encrew of primary productivity. Furthermore, in the deep-sea sediments from Broken Ridge, Easten Indian Ocean, the size of cassidulinid species (e.g., Globocassidulina globosa - G. subglobosa series) increases gradually from the Middle Miocene, which epoch is characterized by showing the temperature drop as clarified by oxygen isotopic analysis. Varied test as well as apertural shape; recognized in cassidulinid species occurred from the Middle Miocene and later ages are considered to indicate a strategic adaptation to the environment characterized by cold bottom water are increased organic accumulation. Less