白亜紀海生二枚貝フォーナと層序  [in Japanese] The bivalve faunas and their biostratigraphy of the Cretaceous in Japan  [in Japanese]

In the Cretaceous deposit of Japan, many characteristic bivalve faunas which are illustrated in Text-fig. 1, have been recognized from the localities of various depositional environments, paleogeographic settings, and of different geologicages. The biostratigraphic ranges of each bivalve species are statisfactorily determined as shown in Text-figs. 2-4, based on the occurrences of the bivalve species from variable Cretaceous stratain Japan as shown in Text-figs. 5-7. An important faunal change is noted between the faunas of Cenomanian and Turonian. Many bivalves occurring from the Hauterivian to Cenomanianin Japan, are usually common with those of the Cretaceous "Tethyan" fauna in their generic composition, e.g., Gervillia, Gervillaria, Neithea, Anthonya, Pterotrigonia (s.s.), Hanaia etc. On the other hand, the bivalves from the post-Turonian of Japan closely resemble with those of the Cretaceous "Circum-Pacific" fauna in the same composition, e.g., Yaadia, Apiotrigonia, Hetrotrigonia, Sphenoceramus, Glycymerita, Agnomyax etc.. This remarkable faunal change coincides probably with the initiation of so-called "Urakawan Transgression", resulting in the change of paleo-biogeography from the Tethyan bivalve fauna to Circum-Pacific bivalve fauna in the Turonian of Japan. The Aptian fauna is also remarkable in the first apperrance of bivalve genera, e.g., Anthonya, Nemodon, Pectinucula, Glycymeris, Goshoraia, Nippononectes etc., and in the degression of several bivalve genera, e.g., Cosmetodon, Amygdalium, Ptychomya etc., which are well developed in the Barremian faunas in Japan. The Cenomanian and Campauio-Maastrichtian bivalve faunas in Japan are very characteristic in the occurrences of variable species in the same lineage genera or subgenera, e.g., the species of Ptilotrigonia, ?Scabrotrigonia, Anthonya etc., in the former, and those of Cataceramus, Nanonavis, Microtrigonia, Yaadia etc., in the latter. It is also noted that the Cenomanian and Campauio-Maastrichtian faunas are usually found in the coasening-upward sedimentologic facies. It seems that the rapid evolutional development of the selected bivalve genera or subgenera is probably related to the world-wide marine regression of the Cenomanian and the latest Campanio-Maastrichtian.