Vicissitudes of ostracode assemblages and mechanisms involved

来自北海道白垩纪的介形虫化石发表在两篇论文中，证明了当时日本群岛相对于特提斯的位置以及介形虫动物群在日本和欧洲之间的代代性。对分布在金泽市及其周围的更新世奥马组介形类动物群的分析表明，在1.2 Ma时，黑潮流入日本海的峰值明显，在0.12 ~ 0.09 Ma的周期内，水势波动明显（Alcheringa, in press）。此后，我集中收集了仙台市及其周边新近系莫尼瓦组的介形类化石，为1966年的报告提供了补充工作。在仙台市南部边界附近的Tsunaki组介形类动物中，发现了一种目前报道为日本东北部更新世Cornucoquimba moniwensis的形态，与Cornucoquimba moniwensis （s.s.）的形态，两者的甲壳纹饰在形态上有明显的区别。中新世末期，气候变暖导致它们的栖息地扩展到较浅的水域，在那里形成了一种新的形式，至少在更新世的浅海中徘徊。在白垩纪，Krithe属首先出现在Hakobuchi群（Campanian）的最底层。这是日本最古老的记录，可能代表了介形虫动物群的一些变化，允许引入新的介形虫动物群元素，为我们提供了一个非常有吸引力的研究对象。

Fossil ostracodes from the Cretaceous of Hokkaido were published in two papers proving the position of the Japanese Islands in relative to Tethys at the time and hererochroneity of ostracode faunas between Japan and Europe. Analysis of ostracode fauna from the Pleistocene Omma Formation distributed in and around Kanazawa City disclosed a prominent spike of Kuroshio flowing into the Japan Sea at 1.2 Ma, and distinct fluctuations of water conditions in a cycle of 0.12 to 0.09 My (Alcheringa, in press).Afterward, I concentrated in collecting fossils ostracodes from the Moniwa Formation, one of the Neogene distributed in and around Sendai City, to enable work supplementary to the report in 1966. Ostracode faunas from the Tsunaki Formation distributed near the southern border of Sendai City,were found containing the form which has been reported to date as Cornucoquimba moniwensis from the Pleistocene in Northeast Japan, together with Cornucoquimba moniwensis (s.s.), both being morphologically separated clearly by their carapace ornamentation. The dimorphism invoked at this time is shown subject to an increasing warming toward the end of Miocene time, leading their habitats extended to shallower waters, where a new form was established to linger on, at least, in Pleistocene shallow sea. With respect to the Cretaceous, the genus Krithe was found first appearing from the lowest part of the Hakobuchi Group (Campanian). This occurrence, the oldest record in Japan, may represent some changes in ostracode faunas to allow the introduction of a new element of ostracode faunas, offering us one of the very attractive subjects of our study.