Development of lacustrine diatoms in the circum-Pacific region

早中新世至中中新世，日本诸岛湖相辐环藻属（Actinocyclus）物种分布丰富，包括伊基岛的长原组、能登中岛的山田组、日本海大和滩的RC 12 -394活塞岩心和宫城的大内组。根据对日本列岛的放线环藻属（Actinocyclus）的形态特征与北美大陆中新世浮游植物放线环藻属（Actinocyclus）的13个种的比较，将日本列岛的放线环藻属（Actinocyclus）初步划分为8个形态类型。形态类型尚未确定的13种。然而，该种在北美大陆西部和日本群岛的共同丰富性得到了证实，这一证据有力地表明早中新世至中中新世湖相Actinocyclus种的发展几乎同时发生在环太平洋地区;也就是说，中新世放线菌属物种的发展和后来的迅速衰落很可能是环太平洋地区的共同事件。要建立一个完整的环太平洋地区湖相硅藻生物地层格架，需要建立模型，上述证据为建立该格架提供了非常基础的数据/信息。在这项研究中，日本群岛和北美大陆之间的Actinocyclus物种发展的严格一致性尚未得到证实，控制该物种发展和后期衰退的生态/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.