981. DIVERSITY OF CHLAMYS (BIVALVIA) FROM THE PLIOCENE OGIKUBO FORMATION, CENTRAL JAPAN

Abstract

Many specimens of Chlamys were obtained from the Early Pliocene Ogikubo Formation in northern Nagano Prefecture, central Honshu. These are identified with the following six species : Chlamys (Chlamys) cosibensis (Yokoyama), C. (C.) ingeniosa tanakai Akiyama, C. (C.) foeda (Yokoyama), C. (C.) insolita (Yokoyama), C. ("C.") lioica shigaramiensis Amano et Karasawa and C. (Leochlamys) tanassevitschi (Khomenko). The last species has hitherto also been known as Chlamys daishakaensis Masuda et Sawada or C. iwakiana (Yokoyama). The diversity of this chlamid fauna is attributed to the distributional pattern and geologic ranges of the species. Chlamys lioica shigaramiensis and C. tanassevitschi are confined to the northwest flank of the Orihashi Syncline, which was deeper and colder than the southeast flank area where C. foeda occurred. Chlamys ingeniosa tanakai, C. insolita and C. lioica shigaramiensis are Miocene relict forms. They survived in the studied area to the Middle Pliocene, owing to the cold-water embayment conditions. The mixture of Miocene and Pliocene type species accounts for the diversity of Chlamys from the Ogikubo Formation.