Pleistocene Shallow-Water Whale-Fall Community from the Omma Formation in Central Japan

Abstract. We report a single whale bone associated with many molluscan fossils from the Omma Formation, Lower Pleistocene shallow marine deposits, along the Sai-gawa River, Kanazawa City, in central Japan. Most molluscan species which are commonly found in the Omma Formation show disarticulated and/or damaged shells, indicating semi-autochthonous or allochthonous modes of occurrence. However, the assemblage contained chemosynthetic bivalves, such as lucinid, solemyid and thyasirid bivalves, which are rare in the Omma Formation. The lucinids and solemyids show a high articulation ratio, along with some predatory and scavenging gastropods, such as naticids, nassariids and borsoniids whose well-preserved shells indicate an autochthonous mode of occurrence. In addition, most of the lucinid bivalves show an umbo-upward position similar to the life position of Recent species. Recent lucinid, solemyid and most thyasirid bivalves harbor chemosymbiotic bacteria in their gills and are well known members of the chemosynthetic community. These lines of evidence indicate that the community, mainly comprising lucinid bivalves and other autochthonous molluscan species associated with the whale bone, is an ancient whale-fall community. This shallowest fossil whale-fall community differs from deep-water cases in the dominance of infaunal bivalves, such as lucinids, and in the lack of epifaunal and semi-infaunal chemosynthetic bivalves, such as bathymodiolins and vesicomyids. This community supports a previous suggestion that the difference in characteristic species of the whale-fall communities depends on the water depth.