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We describe the oldest Paucituberculata marsupials, from the La Barda and Las Flores localities (Argentina; Late Palaeocene, and Early-Middle Eocene), as well as from the Itaboraí Basin (Brazil; Late Palaeocene). The new taxa are represented by very scarce, although well-preserved, dental remains. A parsimony analysis was performed in order to evaluate the phylogenetic affinities of these taxa. Representatives of both Riolestes capricornicus gen. et sp. nov. and Bardalestes hunco gen. et sp. nov. appear to be basal paucituberculatans, and their molar features give clues on the early evolution of the representatives of this order. Within the Paucituberculata we recognize two major clades: Caenolestoidea and Palaeothentoidea. We conclude that 'pseudodiprotodont' marsupials of the traditional literature (i.e. Polydolopimorphia + Paucituberculata) do not form a natural group.

The Paucituberculata is an endemic group of South American marsupials, recorded from the early Cenozoic up to the present. In this report, the most comprehensive phylogenetic analysis of Paucituberculata to date is presented. Fifty‐seven terminal species were scored for 74 new and re‐examined characters. Homologies of dental characters used in previous systematic studies were critically reviewed to evaluate their inclusion in the analysis. Phylogenetic results corroborated two major paucituberculatan clades, Palaeothentoidea and Caenolestoidea, and the main palaeothentoid groupings: Pichipilidae, Palaeothentidae, and Abderitidae. Taxon sampling and reinterpretations of molar cusp and crest homologies played an important role in the generation of new phylogenetic hypotheses. The main differences with respect to previous phylogenies were focused on palaeothentoid relationships: Palaeothentes boliviensis and Pilchenia lucina are not members of Palaeothentidae but instead clustered with Pilchenia intermedia and P. antiqua, forming the sister‐group of a Palaeothentidae + Abderitidae clade, and Titanothentes simpsoni, previously considered a palaeothentine, is nested within the Acdestinae clade. Based on the time‐calibrated phylogeny, the following stages in the paucituberculatan evolutionary history are suggested: origin of the group, in the Paleocene to early Eocene at the latest, split of Caenolestoidea and Palaeothentoidea clades during the late early to middle Eocene, evolutionary radiation of palaeothentid and abderitid lineages near the Oligocene–Eocene boundary, and decreased diversity and extinction of palaeothentoids during the middle Miocene. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 441–465.

Resumen. En este trabajo se encara la revisión sistemática de dos órdenes de Metatheria (Mammalia) sudamericanos de la Edad Colhuehuapense (Mioceno temprano): Microbiotheria y Polydolopimorphia. En primer lugar, se comentan y discuten las homologías de las cúspides de los molares superiores e inferiores de los Argyrolagoidea (Polydolopimorphia). En segundo lugar, se ofrecen los resultados de un análisis filogenético de los Microbiotheriidae (Microbiotheria). En tercer lugar, se analizan los taxones colhuehuapenses de Microbiotheria y Polydolopimorphia, reconociéndose dos especies nuevas: