Development of lacustrine diatoms in the circum-Pacific region

保存完好的放线藻物种在日本群岛早中新世硅藻土中大量出现：Iki岛的Chojabara组，Noto-Nakajima的Yamatoda组，日本海Yamato-Bank的活塞芯RC12-394，以及宫城的Ouchi组。它们对乔贾巴拉组多个层位的贡献占硅藻瓣总数的近99%。通过对放线藻与北美大陆中新世硅藻土中描述的13种放线藻的形态特征的比较，初步将日本岛屿上的物种划分为8个形态类型。到目前为止，这13个种的形态类型还没有确定。然而，该物种在北美大陆西部和日本群岛的普遍丰富的存在得到了证实，这一证据有力地表明，早中新世湖相放线菌物种的发展几乎同时发生在环太平洋地区，即中新世放线菌物种的发展和后来的迅速衰退很可能是环太平洋地区的常见事件。尽管需要建立一个基于环太平洋地区湖泊硅藻的完整生物地层格架，但上述证据为建立该框架提供了非常基本的数据/信息。在这项研究中，日本群岛和北美大陆之间放线菌物种发展的严格同时性并未得到证实，控制该物种发展和后来衰落的生态/环境因素(S)仍未解决。

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.