PaleoBios

Response to the Eocene doubthouse interval of global climate cooling (53–33.5 Ma) is explored in arcoid bivalves of the families Parallelodontidae, Cucullaeidae, Arcidae, and Noetiidae. An anomalous biodiversity hotspot in the intracontinental Paris Basin of Northern Europe is contrasted with an equally anomalous coldspot at comparable latitude on the tectonically active Cascadia Margin of western North America. Reevaluation of arcoid shell morphology and an annotated glossary of shell features accompanies illustration and discussion of eight exemplar species, identifying new characters and distinguishing those with a strong phyletic signal from those representing functional convergence or developmental differences specific to size or age. Biodiversity anomalies cannot be attributed to any single factor. However, contributing factors include tectonic setting, correlates of bathymetric and sedimentary setting, sediment geochemistry, ocean gateway events, reorganization of current systems and water masses, deepening of the calcium carbonate compensation depth, patterns in the development of sea ice and polar ice storage, changes in sea level, and changes in atmospheric carbon dioxide and the carbon cycle. Opening of the Tasman Gateway and Drake Passage, thermal isolation of Antarctica, and evolution of a Pacific psychrosphere are correlated with the early appearance of cold-water molluscan taxa on the active Cascadia Margin along with the unrelated onset of arc volcanism, subduction, and geochemical changes associated with methane and sulfide seepage. Persistence of a shallow carbonate platform and proliferation of molluscan diversity in spite of global cooling is more difficult to explain, and understanding biogeographic anomalies requires additional climate proxy records. History of the western margin of North America includes an earlier Mesozoic volcanic arc and forearc basin in central and northern California with abundant basal arcoids, negating the need for westward migration out of the Tethyan region to the Cascadia Margin during the Paleogene.