| UDC 564.1:551.77(5-17) |
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The Paleocene-Middle Eocene Stage in the Development of Cenozoic Mollusks in the North Pacific |
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| © 2006 A. I. Kafanov |
| A.V. Zhirmunsky Institute of Marine Biology FEB RAS, Vladivostok |
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| The Paleocene-Middle Eocene bivalves comprise 300 valid species and subspecies in 129 genera and subgenera and 49 families throughout the northern Pacific areas. The Nuculana (Saccella) gabbii – Corbula (Cuneocorbula) Formosa (Kafanov, Ogasawara, 2003) assemblage, that is tentatively diagnosed for the northwestern Pacific, shall be rather defined as the Nuculana (Saccella) gabbii – Acila (Truncacila) decisa assemblage, with regard to the entire North Pacific. The Californian sediment sequences yield its index species of a wider age distribution, which allow us to suggest the Paleocene – Early Eocene ages of the above-mentioned bivalve assemblage. The assemblages of Cowlitz Formation in Washington and Oregon are supposed to be of the same age. The Lucina washingtonensis – Nuculana (Saccella) alaeformis assemblage established for the northwestern Pacific shall be defined as the Nuculana (Saccella) alaeformis – Glyptoactis (Claibornicardia) keenae assemblage of Paleocene throughout the North Pacific. Findings of Corbicula (Batissa) sitakaraensis in Krasnopoliev and the lower Nizhnedui Suites serve as a basis for as to make a tentative correlation between the coastal (lagoon) facies of Shitakara Formation in eastern Hokkaido and the Krasnopoliev Horizon of Lower Eocene in southern Sakhalin. The bivalve fauna of Paleocene-Middle Eocene ages in the North Pacific is poorly differentiated in terms of its biogeographic distribution and represents the single northern Pacific Province of the Boreal Realm including its Eastern Asia – Kamchatka and Alaska – lower Californian Subprovinces. The northern Pacific had wide connections with the Indian Ocean and the Tethys, which resulted in dominating trans-Tethys genera; it is noteworthy, that side-by-side with mainly westward migrations, there were migrations in opposite directions as well, which is testified by the developmental history of Mytilus ex gr. edulis. Repeated sea transgressions in Beringia are supported by findings of Megayoldia (Portlandella) moriyai yielded by the Kamchik Suite in western Kamchatka and the Prince Creek Formation of Danian-Paleocene over the Arctic coasts of Alaska. The genus composition and the species-to-family spectra of Bivalvia, as well as the species-to-genus spectra of thermotrophile taxa (Kafanov, Volvenko, 1997) allow us to assume a paratropical marine climate in the North Pacific in Paleocene through Middle Eocene with the annual average temperatures of surface water about 12–13°C and seasonal amplitudes about 3–4°C. |
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| Keywords: Bivalvia, assemblage, historical biogeography, Paleocene, Eocene, the northern Pacific. |
| Full text [In Russian] |
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