九州の白亜紀前期淡水魚類相

抄録

The Wakino Subgroup in the Kwanmon Group at Kitakyushu City in the northern part of Kyushu, Japan, has a rich yield of freshwater fish fossils. Twenty-one species, 8 genera and 6 families are described in the present study, among them, 15 species are described as new, plus 4 new genera Nipponamia, Aokiichthys, Wakinoichthys and Paraleptolepis; and 2 new families, Chuhsiungichthiidae and Wakinoichthiidae, are established. Three different paleoichthyofaunas are recognized in the Wakino Subgroup. The Nipponamia-Aokiichthys, the Paraleptolepis-Wakinoichthys and Diplomystus-Wakinoichthys faunas are proposed to designate them. The name Wakino fish fauna is proposed for the entire ichthyofauna of the Wakino Subgroup. It is composed of the primitive halecostome orders Semionotiformes and Amiiformes, and the primitive teleostean orders lchthyodectiformes, Osteoglossiformes and Clupeiformes. In numbers of species and individuals, osteoglossiform and clupeiform fishes are most abundant. This fish fauna is the only freshwater one known from the Mesozoic of Japan.

The Nipponamia-Aokiichthys fauna is found in the First Formation (W₁). This fauna consists of Lepidotes macropterus sp. nov. of the order Semionotiformes, Nipponamia satoi gen. et sp. nov. of the Amiiformes, Chuhsiungichthys yanagidai sp. nov. of the lchthyodectiformes, and Aokiichthys toriyamai gen. et sp. nov., A. changae gen. et sp. nov., A. otai gen. et sp. nov. and A. uyenoi gen. et sp. nov., A. praedorsalis gen. et sp. nov., and A. sp. of the Osteoglossiformes. Among these, the order Osteoglossiformes are the most abundant in numbers of species and individuals. The Paraleptolepis-Wakinoichthys fauna is found in the Third Formation (W₃). This fauna is composed of Paraleptolepis kikuchii gen. et sp. nov. and P. elegans gen. et sp. nov. of the order and family incertae sedis, Chuhsiungichthys sp. of the lchthyodectiformes, and Wakinoichthys aokii gen. et sp. nov. and W. robustus gen. et sp. nov. of the Osteoglossiformes. Fishes of the genera Paraleptolepis and Wakinoichthys are the most abundant. The Diplomystus-Wakinoichthys fauna is found in the Fourth Formation (W₄), and composed of Chuhsiungichthys japonicus sp. nov. of the order lchthyodectiformes, Yungkangichthys macrodon sp. nov. and Wakinoichthys aokii gen. et sp. nov. of the Osteoglossiformes, and Diplomystus primotinus, D. kokuraensis, D. altisomus sp. nov. and D. sp. of the Clupeiformes, with the genus Diplomystus being most abundant. No fish fossils have been found in the Second Formation (W₂).

Among the Wakino fish faunas, the Nipponamia-Aokiichthys fauna is closest to the Mesoclupea assemblage from southeastern China. Four families: Semionotidae, Amiidae, Chuhsiungichthiidae and Lycopteridae; and one genus, Lepidotes, are common in both faunas. The Paraleptolepis-Wakinoichthys and Diplomystus-Wakinoichthys faunas are considered to be endemic. In comparison with the Cretaceous freshwater fish faunas of the world, the Wakino fish fauna is characterized by the abundance of the Osteoglossiformes and Clupeiformes.

It is considered that the Nipponamia-Aokiichthys fauna became extinct by the end of the First Formation (W₁) or the beginning of the Second Formation (W₂). On the basis of this study, it is hypothesized that the clupeid fishes of the Fourth Formation entered the Kowakino-ko Lake from sea at the beginning of this period, and took its place as the most abundant fish, Paraleptolepis, of the Paraleptolepis-Wakinoichthys fauna.