Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

The Family Astartidae (Bivalvia) has fairly long history from the Mesozoic to the Recent, however, it has not been found in the lower to middle Tertiary strata of the northern Pacific region including Japan. In contrast, the family has well diversified and developed during the early Tertiary time in Europe. This evidence indicates that the late Tertiary species of the Astartidae in the northern Pacific region are the characteristic Atlantic element.

The migration route of the Astartidae from the Atlantic into the Pacific can be estimated that some ancestral species has invaded into the northern Pacific through the paleo-Bering Strait as mentioned by some authors (Durham and MacNeil, 1967; Hopkins, 1967; Nelson et al., 1977; etc.). But the age of the Astartidae migration into the Pacific is still uncertain. There are controversial opinions of this matter, i, e., one concludes it during the late Miocene (Nelson et al., 1977; etc.), and the other the late Pliocene to early Pleistocene (Menner et al., 1977; Allison, 1977; etc.).

The writer tentatively classified the species of the Astartidae based on the specimens collected from the Japanese Pliocene and Pleistocene strata and tried to establish their biostratigraphy.

Then, the fossil Astartidae in the late Tertiary time of Japanese Islands can be tentatively grouped into four species groups, i.e., the Tridonta (Tridonta) borealis, Tridonta (Tridonta) rollandi, Tridonta (Elliptica) alaskensis and Astarte (Astarte) hakodatensis.

The stratigraphic horizons of these fossils can be allocated into the Pliocene to Pleistocene based on some recent data of biostratigraphy and chronology (Tsuchi, ed., 1979, 1981; Ikebe and Tsuchi eds., 1984).

It is noteworthy that the fossil species of the Astartidae are restricted in the Omma-Manganzi and Takikawa-Honbetsu faunas distributed in the Sea of Japan borderland and Hokkaido, respectively. The distributions of two fossil species, Tridonta borealis and T. alaskensis, are shown in Fig. 1 based on some publications on the Pliocene to Pleistocene molluscan faunas in northern region.

The fossil data of northwestern Pacific including Japan, Sakhalin, the Kuril Islands, Kamchatka, Chukot Peninsula and Alaska may indicate that the age of migration of the Tridonta species can be inferred to be the Pliocene which may correspond to that of the Beringian Transgression of Hopkins (1967).

It is most interesting to study how the late Miocene marine connection between the Arctic and Pacific oceans had influenced the marine fauna and flora, as pointed out by Hopkins (1967).