MICROBIOTHERIIDAE (MAMMALIA, MARSUPIALIA) FROM THE PINTURAS FORMATION (EARLY MIOCENE, ARGENTINA).

The Pinturas Formation (early Miocene, northwestern Santa Cruz Province, Argentina) is a continental succession that can be divided into three sequences based on its major intraformational erosive surfaces and lithological changes. Previous studies concluded that the lower and middle sequences have an absolute age of more than 16.5 Ma and they bear a mammal assemblage (Astrapothericulan fauna) different from and older than that typifying the Santacrucian Land Mammal Age. Recently, the first microbiotheriid marsupial species, Pachybiotherium illuminatum Goin, Tejedor, Abello, and Martin, was described from the middle sequence of this unit. In this contribution, new microbiotheriid specimens from the same sequence, represented by dental elements, are described from three localities of Santa Cruz Province: Cerro de los Monos, Portezuelo Sumich Norte, and Portezuelo Sumich Sur. These specimens are referred to Microbiotherium tehuelchum Ameghino, Microbiotherium divisum Ameghino, and Microbiotherium cf. M. gallegosense Sinclair. Similarly to what was observed for other mammals from these levels of the Pinturas Formation, the microbiotheriid association includes a combination of taxa previously known from the Santacrucian SALMA, others known from the Colhuehuapian SALMA, and a taxon recorded exclusively in these levels (Pachybiotherium illuminatum). This combination of taxa reinforces the idea of the Pinturan assemblage as distinct and intermediate in age between the Colhuehuapian and the Santacrucian. The four microbiotheriids represent the most diverse community so far known for this marsupial group as they positively occur in a single locality and stratigraphic level (i.e., Portezuelo Sumich Sur).