Early Cretaceous Freshwater Fish Fauna in Kyushu, Japan

246 Yoshitaka Yabumoto
stratigraphic correlation of fish fossil localities in the Wakino Subgroup is clear, the
changes of the ancient lake, Kowakino-ko, which covered the area from the present
northern part of Kyushu to the western part of Yamaguchi Prefecture, can be
restored on the basis of the changes of fish faunas in the Wakino Subgroup.
In the period of the First Formation, the fauna of the Kowakino-ko Lake was
similar to faunas of the lakes of the southeastern China in some extent, because the
Nipponamia-Aokiichthys fauna slightly resembles the fauna of the early to middle stage
of the Early Cretaceous in the southeastern China. The fishes of this period of the
Kowakino-ko Lake became extinct at the end of the First Formation or the beginning
of the Second Formation, because no fish fossils have been found in the Second
Formation, and the fish fauna of the First Formation is entirely different from other
two faunas.
The Second Formation represents the age of volcanic activity (Y. Ota, 1960b).
The fishes of the Third Formation had developed without relation to the fishes of the
First Formation in a period of volcanic inactivity after the extinction of the fishes of
the First Formation.
Most fishes of the Paraleptolepis-Wakinoichthys fauna became extinct at the end of
the Third Formation. There is a possibility that the clupeid fishes of the Fourth
Formation came into the lake from the sea at the beginning of the period of the
Fourth Formation and take place of the most abundant fish, Paraleptolepis of the
Paraleptolepis-Wakinoichthys fauna. Wakinoichthys survived in the period of the Fourth
Formation. Ichthyodectiform fishes, Chuhsiungichthys, appear in all fish faunas of the
Wakino Subgroup. Most members of the Ichthyodectiformes are marine and some
fossils are found in brackish and freshwater deposits. Fishes of the genus Chuhsiung
ichthys of each formation are considered to be originated in marine or brackish water
ichthyodectiform fish.
Ostracoda and conchostracan fossils are yielded from the Third and Fourth
Formations with fish fossils, but they are not found from the First Formation at the
Tokuriki fish fossil site. Extant conchostracans live in shallow water. The exist
ence of conchostracan fossils indicates that the depth of water at fossil sites of the
Third and Fourth Formations are shallow, and the absence indicates that the fossil
site of the First Formation is comparatively deep. Many juvenile fish are yielded
from the site of the Third Formation with numerous ostracods. This indicates that
the depth of the water was shallower at the site of the Third Formation.
3. Record of freshwater fish fossils in Japan.
The Mesozoic freshwater fish fossils have been very meager in Japan with the
exception of the fish fossils of the Wakino Subgroup which are described in the
present study. The only record isa few fragments from the Lower Cretaceous Tetori
Group in Ishikawa Prefecture. These fragments consist ofone lower jaw and several
isolated scales belonging to the holostean fishes (Azuma and Hasegawa, 1989).