New portunoid fossils from southern Argentina and from the west coast of North America permit the reevaluation of the generic and family relationships within the Portunoidea Rafinesque, 1815. It has previously been suggested that the Portunidae and the Geryonidae Colosi, 1923, are closely related families (Manning and Holthuis, 1989). The new fossils suggest that the Geryonidae may in fact be derived from a portunid progenitor, Proterocarcinus Feldmann, Casadío, Chirino-Gálvez, and Aguirre Urreta, 1995, through a process of peramorphosis in which juveniles of the geryonid species Chaceon peruvianus (d'Orbigny, 1842) resemble adults of Proterocarcinus latus (Glaessner, 1933). Examination of several genera within the portunid subfamily Polybiinae Ortmann, 1893, including Imaizumila Karasawa, 1993; Megokkos new genus; Minohellenus Karasawa, 1990; Pororaria Glaessner, 1980; Portunites Bell, 1858; and Proterocarcinus, suggests that the subfamily had an amphitropical distribution early in its history. New taxa reported here include Megokkos new genus and Portunites nodosus new species. New combinations include Chaceon peruvianus (d'Orbigny, 1842), Imaizumila araucana (Philippi, 1887), Megokkos alaskensis (Rathbun, 1926), Megokkos hexagonalis (Nagao, 1932), Megokkos macrospinus (Schweitzer, Feldmann, Tucker, and Berglund, 2000), Minohellenus triangulum (Rathbun, 1926), and Proterocarcinus latus (Glaessner, 1933).