Early Norian silicified bivalves from Hells Canyon in the Wallowa terrane of northeastern Oregon are part of a rich molluscan biota associated with a tropical island arc. The Hells Canyon locality preserves lenses of silicified shells formed as tempestites in a shallow subtidal carbonate environment. These shell assemblages are parautochthonous and reflect local, rather than long-distance, transport. Silicification at this locality involved small-scale replacement of original calcareous microstructures, or small-scale replacement of neomorphosed shells, without an intervening phase of moldic porosity. This incremental replacement of carbonate by silica contrasts markedly with void-filling silicification textures reported previously from silicified Permian bivalve assemblages.
The bivalve paleoecology of this site indicates a suspension feeding biota existing on and within the interstices of coral-spongiomorph thickets, and inhabiting laterally adjacent substrates of peloidal carbonate sand. The bivalve fauna is ecologically congruent with the reef-dwelling molluscs associated with Middle Triassic sponge-coral buildups in the Cassian Formation of the Dolomites (Fuersich and Wendt, 1977). Hells Canyon is a particularly important early Norian locality because of the diversity of substrate types and because the site includes many first occurrences of bivalves in the North American Cordillera. These first occurrences include the first documentation of the important epifaunal families Pectinidae and Terquemiidae in Triassic rocks of the North American Cordillera.
The large number of biogeographic and geochronologic range extensions discovered in this single tropical Norian biota indicates that use of literature-based range data for Late Triassic bivalves may be very hazardous. Many bivalve taxa formerly thought to have gone extinct in Karnian time have now been documented from Norian strata in this arc terrane. These range extensions, coupled with the high bivalve species richness of the Hells Canyon site, suggest that the Karnian mass extinction in several literature-based compilations may be an artifact of incomplete sampling. Even for the Norian, present compilations of molluscan extinction may have an unacceptably large artifactual component.
Thirty-five bivalve taxa from the Hells Canyon locality are discussed. Of these, seven are new: the mytilid Mysidiella cordillerana n. sp., the limacean Antiquilima vallieri n. sp., the true oyster Liostrea newelli n. sp., the pectinacean Crenamussium concentricum n. gen. and sp., the unioid Cardinioides josephus n. sp., the trigoniacean Erugonia canyonensis n. gen. and sp., and the carditacean Palaeocardita silberlingi n. sp.