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The extinct genus Sparassocynus is known from the Montehermosan, Chapadmalalan, and Uquian stages of Argentina, spanning the Pliocene-Pleistocene transition, and a relative occurs in the older Huayquerian. After a taxonomict review, the morphology of skull, mandible, and dentition is described. The short, triangular face, wide zygomata, broad cranium with large completely bone-enclosed hypo- and epitympanic sinuses, and carnassialized molars are unique among known American marsupials. All these characters are functionally convergent toward living Australian phascogaline dasyurids, small marsupial carnivores, although phylogenetically Sparassocynus clearly represents a moderately specialized offshoot of the American family Didelphidae. It represents a small steppe carnivore now ecologically replaced, at least in part, by placental carnivores that spread from North America in the late Pliocene and Pleistocene.