No. 1 ] Proc. Japan Acad., 64, Ser. B (1988) 1

1. The Cephalopods and from the Yokokurayama Formation, Shikoku, Japan*'

By Teiichi KOBAYASHI,M. J. A. (Communicated Jan. 12, 1988)

The Older Palaeozoic Formation of Mt. Yokokura in Kochi Prefecture, ShikokuIsland is very fossiliferousas a rich fauna has been described in 1974. This formationis separated from the Ochi Formationcontaining Upper Devonianplants by a strike fault on its south side. Someimportant facts were reported since then on the formation. Koizumi (1975) has reported an occurrence of Lailioceras sp. from the YokokurayamaFormation and Kato and Yasui (1975) discoveredgraptolites resemblingDictponema delicatulum (Llandovery-Ludlow) in the limestonemem- ber. Kuwano(1976, 1980, 1983) found variousconodonts from latest Llandoverian to early MiddleLudlovian in the so-calledKurosegawa tectonic belt where Mt. Yokokura is located. In finding reworked Ordovicianconodonts, he presumed their derivationfrom the underlyingportion of the Yokokurayamasequence now unexposed.According to Furutani (1983) Radiolariansof the Palaeoscenidiidae from the middlepart of the G4 :formationmay be early to middle Devonianin age. Finally Yoshikura (1982) reported that biotite in granitic boulders con- tained in the conglomeraticportion of the formation (G„5)measured 364-11 Ma accordingto Shibata et al. (1979). The age of the Yokokuratrilobites is however, not so young as late . The descriptive work of the Yokokurayamafauna was commencedwith Scutellum (subgen. nov.) japonicum Kobayashi et Hamada, 1965 for which PHbyl and Vanek (1960) proposed Japanoscutellum. The second is Cerauroides orientalis for the age of which Kobayashiand Hamada (1973) suggested early Ludolovian. Katto (1984) carried out a detailed geologicalsurvey of the Shiroishivalley where Gomi quarry is located. He divided the Yokokurayamafor- mationexposed along the valleyinto three units and Kobayashi(1984) described the cephalopodsfrom the upper unit as follows: Michelinocerasalticameratum Kobayashi Michelinocerasmizobuchii Kobayashi Arionocerasdensiseptum Kobayashi Leurocycloceras(?) sp. indet. Orthocyclocerasgomiense Kobayashi Orthocyclocerasaff, gomiense Kobayashi Protocycloceras(?) f essicancellatumKobayashi These cephalopods,particularly Arionoceras and Orthocyclocerasreveal that the trans-Eurasiatic faunal connectionmust have been maintained during the middleto late Silurian period from Bohemiaor Central to the Far East through Kazakhstanand Afghanistan. T *' Studiesof JapaneseTrilobites and AssociatedFossils-LIX. 2 T. KOBAYASHT [Vol. 64(B),

In a monograph on "Silurian Trilobites of Japan" in 1974 some 30 species of trilobites inclusive of exactly indeterminable forms were described. Among them Encrinurus nodai is a solitary species from the summit of the mountain. Most of them were derived from Gomi quarry. Four species of trilobites were obtained from Sugihara adjacently to the east of the quarry. They are pene- contemporaneous to the Gomi trilobites, because three of them occur also at Gomi.

Kosovopeltis angusticostata Kobayashi et Hamada, 1974 Mic'roscutelluin primigcniuni K. et H., 1974 Microscutellunz sp. nov. 1974 Japonoscutellum japonicum K. et H., 1965 J. japonicum puteatum K. et H., 1986 .1. japonicum laticephalum K, et H., 1986 Japonoscutellum geniculatum K. et H., 1985 Japonoscutellum tumidum K. et H., 1987 Opoa (?) trinodosa K. et H., 1986 T osacephalus f ungi f ormis K. et H., 1974 Illaenoscutellum platyceps K. et H., 1974 glomerosus K. et H., 1974 Bumastus agmakros K. et H., 1984 Bumastus kattoi K, et H., 1984 Bumastus sakoi K, et H., 1984 Bumastus aff, barriensis Murchison, 1939 Bumastus (Bumastus) sub quadratus K. et H., 1974 Bumastus (Bumastella) spiculus K. et H., 1974 Bumastus (Bumastella) bipunctatus K. et H., 1974 Bumastus (Bumastella) aspera K, et H., 1974 Illaenoides (?) magnisulcatus K. et H., 1986 Illaenuroides (?) abnormis K. et H., 1986 Proetus (Proetus) subovalis K. et H., 1974 Proetus (Gerastos) sugiharensis K. et H., 1974 Proetus (Gerastos) subearinatus K, et H., 1974 Proetus (Bohemiproetus) magnicerviculus K. et H., 1974 Cyphoproetus latiaxis K, et H., 1986 Latiproetus bilobus (K, et H., 1974) Gomites granulatus (K, et H., 1974) Pseudocheirurus gomiensis K. et H., 1985 Ceruroides orientalis K. et H., 1973 Ceruroides elongatus K. et H., 1983 Sphaerexochus hiratai K. et H., 1974 (1986) S. hiratai robustus K. et H., 1974 Sphaerexochus planirachis K. et H., 1974 Encrinurus yokokurensis K, et H., 1974 Encrinurus mamelon K, et H., 1974 Encrinurus nodai K. et H., 1974 Encrinurus tosensis K, et H., 1974 Encrinurus subtrigonalis K. et H., 1985 Encrinurus similis K, et H., 1985 Encrinurus stenorhachis K. et H., 1985 Dindymene (?) megacranidia K. et H., 1985 Dindymene (?) sp, indet. 1986 Ichiyamella subglobula K. et H., 1986 1 chiyamella megaeranidi.a K. et H., 1986 Staurocephalus trichoehin K, et H., 1985 No. 1] Silurian Yokokura Formation Japan 3

