The large euthycarcinoid arthropod Mictomerus melochevillensis from the middle Cambrian-Furongian Potsdam Group of Quebec occurs as threedimensional casts at the end of Cruziana-and Didymaulichus-like trace fossils. This association provides a rare opportunity to test functional morphological hypotheses about these animals, it provides a framework for understanding how arthropods can be sand-cast in three dimensions, and it suggests that euthycarcinoids may have burrowed into mud as an antidesiccation strategy. In the coeval Elk Mound Group of Wisconsin, the phyllocarid arthropod Arenosicaris inflata occurs on the same beds as Cruziana-like and Rusophycus traces; together with morphologically similar ropelike traces, they are interpreted as having been produced by phyllocarids. These traces preserve the earliest known ethological evidence of phyllocarid crustaceans, and imply that Cambrian phyllocarids employed antidesiccation and possibly feeding strategies still used by modern intertidal leptostracan phyllocarids.