Development of lacustrine diatoms in the circum-Pacific region

保存较好的湖相放线素环虫种类大量出现在日本岛屿早中新世至中中新世的硅藻土中：Iki岛的Chojabara组、noto -中岛的Yamatoda组、日本海yamatobank的Piston-core RC12-394和宫城县的Ouchi组。它们对乔贾巴拉组几个层位的贡献占总硅藻瓣的近99%。通过与北美大陆中新世硅藻土中发现的13种放线素环孢属植物的形态特征比较，将日本岛屿的放线素环孢属植物初步划分为8种形态类型。到目前为止，这13个物种的形态类型尚未确定。然而，该物种在北美大陆西部和日本列岛均有丰富的分布，这一证据有力地表明，早中新世至中新世中期湖相放线环虫的发育几乎同时发生在环太平洋地区；也就是说，中新世放线菌的发展和后来的快速衰落很可能是环太平洋地区的共同事件。虽然在环太平洋地区湖泊硅藻的基础上建立完整的生物地层格架还需要进行年龄模型的制作，但上述证据为框架的制作过程提供了非常基础的数据/信息。在本研究中，放线菌属物种在日本列岛和北美大陆之间的发育并没有严格的同时性，控制该物种发展和后来衰落的生态/环境因素仍未得到解决。

Well preserved lacustrine Actinocyclus species occurred abundantly from Early to Middle Miocene diatomite from the Japanese Islands: Chojabara Formation in Iki Island, Yamatoda Formation in Noto-Nakajima, Piston-core RC12-394 from Yamato-Bank in the Sea of Japan, and Ouchi Formation in Miyagi. Their contribution to several horizons of Chojabara Formation account for nearly 99% of the total diatom valves.Based on a comparison of the morphological characteristics of the Actinocyclus species with thirteen Actinocyclus species described from Miocene diatomite in the North American Continent, the species from the Japanese islands have classified tentatively into eight morphological types. The morphologic types have not been so far identified with the thirteen species. However, the common-abundant occurrences of the species of the species in both western part of North American Continent and the Japanese Islands is confirmed, and this evidence strongly suggested that development of lacustrine Actinocyclus species in the Early to Middle Miocene occurred nearly simultaneously in the circum-Pacific region ; namely it is highly probable that development and later rapid decline of Actinocyclus species in the Miocene were common events in the circum-Pacific region.Although an age-model making is needed for making a complete biostratigraphic framework based on the lacustrine diatoms in the circum-Pacific region, the above evidences provide very basic data/information in a procedure to make the framework. In this study, a strict simultaneity in the development of Actinocyclus species between the Japanese Islands and the North American Continent is not confirmed, and an ecological/environmental factor(s) controlling the development and later decline of the species are still unsolved.