Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 7

Recent decapod crustacea is one of common macrobenthos and includes a great number of taxa. Stuidies of fossil decapods in Japan, however, are not so common because of meagerness of material. I have studied decapod fossils from the Miocene formations in Southwest Japan on the basis of newly obtained specimens.

I collected decapod fosslis from 19 formations at 30 localities in Southwest Japan. 64 species of 32 genera among 18 families are identified in these specimens. 16 assemblages are recognized and each of them is characterized by the association of several dominant species. Paleoecology of 16assemblages is inferred, referring to ecological data of recent decapods (Table 2).

Association of species in each assemblage of lower sublittoral to upper bathyal zones is unchanged during Early to Middle Miocene. On the other hand, component species in each assemblage of intertidal to upper sublittoral zone, especially, of intertidal muddy bottom, change between Early Miocene and Middle Miocene. The assemblages of intertidal muddy bottom in the Middle Miocene contain Thalassina anomala which is the inhabitant in the mangrove swamp and reflects the tropical paleoenvironment in Southwest Japan.

The component species in the four assemblages which reflect the mangrove swampy environments seem to indicate different biotopes each other, judging from their respective modes of occurrence and ecology of correlative living fauna. Therefore, it seems that vertical distribution of each assemblage has been different in the intertidal zone respectively.