Title Permian Bivalves of Japan Author(s) Nakazawa, Keiji; Newell, D. Norman Memoirs of the Faculty of Science, Kyoto University. Series of Citation geology and mineralogy (1968), 35(1): 1-108 Issue Date 1968-10-11 URL http://hdl.handle.net/2433/186552 Right Type Departmental Bulletin Paper Textversion publisher Kyoto University MEMOIRs oF THE FAcuLTy oF SclENcE, KyOTo UNIvERslTY, SERIEs oF GEoL. MINERAL.& Vol. XXXV, No. 1, pp. 1-108, Pls. 1-11 Permian Bivalves of Japan By Keiji NAKAzAwA and Norman D. NEwELL* (Received Aprii 28, 1968) Abstract Permian bivalve molluscs ofJapan shed interesting new light on the paleontological com- position 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 ofthe world by a hiatus. However, there is a noteworthy break, even inJapan, 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 theJapanese Perrnian. Among these, 58 species, ofwhich 18 are new, are described in this paper. TowaPteria, Ensi teria, Tambanelta, HayasakaPecten, and Gnjocardita 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 con- tains a mixed fauna of Permian and Triassic aspect. Some Mesozoic types, such as Neoschizodus, Costatoria and WaagemoPerna, 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 Perrnian 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 sedirnentary regions or facies, eugeosynclinal, miogeosynclinal and carbonate shelffacies. Eugeosynclinal and carbonate shelffacies are chara- cterized by fusuline fossils, whereas the miogeosynclinal facies contains various kinds ofmolluscs 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 geosynclina] 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 ofthe Permian is herein adopted. These divisions are the Sakamoto- sawan (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 * TheAmerican Muscumof Natural History, New York, U.S.A. 2 Keiji NAKAzAwA and Norman D. NEwELL Misellina claudiae zone), the Parafusttlina kaerimizensis zone (or IVeoschwagerina simPlex zone) to the Yabeina globosa zone (or Yabeina shiraiwensis-Lepidolina tori amai zone), and a post-Yabeina zone, respectively. Bivalve molluscs from all these divisions are considered. Introduction An important biological revolution took place at the close of the Paleozoic era. The span of time involved in the replacement of Permian by Triassic biotas is as yet unknown, but clearly it was geologically short. During late Permian time, approximately fifty percent of all animal families known to be then living became extinct (NEwELL, 1967 b). The resulting strong contrast between Permian and Triassic fossil faunas is very great and this contrast doubtless is accentuated wherever a hiatus separates the two systems (NEwELL, 1967a). Some of the Permian sequences of Asia are of exceptiona! interest in this con- nection because they have been well subdivided by rapidly evolving fusuline faunas and are thought by many to range higher than the most complete sequences of America and Europe. Much semantic confusion results from the partition of local sequences into two or three major divisions as Lower, Middle and Upper Permian, because these are employed with disparate biostratigraphic and time values in different parts of the world. In this report, we shall employ a biostratigraphic framework based on fusuline zones. Most families of bivalves have been conservative in their evolution throughout their history. For example, many Permian genera originated during the early Carboniferous, enduring with only insignificant changes more than 1OO million years. Yet, only a few of these survived into the earliest Triassic system, the rocks of which are notable for their curious depauperate but cosmopolitan benthonic faunas and their ceratite swarms. In Japan, a number of bivalve genera which elsewhere characterize the Tri- assic appear high in the Permian in association with brachiopods and fusulines of Permian aMnities. Especially the Gujo Formation of Kyoto Prefecture, younger than the Yabeina-LePidolina zone, is exceptional for the large number of Triassic elements among what is otherwise an assemblage of Permian brachiopods, bryozoans and bivalves. Stratigraphic position and faunal content suggest to us a geologic age quite high in the Permian, perhaps as high as any Permian faunas thus far known. The Japanese Permian lies within the northeastern extension of the Tethyan province. Our discovery here of several bivalve genera characteristic of the Permian cold-water Gondwana province of eastern Australia is unexpected and suggests deep-water depositiQn Qr Pcrmian climatic QscillatiQns in Japan, Permian Bivalves ofJapan 3 Previous Studies of Permian Bivalves in Japan The study of Permian bivalves in Japan has been neglected as compared with gther invertebrate fossils such as fusulines, corals, and brachiopods. The chief reasons for this neglect ar.e the scarcity of fossils, poor state of preservation, and consequently their diminished biostratigraphical utility. However, as a matter of .fact, Permian bivalves are not uncommon, although their occurrence is almost limited' in distribution to mudstones and sandstones. Pioneer studies were made by HAyAsAKA (1923, l924, 1925) mainly on 'collections from the southern Kitakami massif, northeast Japan, where the clastic rocks of shallow shelf seas are widely distributed. In later years fossil pelecypods have been neglected. In 1951, IcH!KAwA described a rather unusual community of fossil inverte- brates from the high Permian of Shikoku Island, southwest Japan, consisting of bivalves, gastropods and scaphopods. Other major groups, such as brachiopods, corals, and fusulines, that flourished elsewhere inJapan during the Permian period, were lacking in his collections. Later NAKAzAwA (1959, 1960) noted the oc- currence of Permian myophoriids and bakevelliids in the Maizuru Belt, southweSt Japan. More recently, MuRATA (1964) described Aviculopectinidae from the southern Kitakami massif, and HAyAsAKA (1967a, b) added several other bivalve specles. Those of the Takakurayama Group in the Abukuma massif were briefly des- cribed by KoBiyAMA (l961), and recently by YANAGisAwA (1967) together with many other invertebrate fossils. A considerable number of pelecypods have been listed from the southern Kitakami massif but none of these, excepting those published by HAyAsAKA and MuRATA, was described. Consequently, much work remains to be done on the bivalve faunas of the Permian ofJapan. Permian System in Japan Sedimentary Provinces Three major sedimentary provinces can be distinguished in the Permian sedi- ments in Japan. They are eugeosynclinal, miogeosynclinal and carbonate shelf facies (Figure 1). The first facies is represented by thick shales, cherts, and basic volcanic tuffs. Sandstones are also important. Limestones usually are intercalated as relatively thin lenticular masses but in some places are as thick as several hundred meters. The total thickness is not less than ten thousand meters. Probably most ofJapan was covered by eugeosynclinal deposits of Permian age. Fossils are extremely rare 4 Keiji NAKAzAwA and Norman D. NEwELL in these deposits excepting fusulinids in sparse limestones. The miogeosynclinal sediments* are composed of terrigenous and carbonate rocks: shales, sandstones and limestones, with subordinate conglomerates. Cherts and basic tuffs are absent or subordinate. Molluscs and brachipods are mainly Iimited to the clastic rocks ofthis facies, fusulines and corals to the carbonate rocks. The thickness attains several thousand meters. The Permian of southern Kita- kami massif, Abukuma massifand Hida massif provide examples of miogeosynclinal deposits. The carbonate shelf facies is made up almost exclusively of limestone units up to several hundred meters thick. These rocks are found in southwestJapan in Omi, Akasaka, Ibuki, Atetsu, Taishaku and Akiyoshi. The limestone masses were once considered to be remnants of a large thrust sheet that moved from the north (KoBAyAsHi, 1940) but recent studies indicate that they are more or less