Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, Motooka, Nishi-ku, Fukuoka, Japan.
Mifune Dinosaur Museum, Mifune, Kumamoto, Japan.
PLoS One. 2018 Jun 5;13(6):e0198003. doi: 10.1371/journal.pone.0198003. eCollection 2018.
Fossil evidence indicates that modern assemblages of temperate nonmarine planktonic diatoms began near the middle/late Miocene boundary when the genus Actinocyclus, an important constituent of lacustrine planktonic diatom assemblages during the early to middle Miocene, was replaced by genera of the family Stephanodiscaceae. This floral turnover has been confirmed in many regions of the world, except eastern Asia where taxonomic data about early and middle Miocene planktonic diatom assemblages have until recently been scarce. Our analysis of Lower and Middle Miocene lacustrine diatomaceous rocks in Japan confirms that species of nonmarine Actinocyclus were important constituents of lake phytoplankton there as well. The appearance of nonmarine Actinocyclus species near the beginning of the Miocene may have resulted from the introduction of euryhaline species into lacustrine environments during a highstand of sea level at that time. Similarly, it is possible that species of Stephanodiscaceae evolved from marine thalassiosiroid ancestors that invaded high latitude lacustrine environments during multiple Paleogene highstands, resulting in a polyphyletic origin of the family. The turnover from nonmarine Actinocyclus to Stephanodiscaceae genera near the middle/late Miocene boundary may be linked to a contemporaneous increase in silica concentrations in lakes caused by active volcanism, increased weathering of silicate rocks due to orogeny, and the expansion of C4 grasslands. This turnover may also have been influenced by enhanced seasonal environmental changes in the euphotic zone caused by the initiation of monsoon conditions and a worldwide increase in meridional temperature gradients during the late Miocene. Morphological characteristics of Stephanodiscaceae genera, such as strutted processes and small size, suggest their species were better adapted to seasonal environmental changes than nonmarine species of Actinocyclus because of their superiority in floating and drifting capabilities and possibly metabolism, intrinsic growth rate, and reproductivity. As climates deteriorated during the late Miocene, Stephanodiscaceae species may have spread from high latitudes to temperate lakes where they diversified, ultimately displacing Actinocyclus.
化石证据表明，现代温带淡水浮游硅藻组合大约在中新世中晚期开始出现，当时，中中新世至早中新世期间湖泊浮游硅藻组合的重要组成部分 Actinocyclus 属被 Stephanodiscaceae 属所取代。这种植物更替已在世界许多地区得到证实，除了东亚，那里关于早中新世和中中新世浮游硅藻组合的分类学数据直到最近才变得丰富。我们对日本下中新统和中中新统湖相硅质岩的分析证实，淡水 Actinocyclus 物种也是那里湖浮游植物的重要组成部分。中新世早期非海洋 Actinocyclus 物种的出现可能是由于当时海平面高水位期间，广盐性物种被引入湖泊环境所致。同样，Stephanodiscaceae 物种也可能是从海洋 thalassiosiroid 祖先演化而来，它们在多次古近纪高水位期间侵入高纬度湖泊环境，从而导致该科的多系起源。从中新世中晚期边界附近的非海洋 Actinocyclus 到 Stephanodiscaceae 属的更替可能与湖泊中硅浓度的同时增加有关，这种增加可能是由活火山作用引起的，也可能是由于造山作用导致硅酸盐岩石风化增强，以及 C4 草原的扩张。这种更替也可能受到季风条件开始和晚中新世全球子午线温度梯度增加引起的透光带季节性环境变化增强的影响。Stephanodiscaceae 属的形态特征，如结构突起和较小的体型，表明它们的物种比非海洋 Actinocyclus 物种更能适应季节性环境变化，因为它们在漂浮和漂流能力以及可能的新陈代谢、内在生长率和繁殖力方面具有优势。随着晚中新世气候恶化，Stephanodiscaceae 物种可能从高纬度扩散到温带湖泊，并在那里多样化，最终取代 Actinocyclus。
