Giant alatoform bivalves in the Upper Triassic of western North America

ABSTRACT. Large, alatoform bivalves, Wallowaconcha raylenea gen. et sp. nov., from the Upper Triassic of north‐eastern Oregon, are described and placed in a new family, Wallowaconchidae, within the Megalodontoidea, which also contains the families Megalodontidae and Dicerocardiidae (herein transferred). Major character innovations of wallowaconchids are the internal partitioning of wings and development of non‐articulating thin vanes on the hingeplate. The wallowaconchid hinge, which changed during ontogeny, differs greatly from the hinge of other bivalves. Wallowaconchids probably evolved from a species of the megalodontid Triadomegalodon. Although wallowaconchids are homeomorphs of the alate Permian alatoconchid bivalves, hinge structure shows that these two groups are unrelated.Large wings on wallowaconchids were used for snowshoe support. They may have utilized elaborate vanes and wing chambers to culture microbial symbionts, either microalgae or bacteria. These bivalves are endemic to displaced island arc terranes in western North America, occurring in Yukon, Canada (Stikine terrane), Oregon, USA (Wallowa terrane), and Sonora, Mexico (Antimonio terrane). They occupied environmental niches similar to those of large megalodontid bivalves of Triassic tropical provinces.