Vicissitudes of ostracode assemblages and mechanisms involved

北海道白垩纪的介形类化石被发表在两篇论文中，证明了当时日本岛屿相对于特提斯的位置以及日本和欧洲之间介形虫动物群的先代性。通过对分布在金泽市及周边地区的更新世Omma组介形类动物群的分析，发现1.2 Ma时有一个明显的黑潮涌入日本海的尖峰，水情在0.12~0.09 My(Alcheringa)的周期内波动明显。之后，我集中精力从分布在仙台市及周边的新近系Moniwa组收集介形虫化石，以便对1966年的报告进行补充。在仙台市南部边界附近的Tsunaki组发现的介形虫动物群，含有迄今报道的来自日本东北部更新世的Cornucoquimba moniwensis形式，以及Cornucoquimba moniwensis(S.S.)，它们的甲壳装饰在形态上是明显分开的。在中新世末期，随着气候变暖，它们的栖息地扩展到较浅的水域，在那里建立了一种新的形式，至少在更新世的浅海中逗留。关于白垩纪，Krithe属最早出现在Hakobuchi群(Campanian)的最低部分。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，从而引入了介形虫动物群的新元素，为我们提供了一个非常有吸引力的研究对象之一。

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.