Neogene molluscan biostratigraphy and paleoecology of the Hokuriku district

从生物地层学和古动物地理学的角度，参照新的微生物年代学，特别是硅藻年代学，对北陆地区新近系软体动物化石进行了研究。北陆地区新近系软体动物动物群可分为4个：1)Kurosedani动物群（早中新世晚期，16.5 ~ 15.5 ma）， 2) Nanao动物群（中中新世早期，15.5 ~ 14Ma）， 3) Otogawa动物群（晚中新世，约9ma）， 4) Omma动物群（晚上新世~早更新世，2 ~ 1ma）。这些动物群分别在热带至亚热带（16.5 ~ 15.5 ma）、暖温带（15.5 ~ 14ma）、温和至冷温带（约9Ma）和亚北温带至冷温带和暖温带混合环境下生长。其中，Otogawa动物群属于日本特有动物群，可能是在中新世海水冷却时期形成的。虽然Otogawa动物群一直被认为是冷水型的，但一些温水起源残存的软体动物元素表明动物群是温和到冷温带的。上新世可能首次出现与最近的亲潮（亲潮）相媲美的亚北方海洋环境。因此，北陆地区中新世至早更新世的动物变化与可能在晚中新世至早更新世逐渐发生的全球变冷很好地对应。

Neogene molluscan fossils distributed in the Hokuriku district have been studied from the view points of biostratigraphy and paleozoogeography in refer to the new microbiochronology, in particular the Diatom biochronology.The Neogene molluscan faunas of the Hokuriku district can be divided into four as 1). the Kurosedani fauna (late Early Miocene; 16.5 - 15.5Ma), 2) the Nanao fauna (early Middle Miocene; 15.5 - 14Ma), 3) the Otogawa fauna (Late Miocene; ca. 9ma), and 4) the Omma fauna (Late Pliocene to Early Pleistocene; 2-1Ma), respectively.These faunas can be assumed that the faunas had been fourished under the tropical to subtropical (16,5-15,5Ma), warm-temperate (15.5-14Ma), mild to cool-temperate (ca. 9Ma) and mixed of subboreal to cool-temperate and warm-temperate, respectively. Among them, the Otogawa fauna can be assigned to the Japanese endemic one which might be established during Miocene sea-water cooling. Although the Otogawa fauna has been considered to the cold-water type, some molluscan elements which are survivals of the warm-water origin suggest that the fauna is mild to cool-temperate ones. The first appearence of the subboreal marine environment which is comparable to those of the Recent Oyashio (Oyashio Current) might be in the Pliocene time. Consequently, faunal changes of the Miocene to the Early Pleistocene of the Hokuriku district well corresponded to worl-wide cooling that was probably taking place gradually through the late Miocene to early Pleistocene.