Euthycarcinoids are one of the most enigmatic arthropod groups, having been assigned to nearly all major clades of Arthropoda. Recent work has endorsed closest relationships with crustaceans or a myriapod-hexapod assemblage, a basal position in the Euarthropoda, or a placement in the Hexapoda or hexapod stem group. Euthycarcinoids are known from 13 species ranging in age from Late Ordovician or Early Silurian to Middle Triassic, all in freshwater or brackish water environments. Here we describe a euthycarcinoid from marine strata in Argentina dating from the latest Cambrian period, extending the group's record back as much as 50 million years. Despite its antiquity and marine occurrence, the Cambrian species demonstrates that morphological details were conserved in the transition to fresh water. Trackways in the same unit as the euthycarcinoid strengthen arguments that similar traces of subaerial origin from Cambro-Ordovician rocks were made by euthycarcinoids. Large mandibles in euthycarcinoids are confirmed by the Cambrian species. A morphology-based phylogeny resolves euthycarcinoids as stem-group Mandibulata, sister to the Myriapoda and Crustacea plus Hexapoda.