| Budget Amount *help |
¥1,700,000 (Direct Cost: ¥1,700,000)
Fiscal Year 1990: ¥200,000 (Direct Cost: ¥200,000)
Fiscal Year 1989: ¥1,500,000 (Direct Cost: ¥1,500,000)
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| Research Abstract |
During the course of this study, many fossils were collected from twenty outcrops or more of ten formations which are classified into four stratigraphic horizons - Cretaceous, Lower Miocene, Middle Miocene and Pleistocene. Their occurrence, mineral and atomic composition of shell and surrounding matrices and internal shell structure were also investigated. The shell structure of four Cenozoic extinct species were compared with some recent relatives. Although a new type of microstructure or structural combination is not found in those shells, their phylogenetic positions are taken into consideration. For instance, the shell structure of Vicarya japonica, one of extinct potamids, is very similar to those of some recent species of the genera Cerithidea and Terebralia. They might have evolved from a common ancestor who have the same structure as that of Batillaria. While, all glycymerid species examined in this study have a single type of shell construction without regard to their living ages. No change of the shell structure has probably occurred in this family throughout the Cenozoic era. The preservation of the primary structure in the fossilization was also examined. The calcitization is one of the most current courses in the fossilization of molluscan shells. In this course aragonitic microstructures are sometimes preserved in or with calcite crystals. When a carbonate shell is replaced with a siliceous matter such as quartz, internal structures are preserved very well. Some other preservation patterns were also described.
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