Evolutionary paleoecology of the Cretaceous brackish-water bivalves of southwest Japan, and the origin of the Modern Evolutionary Fauna

研究了日本西南部白垩系微咸水化石组合的组成、地层和沉积相。研究范围包括德岛县胜浦地区、和歌山县汤浅地区和熊本县田原海岸。在这些地区，白垩纪沉积序列产生半咸水双壳类，由潮汐水道形成的厚砂岩组成，上面覆盖着较细的沉积物，偶尔显示波浪状和闪光状的层理，表明有水流。根据潮汐通道形成的旋回性，重建了咸淡水双壳类的生境。对胜浦地区Tatsugatani剖面进行了详细的古生态分析。本研究首次在原位保存的咸水双壳类中鉴定出Isodomella shiroiensis、Costocyrena radiatostriata、Tetoria sanchuensis、Pulsidis sp. affer .nagatoensis和Pharella sp.五个类群，包括Crassostrea、Ostrea-Pulsidis、Ost . More rea Pyrgurifera、Isodomella-Pharella和Costocyrena- ostrea类群。从组成种和原位保存种的相对丰度上看，最常见的是Costocyrena-Ostrea Association，主要由水栖双壳类、辐射型Costocyrena radiatostriata以及水栖表面Ostrea sp组成。水栖双壳类的个体比例最高，栖息地最不稳定，具有频繁的侵蚀和沉积特征。相比之下，长牡蛎群大多是单特异性组合，由长而扁平的壳上长牡蛎组成。这种结合体的栖息地被解释为具有相对高盐度的稳定底物，可能是中盐和短盐。以前，从低盐、中盐和短盐的盐度制度来理解苦咸水底栖动物的关系。然而，我们的研究表明，侵蚀和沉积等物理因素是一个严重的限制因素，与海洋底栖生物群落相似。少

Composition of brackish-water fossil assemblages and stratigraphy and sedimentary facies of the Cretaceous in southwest Japan were investigated. Study are includes katsuura area in Tokushima Prefecture, Yuasa area in Wakayama Prefecture and Tanoura coast in Kumamoto Prefecture. In these areas, the Cretaceous sedimentary successions yielding brackish-water bivalves, consist of thick sandstone of tidal channel origin overlain by finer sediments occasionally displaying wavy- and flaser bedding indicationg tial current. Habitats of brackish-water bivalves were reconstructed in relation to the cyclicity originated from tidal channel formation.Detailed paleoecological analysis was conducted in Tatsugatani section in Katsuura area. In-situ preserved brackish-water bivalves were identified firstly in this study for Isodomella shiroiensis, Costocyrena radiatostriata, Tetoria sanchuensis, Pulsidis sp.aff.nagatoensis and Pharella sp. Five associations, comprising Crassostrea, Ostrea-Pulsidis, Ost … More rea Pyrgurifera, Isodomella-Pharella and Costocyrena-Ostrea Associations, were identified, on the basis of the relative abundances of the constituent species and in-situ preserved species The most common are Costocyrena-Ostrea Association, consisting mainly of infaunal bivalve, Costocyrena radiatostriata, along with epifaunal Ostrea sp. The ratio of individuals of infaunal bivalves is highest, and the habitat is interpreted as most unstable, being characterized by frequent erosion and deposition. In contrast, Crassostrea Associations are mostly monospecific associations, consisting of epifaunal Crassostrea sp.having elongated and flat shell. The habitat of this association is interpreted as stable substrate with relatively high salinity, probably of meso- and brachyhaline.Formerly, brackish-water benthic associations have been understood in terms of salinity regime, from oligohaline, mesohaline and brachyhaline. However, our study indicates that physical factors such as substrate unstability, such as erosion and deposition acted as a serious limiting factor, being similar to those in marine benthic associations. Less