Densely populated communities of soft-bottom-dwelling taxa similar to those found today along subduction zones off the coasts of Japan and Oregon have been discovered in very localized deep-water limestones of late middle to late Eocene age along the southwestern margin of Washington. Subduction was prevalent in this area during this time, and compressive forces squeezed subsurface methanerich waters onto the ocean floor, where opportunistic bivalves (especially Modiolus, Calyptogena, and Thyasira), vestimentiferan? tube worms, serpufid tube worms, siliceous sponges, very small limpets, trochid and turbinid archaeogastropods, and other macrobenthos colonized. These assemblages are the earliest recorded biologic communities formed in response to methane seeps in subduction zones.

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