Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 7

The fish faunas of the Setouchi Miocene Series in the east Setouchi region, Southeast Japan are studied. Following results are obtained.

1. Two fish faunas are recognized in the first stage of deposition of the Setouchi Miocene Series. One is composed of fresh water fishes such as Families Cyprinidae, Siluridae, Bagridae and Percichthyidae and is found in the Hachiya and Nakamura formations of the Mizunami Group. The other is a marine fauna from basal parts of the Morozaki and Ichishi Groups

2. The Morozaki Group, which is composed of tuffaceous sandstone and mudstone, seems to be deposited in deep sea environments such as the continental shelf edge or the continental slope. Fossil fishes of the Morozaki Group are grouped into three assemblages as follows;

a. micronekton assemblage composed of small fishes which have luminous organs of Fami1ies Myctophidae and Gonostomatidae inhabiting in warm sea area where the Kuroshio current affected to

b. benthonic assemblage composed of moderate-sized fishes such as Families Scorpaenidae, Notacanthidae and Pleuronectidae inhabiting in cold sea water in the northern part of Japan

c. nekton assemblage composed of rapidly moving fishes such as Family Scombridae, for example, Scomber sp.

3. Fossil specimens of the micronekton assemblage of the Morozaki Group can be correlated with living fishes inhabiting in the Pacific coast of Japan under the Kuroshio current on the basis of their good condition of preservation. They might be adaptive to deep sea environment and have ecological niches as the micronekton.

4. Similar depositional conditions are assumed in the Ichishi and Shidara Groups. Fossils of Pterothrissus spp. of Family Albulidae, an index species of deep waters, are commonly found in the second stage of the sequence of the Ayugawa, Chikusa, Mizunami and Tomikusa Groups deposited in surrounding shallow sea basins.

5. In the sequence of the second stage of the Setouchi Miocene Series, the Mizunami Group which consists of tuffaceous clastic sediments bears neritic fish assemblages in shallow seas of temperate water. Scomberomorus sp., a large specimen over 3 meters in length, Acanthocybium sp. and Scomber spp. were found from the Yamanouchi Formation.

Many neritic species of Order Perciformes and oceanic species such as Albula sp. of Family Albulidae in tropical to subtropical waters are found in the Shukunohora bed, the uppermost part of the Akeyo Formation. ln a word, the fish assemblage of the Shukunohora bed is a mixed one of fishes of shallow and deep seas.

The Oidawara Formation, the third stage deposits, overiles the lower formations with unconformity. The fish assemblage of the formation is a mixed one composed of neritic to oceanic and deep sea species. The former consists of warm water inhabitants and the latter is micronektonic species which inhabit around the continental shelf such as Families Myctophidae and Macrouridae.

6. The deep water fish assemblage seen in the sequence of the eastern part of the Setouchi Miocene Series can not be found in the deposits to the west of the Ayugawa Group. The Bihoku and Katsuta Groups, the western part of the Setouchi Miocene Series exhibit the third stage of Series. They include fishes of the Family Gobiidae of shallow waters.

7. The fish assemblage of the Oidawara Formation of the Mizunami Group is found in the Kurosedani Formation of the Hokuriku Group. The constituents are same one another belonging to Families Scombridae, Sillaginidae, Myctophidae and Macrouridae.

8. The species of the Miocene fish assemblage in Setouchi Series are different with ones of the Pliocene fish assemblage in Shimajiri, Tonohama and Kakegawa Groups developed along the Pacific Ocean side of Japan. The difference is remarkable in shallow water fishes, e.g. Famliy Sciaenidae. On the other hand, deep water fishes have similar character except Pterothrissus spp. of Family Albulidae.

9. Similar phenomenon to the former is seen between the Miocene and Pliocene Series in Hokuriku district on the Japan Sea coast. The Miocene assemblage is similar to the assemblage of the Oidawara Fomation of the Mizunami Group but the Pliocene assemblage is mainly composed of species belonging to Families Cottidae and Scorpaenidae of Scorpaeniformes inhabiting in cold water of the Sea of Japan. It is remarkable in the Shigarami Formation of the Hokushin Group and the Otogawa Formation of the Hokuriku Group.

10. The Miocene otolith assemblage in Japan is correlative with the Eocene to Miocene assemblages in Europe and New Zealand. It is clear that assemblages in three regions were established in the beginning of the Early Miocene.