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Keywords:

  • Chemosynthesis;
  • crustacean microcoprolites;
  • decapod crustaceans;
  • Eocene;
  • seeps;
  • hydrocarbons;
  • Washington State

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 Brönnimann & 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.