Content area
Full Text
ABSTRACT-The presence of the bivalve mollusks Astarte (Tridonta) borealis Schumacher and A. (T.) hopkinsi new species, in uppermost Miocene or lower Pliocene strata of the Milky River Formation on the Alaska Peninsula, southwestern Alaska, signals the earliest opening of Bering Strait. These species migrated from the Arctic Ocean into the North Pacific when the Bering Strait first flooded and, along with co-occurring marine diatoms, are primary evidence for the earliest opening of the strait. in the latest Miocene or early Pliocene. These paleogeographically important Alaskan Astarte have been cited in this context, but have not been previously illustrated or discussed.
INTRODUCTION
THE BIVALVE mollusk Astarte is common in many middle- and high-latitude North Pacific, Arctic Ocean, and North Atlantic faunas, as living shells and Neogene fossils. The oldest known Astarte lived in the Jurassic of northern Siberia; the genus became cosmopolitan during the early Cretaceous, then lived primarily in the North Atlantic and Arctic oceans after the middle Cretaceous (Zakharov, 1970; Bernard, 1979). There is no well-documented occurrence of Astarte in the North Pacific prior to the earliest opening of Bering Strait, although it has been reported in Cretaceous-Tertiary boundary sediments on Sakhalin (Kalishevich et al., 1981).
During the Cenozoic Astarte also lived in northwestern Europe (Deshayes, 1860; Speyer, 1862; Glibert, 1945), southern Russia (Klyushnikov, 1958; Mironova et al., 1962; Makarenko et al., 1968), Africa, northeastern North America, and western India (Davies, 1971), as well as along the arctic coast of Alaska (Dall, 1920; MacNeil, 1957) (Fig. 1). The opening of Bering Strait at the end of the Miocene or early in the Pliocene (Marincovich and Gladenkov, 1997, 1999; Marincovich, 2000) allowed Astarte to extend its geographic range into the middle- and high-latitude North Pacific, while it continued to dwell in the Arctic and Atlantic oceans.
The oldest documented Astarte in the North Pacific occurs in uppermost Miocene or lower Pliocene strata of the Milky River Formation of the Alaska Peninsula in southwestern Alaska (Fig. 2). These strata had been assigned to the Bear Lake Formation (Detterman et al., 1996), and were determined to have an age range of 4.8-5.5 Ma, based on the occurrence of marine diatoms that correlate with Neodenticula kamtschatica Subzone B of the North Pacific diatom biochronology (Marincovich and Gladenkov,...