Abderitid marsupials are common in vertebrate-bearing deposits from the middle Miocene of Argentine Patagonia. Recent collections from the inland Pinturas Formation and slightly younger coastal Santa Cruz Formation have dramatically increased the number of abderitid specimens. These new collections permit a re-assessment of abderitid taxonomy as well as an investigation of the dietary habits of these unique small mammals. The vast majority of new specimens represent Abderites meridionalis; Pithiculites minimus is rare. Patterns of macrowear on the double-bladed, plagiaulacoid shearing complex suggest that abderitids used these teeth to prepare a variety of resistant food items as do modern marsupials with double-bladed shearing systems. Data summarizing molar-shearing morphology and body size further suggest that A. meridionalis was a frugivore. The diet of the small P. minimus is equivocal, although it may represent a mixed feeder (frugivore/faunivore). A comparison of relative species richness and dietary adaptation between abderitids and palaeothentids (a closely related caenolestoid family that lacks the highly specialized shearing complex of abderitids) reveals distinct evolutionary patterns within the two lineages. Abderitids exhibit low species diversity. In contrast, palaeothentids are represented by 17 species, lack highly specialized shearing mechanisms, and typically exhibit molar morphologies that range from frugivory to faunivory and include mixed feeders. Both temporal and geographic variation are introduced as possible factors affecting differences in the relative abundance of abderitids and palaeothentids in the Pinturas and Santa Cruz Formations.