|Budget Amount *help
¥1,800,000 (Direct Cost : ¥1,800,000)
Fiscal Year 1997 : ¥800,000 (Direct Cost : ¥800,000)
Fiscal Year 1996 : ¥1,000,000 (Direct Cost : ¥1,000,000)
The shallow molluscan fauna of the Japan Sea borderland during 4-3Ma can be ecologically subdivided into two types. The embaymental associations from the Ogikubo, Joshita, lower Mita Formations are dominated by the cold-water species, including many Miocene-type relicts.On the other hand, the open sea assemblages from the Tentokuji, Kuwae and upper Mita Formations includes many warm-water species, but very few relicts.
The open water assemblages were directly affected by the sudden influx of warm Paleo-Tsushima current into the Japan Sea during this period. The genus Ranella, not living around the Japanese waters was introduced by this influx (Amano, 1997a). Many Miocene-type species in the assembages suffered extinction by this event. But many relicts survived in the cold embayment where was not directly affected by the warm-water current.
During this age, the deep water taxa such as Portlandia toyamaensis, Ancistrolepidenae and Neptunea were not affected by the above events and the succeeding cold event by the spreading of glaciation (Amano, 1996a, b ; Amano et al., 1996 ; Amano, 1997b ; 1997MS).
Consequently, it becomes clear that the cold embayment acts as an ecological refuge of shallow marine relicts.