Abstract
Molluscan fossils from the Omma Formation in Oyabe City, Toyama Prefecture, central Honshu, were examined for investigating influence on the fauna by the environment change throughout the early Pleistocene. The age of the Omma Formation herein can be assigned to 2.02 Ma to 0.99 Ma, based on the calcareous nannofossils. From this formation, 169 species of mollusks have been recovered from 12 localities. The lower part and the lower upper part of the Omma Formation were deposited in lower sublittoral zone whereas the middle to upper horizons of upper part of the Omma Formation was mostly deposited in the upper sublittoral zone. As a result of examination of the succession of Omma-Manganji fauna, Mizuhopecten yessoensis, Lirabuccinum japonicum, Tachyrhynchus asatoi first appeared by the cooling event at about 1.2 Ma. In contrast, Pecten albicans added to the fauna by the warming event at about 1.0 Ma. It has cleared that the diversity of Omma-Manganji fauna increased by these events.
