Zoo and phyto biostratigraphy of the Tetori Group and evolutionary significance of terrestrial paleoecosystem

Abstract

The Tetori Group is one of the more important Mesozoic terrestrial deposits in East Asia, with taxa common throughout the area. Biostratigraphic resolution of these non-marine species as chronological indices is a moot problem which, to be answered adequately, requires thorough paleoecological and paleoenvironmental analysis. Hitherto, regional formation names of the Tetori Group were given separately for sequences in the Shokawa, Oshirakawa, Shiramine and Takinamigawa areas of the greater Mount Hakusan region which can be considered the main "type" section for the group. Based on geological correlation between these separate areas, the group was shown to consist of a coherent stratigraphic sequence thoughout the whole region and the geological map was revised to reveal the correct distribution of these formations, thus, we can discuss and evaluate zoo- and phyto- fossil assemblages as chronological and ecological indices and as geologically meaningful framework. Terrestrial vertebrates and freshwater bivalves occur mainly in the Kuwajima and Okurodani formations, deposited in flood plain and lake environments. The rich terrestrial plant fossils are also obtained from the Kuwajima and Okurodani formations. The very limited occurrence of these fossils in others fluvial formations, suggests that these appearances are controlled by environmental factors. There are two types of zoo-assemblages in the Kuwajima and its coeval formations; vertebrate species whose ancestors flourished in the Jurassic and their descendants in the late Cretaceous periods. As the latter is more superior in numbers than the former, changeable environments at the continental margin probably stimulated adaptation of the later species.