Phacops metacernaspis K. et H., 1974 Dicranopeltis tricornis K. et H., 1986 Apolichas truncatus K, et H., 1974 Apolichas perconvexus K, et H., 1985 Metaleiolichas tuberculatus K, et H., 1987 Paraleiolichas globurus K. et H., 1987

Thus the Yokokurayama fauna is such a rich one comprizing about 50 species of trilobites in 23 genera of seven families viz. the Scutelluidae, Illaenidae, Proetidae, Cheirurida,e, Encrinuridae, Phacopidae and the Lichidae. Among them Japanoscutellum, Tosacephalus, Illaenoscutellum, Ichiyamella, Apoli- chas, Metaleiolichas and Paraleiolichas are new genera, probably endemic to Japan. Of Bumastus s. str., Sphaerexochus, Encrinurus and Bumastus (Buma- sella) the generic range is known to be Silurian and older, while the range of Proetus (Gerastos, Bohemiproetus and Coniproetus) and Microscutellum is Silurian and younger. According to Pillet (1965) Proetus (Proetus) is restricted to the Silurian (Calcaire de Gotland). Phacops metacernaspis is an intermediate form between Llandoverian Acernaspis and late Llandoverian-late Ludlovian Ananaspis. For geochronology Cheiruroides orientalis is particularly important, because it is intermediate between C. hawlei in Bohemia and C. pro pincus of the Carnic Alps morphologically. In weighing these facts, the Gomi trilobites must be early Ludlovian, if not latest Wenrockian in age. Encrinurus nodai from the summit may be late Wenlockian, because the Yokokurayama formation is thought to be overturned. As mentioned already, it is probable that the Yokokurayama formation is underlain by the limestone, whereas its youngest part may be early or even middle Devonian. Because the formation consists of not only lime- stone, brecciated limestone and limestone conglomerate, but also siliceous slate, tuffaceous sandstone and other :rock intercalations and they are cut by faults, streneous work is required to clarify its geology. Nevertheless, it is known now that the Gomi fauna is extraordinarily rich one. Various trilobites and cephalo- pods were propagated in the Silurian reef sea.

References

Furutani, M. (1983) : Middle Palaeozoic Palaeoschenidiidae (Radiolaria) from Mt. Yokokura, Shikoku, Japan, Part 1. Trans. Proc. Palaeont. Soc. Japan, N.S., no. 131, pp. 96-116, pls. 21-27. Katto, J. (1984) : New observation of Silurian rocks in Shiroishi-ohsawa, Mt. Yokokura, Kochi Prefecture, Shikoku. Research Rep. Kochi Univ., 32, Nat. Sci., 236-239, pls. l-2. Kobayashi, T. (1983) : On the Silurian cephalopod faunule from Mt. Yokokura, Kochi Prefecture, Shikoku, Japan. Proc. Japan Acad., 59B, 293-296. (1985) : On two Silurian trilobite genera, Prantlia and Latiproetus. ibid., 61B, 419-421. Kobayashi, T., and Hamada, T. (1973) : Brief note on Silurian trilobites from Mt. Yokokura, Shikoku Island, ibid., 49, 540-544. (1974) : Silurian trilobites of Japan in comparison with Asian, Pacific and other Areas. Palaeont. Soc. Japan, Sp. Pap., no. 18, 155 pp., 12 pls. --- (1984) : Advance report on a new trilobite collection of the Silurian Yokokura- yama fauna, Shikoku Island, Japan. Research Rep. Kochi Univ., 32, Nat. Sc., 253-258, pl. 5. -- (1985) : On the Silurian trilobites and cephalopods of Mt. Yokokura, Shikoku, 4 T. KOSAYASHT [Vol. 64(B),

Japan. Proc. Japan Acad., 61B, 345-347. Kobayashi, T., and Hamada, T. (1985b) : Additional Silurian trilobites to the Yoko- kurayama fauna from Shikoku, Japan. Trans. Proc. Paeont. Soc. Japan, N.S., no. 139, pp. 206-217, pls. 2830. (1986) : The second addition to the Silurian trilobite fauna of Yokokura-yama, Shikoku, Japan. ibid., no. 143, pp. 447-462, pls. 90-92. (1987) : The third addition to the Silurian trilobite fauna of Yokokura-yama, Shikoku, Japan, ibid., no. 147, pp. 109-116. Kuwano, Y. (1980) : Silurian conodonts from Yokokura-yama, Shikoku, Japan. Second European Conodont Symposium (EGOS II) Guidebook Abstracts (ed. Schonlaub, H. P.). Abh. Geol. Bund.-Anst., Wien, 35, 201. -- (1983) : Reworked Ordovician conodonts from Yokokura-yama , Shikoku, Japan. Trans. Proc. Palaeont. Soc. Japan, N.S., no. 89, pp. 245-248.