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Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, part IV: The anomalodesmatans

The late Eocene–early Oligocene Keasey Formation in Northwestern Oregon contains a unique fauna of deep-water (>200 m) marine bivalves preserved in sparsely fossiliferous massive tuffaceous siltstone as well as in several distinctive cold-seep limestone bodies and carbonate layers. The Keasey gastropod fauna has been described previously, but this treatment of the anomalodesmatan bivalves is the first systematic account for any portion of the bivalve fauna. Cenozoic evolutionary radiation and history of anomalodesmatans is less well known than their deep Paleozoic and Mesozoic history. Because internal relationships are not well resolved, ranked classification is not used above the family level, while recognizing that these rare and unusual bivalves do represent a monophyletic assemblage nested within the basal heterodonts. Six species in four anomalodesmatan families in the Keasey Formation shed new light on the Cenozoic history of the group as well as the Eocene appearance of precursors of the living deep-water fauna of the northeastern Pacific. The families represented are Pandoridae, Thraciidae, Periplomatidae, and Cuspidariidae. The new species are Pandora eocapsella, Thracia keaseyensis, Cardiomya anaticepsella, and Cardiomya pavascotti. Aperiploma? n. sp. is described in open nomenclature pending discovery of additional and more complete material. Although shells are frequently crushed and the exterior shell layers are often poorly preserved, the interior nacre is distinctive, well-preserved and useful for recognizing fragments in the field. Characteristic anomalodesmatan granules are well preserved on many specimens of the new thraciid, and the fine-grained matrix at some localities preserves shell features on external molds where no original shell material remains. High-resolution images of uncoated shell encourage greater attention to collection of fragments and imperfect specimens in the fine-grained deep-water facies on the active margin of the Pacific.