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纪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的胜浦地区、和歌山县的汤浅地区和熊本县的田原海岸。在这些地区，白垩纪沉积序列产生了微咸水双壳类，由潮汐水道成因的厚砂岩组成，上面覆盖着偶尔显示波浪状和波浪状层理指示海流的细小沉积物。根据潮汐河道形成的旋回性重建了微咸水双壳类的栖息地，并对Katsuura地区Tatsugatani剖面进行了详细的古生态分析。本研究首次将现场保存的微咸水双壳贝类鉴定为等藻类、放射状Costcyrena属、三楚类、Pulsidis sp.aff.nagatoensis和Pharella sp.。五个群丛，包括Crassostrea，Ostrea-Pulsidis，Ost…根据组成种和原位保存种的相对丰度，确定了更多的Rea Pyrgurifera、Isodomella-Pharella和Costcyrena-Ostrea群丛。最常见的是Costcyrena-Ostrea群丛，主要由非动物群的双壳类、放射状Costcyrena Radiatstriata和上纲动物群Ostrea sp.组成。底栖双壳类个体比例最高，生境最不稳定，具有频繁侵蚀和沉积的特点。相比之下，牡蛎群丛多为单一种群丛，由具细长扁平壳层的近缘牡蛎组成。该群落的生境被解释为盐度相对较高的稳定底质，可能是中盐度和短盐度。以前，微咸水底栖动物群落的盐度状况被理解为低盐度、中盐度和短盐度。然而，我们的研究表明，底物不稳定性等物理因素，如侵蚀和沉积，是一个严重的限制因素，与海洋底栖生物群落的情况相似。较少

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