Litopterna (mammalia) from the Pinturas Formation, early-middle miocene of Patagonia

LITOPTERNA (MAMMALIA) FROM THE PINTURAs FORMATION, EARLY-MIDDLE MIOCENE OF PATAGONIA. The Litopterns from the Pinturas Formation (Ameghino's "Astrapothericidan beds") are examined. Four species referable to the Proterotheriidae are recognized for the lower and middle sequences of this unit: Picturotherium migueli gen. et sp. nov, Lambdacomus laceruni (Ameghino), Tetramerorhimis prosistells (Ameghino), and T. fleaglei Soria. These species retain plesiomorphic dental features, as the labial folds of the paracone and metacone in the upper molars and the little development of the neoparaconid - neoparalophid in lower molars, characteristic of the Deseadan and Colhuehuapian proterotheriids (Soria's "Oligocene Group"). Proterotheriids with more derived dental morphotypes, Tetramerorhinus cingulatum (Ameghino) and Diadiaphorus? caniadensis sp. nov., are recorded at those localities referred to the upper sequence. This stratigraphic distribution agree with that of the hystricognath rodents and palaeothentid marsupials, and reinforces the hypothesis that the lower and middle sequence of the Pinturas Formation bear a mammal assemblage different from and older than those derived from the typical Santacrucian localities of the Santa Cruz Formation. Macraucheniids are only recorded in those levels interpreted as belonging to the upper sequence of the Pinturas Formation. The absence of macrauqueniids in the lower and middle sequences of the Pinturas Formation could be the consequence of peculiar and still not well determined palaeoenvironmental conditions during early Pinturas times.