中・古生界境界問題について

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The Permian-Triassic boundary problems are grouped into two, the one concerning the boundary itself, and the other one related to the so-called mass extinction at or near the erathem boundary. As to the former, bio-or chronostratigraphic classification and correlation are discussed, and in the latter one, case histories of bivalves in the world during the Late Permian and Early Triassic times and also of faunal change in Japan during the Late Permian are examined. There are several schemes of classification and ammonite zonation of the Early Triassic, such as those proposed by SPATH (1934), KUMMEL (1957), KIPARISOVA and POPOV (1956, 1964), and TOZER (1956, 1964). TOZERs classification and ammonite zones are based on the data derived from the Arctic province, while the other ones are made by compilation of date from various places. It is necessary for settling the international standard to establish the ammonite zones in the Tethys, especially in Kashmir and Himalayas, and to correlate them with those of the Arctic. The division and correlation of the Upper Permian is still more in dispute because of the provinciality due to the worldwide regression. From the recent investigations in Iran, India and others by the writer and his collaborators, the Upper Permian is tentatively divided into two series, the Guadalupian and the Dzhulfian (s.1.) in this paper, and the latter is further subdivided into three stages, that is, the Abadehian, the Dzhulfian (s.s.), and the Dorashamian in ascending order, although the characteristics of the Abadehian fauna and the relation to the Amarassian are to be further examined. The correlation is shown in Table 7. The base of the Triassic is referred here to the base of Otoceras woodwardi zone in the Himalayas. A conventional plot of the number of bivalves of various taxonomic levels at different times shows that the marked change took place mainly at genus and species levels, and that the extinction happened not instantaneously but went on through the Late Permian and the great diversity did not occur until the Middle Triassic or later. The compositional change of faunal assemblages during the Late Permian in Japan and Malay Peninsula as shown in P-G-O diagrams (Figs. 10 A and B), suggests that the change may be explained by an increasing degree of embayment condition, which is harmonious with the paleogeographic change which is presumed from geological observations. Many opinions have been presented on the cause of mass extinction, but it is difficult to explain by a simple cause. Thinking the accessible data stated above collectively, the mass extinction at the close of the Paleozoic is considered to be most related to the aggravation of environments for organisms, such as the worldwide regression, especially of shallow epicontinental sea, predominance of dry climate, presumable change of salinity or other chemical components of the ocean. All these must have resulted in excessive competition of organisms and change of ecosystem, which further caused the extinction of many organisms.
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中・古生界境界問題について

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