The occurrence of the fossil mytilid bivalve Mytilus tichanovitchi Makiyama from the Miocene Shimonita Formation, Gunma Prefecture, central Japan: Re-examination of its biostratigraphic and marine paleoclimatic significance

Abstract

The occurrence of the fossil mytilid bivalve Mytilus tichanovitchi Makiyama is reported from the Iwayama Sandstone Member of the Shimonita Formation in the Shimonita district, Annaka-Tomioka area, Gunma Prefecture, central Japan. The mode of fossil occurrence and its associated molluscs indicate that M. tichanovitchi lived in sandy bottom of the upper sublittoral zone. Compilation of the published bio-and chrono-stratigraphic data indicates that the stratigraphic range of this species in Japan is evidently lower than that of the warm-water Kadonosawa-type Fauna and is restricted to the Lower Miocene. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of M. tichanovitchi shell from the Shimonita Formation also suggest Early Miocene age. Faunal character of M. tichanovitchi-bearing molluscan assemblages in Japan indicates the mild- to cool-temperate marine condition. The occurrence of M. tichanovitchi in central Honshu may have been related to a global cooling event at approximately 18 Ma.