FOSSIL DECAPOD CRUSTACEANS FROM THE ASTORIA FORMATION, WASHINGTON AND OREGON, U.S.A.: PALEOBIOGEOGRAPHIC IMPLICATIONS

Recent collections of the Astoria Formation, within the Newport Embayment and in southwesternmost Washington state, has yielded a unique fauna of fossil decapod crustaceans. The Miocene, of the Pacific Northwest, was previously believed to mark a major decrease in fossil decapod crustacean diversity. Of the seven marine deposits associated with the Miocene of Oregon and Washington only the Astoria Formation and the Clallam Formation have had fossil decapod crustaceans previously described from them. Correlations based upon Miocene molluscan stages between California, Oregon, and Washington suggests a warming trend in the late early and middle Miocene, which is evidenced by warm temperate and subtropical molluscan faunas of California reaching as far north as southern Alaska. Known occurrences of fossil decapod crustaceans from the Astoria Formation do not reflect this migrational pattern. Many of the genera have also been collected in Oligocene and younger strata from the Pacific Northwest, many of which do not occur in the northeastern Pacific Ocean today and are instead often found in the subtropical to tropical waters. However, with the descriptions of thirteen new species of fossil decapod crustaceans from the Astoria Formation, it is apparent that fossil decapod crustacean diversity remained high within the northeastern Pacific Ocean up until, and at least through, the middle Miocene. In addition, fossil decapod crustaceans between the northeastern and northwestern Pacific Ocean, which have indicated a strong interchange during the Eocene and Oligocene in terms of numbers of genera, continued through the middle Miocene. The genera Tymolus, Macrocheira, Cancer, Imaizumila, and possibly Globihexapus indicate a strong interchange between the northern Pacific Ocean during the early and middle Miocene. These faunal changes are interpreted to reflect changes in water circulation within the Pacific Ocean and between the tropical Pacific and Indian Ocean, drastically reducing the number of genera and diversity both in fossil decapod crustaceans and in molluscan diversities between the middle and late Miocene of the northeastern Pacific Ocean.