Permian Bivalves of Japan

Abstract

Permian bivalve molluscs of Japan shed interesting new light on the paleontological composition of the youngest, post-fusulinid, marine faunas of the highest Paleozoic, and the bivalves tend to bridge the gap between the uppermost Permian and the lowest Triassic occupied in most regions of the world by a hiatus. However, there is a noteworthy break, even in Japan, between the Permian and Triassic systems where there are no bivalve species in common between the Permian and Triassic. Ninety-six species belonging to 46 genera of bivalves are distinguished in the Japanese Permian. Among these, 58 species, of which 18 are new, are described in this paper. Towapteria, Ensipteria, Tambanella, Hayasakapecten, and Gujocardita are proposed herein as new genera, and Permoperna is presented as a new subgenus of Waagenoperna. It is worthy of note that the bivalves did not decrease in number and diversity during the late Permian, in contrast to some other invertebrates, and that the late Permian Gujo Formation contains a mixed fauna of Permian and Triassic aspect. Some Mesozoic types, such as Neoschizodus, Costatoria and Waagenoperna, appeared in the middle Permian Kanokura Series. The early Triassic fauna, comprising 45 species in 17 genera, is rich in cosmopolitan species, and not so differentiated as are the Permian forms. A strong diversification of the bivalves did not take place until late Triassic time. Clearly, a marked change occurred among the bivalves at the close of the Permian. World changes in paleogeography and consequent climatic effects are considered to be a probable cause of the Permian extinctions, but these extinctions affected the bivalves less than most other Permian invertebrates, such as the brachiopods and cephalopods. The Japanese Permian is divisible into three sedimentary regions or facies, eugeosynclinal, miogeosynclinal and carbonate shelf facies. Eugeosynclinal and carbonate shelf facies are characterized by fusuline fossils, whereas the miogeosynclinal facies contains various kinds of molluscs and brachiopods. In many places, the late Permian rocks are represented by thick shales and sandstones and frequent conglomerate beds. These sediments are considered to have been accumulated in a relict, inland sea during the final stage of geosynclinal development. Fusulinid zonation is well established in the carbonate shelf and eugeosynclinal terrains. The bivalves are mostly confined to the miogeosynclinal facies of the southern Kitakami massif where a three-fold division of the Permian is herein adopted. These divisions are the Sakamotosawan (P 1), the Kanokuran (P 2), and the Toyoman Series (P 3), in ascending order. They roughly correspond to the Pseudoschwagerina morikawai zone to the Pseudofusulina ambigua zone (or Misellina claudiae zone), the Parafusulina kaerimizensis zone (or Neoschwagerina simplex zone) to the Yabeina globosa zone (or Yabeina shiraiwensis-Lepidolina toriyamai zone), and a post-Yabeina zone, respectively. Bivalve molluscs from all these divisions are considered.