The crustacean ichnofossil Palaxius associated with callianassid body fossils in an Eocene methane-seep limestone, Humptulips Formation, Olympic Peninsula, Washington

Crustacean microcoprolites are preserved along with crustacean megafossils in an Eocene limestone within the Humptulips Formation in Grays Harbor County, Washington. The limestone formed at a methane seep, as revealed by early diagenetic carbonate phases resembling those of other seep limestones, δ13Ccarbonate values as low as −27‰, and lipid biomarkers of prokaryotes involved in anaerobic oxidation of methane with δ13C values as low as −110‰. Recognizable crustacean megafossils are chiefly isolated chelipeds, akin to those of callianassid shrimp. The microcoprolites are pierced by ten longitudinal canals with crescent-shaped outlines in cross-section, arranged in two symmetrical groups. The symmetry plan and number of canals of the microcoprolites identify the Humptulips ichnospecies as Palaxius habanensis Bronnimann & Norton. The occurrence of decapod fossils along with microcoprolites suggests that callianassid shrimp were the producers of the Palaxius, supporting earlier work that attributed this ichnogenus to callianassids.