No. 3] Proc. Japan Acad., 53 (1977) 147

40. Outline of in Japan

By Teiichi KOBAYASHI,M. J. A., and Takashi HAMADA University of Tokyo (Communicated March 12, 1977)

In the advance paper in 1974 the authors have distinguished 12 of trilobites in the Fukuj i collection. Subsequently the collection from the Fukuj i and Oise areas, both in the Hida plateau, Central Japan was greatly amplified. As the result it was found that the Devonian trilobites in the Fukuji Series total 24 species and two subspecies, in 12 genera distributed in 7 families, viz. the Scuttelluidae, Odontopleuridae, Lichidae, , , Proetidae and Otarionidae, as shown in the table inserted on page 149. It is provided that the reference of two Hitoegane trilobites to this fauna is still provisional, because a rich collection of Ludlovian trilobites was recently obtained from the Hitoegane area. Various opinions have been expressed on the age of the Fukuj i fauna from to Middle Devonian, but now it is warranted by the trilobites, particularly by the following facts, that the age is in a range from Gedinnian to Eif elian, or probably early Eif elian. and Crotalocephalina (Crotalocephalina) died out in early Middle Devonian and so did Proetus (Coniproetus) by the end of the early Devonian Period. Crotalocephalina (Crotalo~ cephalides) is restricted to occur in the Praguian so far. It is known that Crotalocephalina (Geracephalina) appeared already at the be- ginning of the period as represented by Crotalocephalus expanses from the Kochbaital horizon in Kazakhstan which is referable to this subgenus. The presence of the lowest Gedinnian sediments in the Fukuj i Series is further confirmed with conodonts by Igo, Koike and Igo, 1975. Besides Unguliproetus and Crotalocephalina s. str. vindi- cate the post-Silurian age of the fauna. There would be two or more horizons in the Fukuj i Series, but it requires more field work to allocate them exactly in its sequence, because the geological structure of the series is strongly complicated. Among the Nakazato from the Of unato area, Kitakami mountains, Northeast Japan are distinguishable 4 or 5 species of trilobites in 4 genera in the Scutelluidae, Phacopidae and Dechenel-

Studies on Japanese Trilobites and Associated Fossils, IV. This is an out- come of studies done with a subsidy from the Academy. 148 T. KoBAYASHIand T. HAMADA [Vol. 53, lidae. The senior author has already pointed out in 1957 that the Scutellum (Thysanopeltella) paucispinosa and Dechenella (Deche- nella) minima are Eifelian and Givetian respectively. According to modern the former belongs to Eifelian subgenus, T hysano- peltella (Septimopeltis), but it is more specialized than the Bohemian type-species in its pentagonal outline of the which sug- gests its being a late Eifelian form. Because the paucispinosa horizon lies about 100 m below the minima horizon, the latter's Give- tian age is fairly certain. No trilobite has so far been described from the Omori Series conformably overlying the Nakazato Series, although a few indeter- minable phacopids were found in it. Palaeophillipsia japonica from Choanj i, Sakari district is now known to be Lower , instead of Upper Devonian. The age of the Devonian trilobites in Japan is, therefore, summarized as below. Omori phacopids...... late Devonian Deehenella minima ...... Givetian T. (S.) paucispinosa ..... Eifelian or late Eifelian Fukuj i trilobites ...... Gedinnian to or early Eifelian

Fig. 1. Trilobite localities in the Japanese Devonian (T). U indicates the Upper Devonian localities without trilobites.

The Tobigamori formation of the Nagasaka district, Kitakami mountains, the Nakakura formation in the Abukuma mountains and the Ochi formation in Shikoku Island, Southwest Japan yield Leptophloeum, Cyrtospiri f er and other Upper Devonian fossils, but no trilobite as yet. Thus the Trilobita were greatly flourished in Japan in the early Devonian, but then strongly declined in the mid- dle Devonian till at length the Trilobita were almost nullified in the late Devonian Period. All of the Japanese Devonian trilobites No. 3] Devonian Trilobites in Japan 149

Table I. List of the Devonian trilobites in Japan 150 T. KOBAYASHI and T. HAMADA [Vol. 53, hitherto recorded are described in detail in the authors' recent mono- graph, 1977.

References

Igo, H., Koike, T., and Igo, H. (1975) : On the base of the Devonian system in Japan. Proc. Japan Acad., 5.1, no. 8, p. 653-658. Kobayashi, T. (1975) : Notes on two Devonian trilobites from the Kitakami mountains in Japan. Jour. Fac. Sci., Univ. Tokyo, Sec. 2, 11, pt. 2, p.1-11, p1.1. Kobayashi, T., and Hamada, T. (1974) : On the geological age of the Fukuji formation in the Hida plateau. Proc. Japan Acad., 50, no. 9, p. 760-763. --- (1977) : Devonian trilobites of Japan in comparison with Asian, Pacific and other faunas. Palaeont. Soc. Japan, Sp. Pap., no. 20, 202 pp, 13 pls.