Hystricognath Rodents from the Pinturas Formation, Early – Middle Miocene of Patagonia. Biostratigraphic and paleoenvironmental implications

KRAMARZ, ALEJANDRO; BELLOSI, EDUARDO

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Año: 2005 p. 199 - 212

0895-9811

The Pinturas Formation is a continental succession, characterized by eolian sediments (mainly tuffaceous) and paleosols, originated in an upland setting of west central Patagonia. The main intraformational erosive surfaces and lithological changes define three sequences. This Formation bears a rich mammalian association (Florentino Ameghino´s Astrapothericulan fauna) which age in relation to the Santacrucian SALMA (middle Miocene) is still controversial. Recent collections at Pinturas Formation, performed with stratigraphic control, allow differentiating two distinct hystricognath rodents associations. The lower and middle sequences bear a particular combination of Colhuehuapian (early Miocene) and Santacrucian genera, mostly represented by species exclusively known for the Pinturas Formation ("Pinturan" association). The upper sequence bears typical Santacrucian species, more derived than its “Pinturan” counterparts. On the basis of the rodents record, the lower and middle sequences of the Pinturas Formation are older than the base of the Santa Cruz Formation exposed at Monte Observación and Monte León, and the upper sequence could be correlated to the lowermost levels of the Santa Cruz Formation and to the deposits exposed at Karaiken bearing Ameghino´s "Notohippidian" fauna. These correlations agree with the more recent radiometric datations and other biostratigraphic evidences, supporting the original Ameghino´s hypothesis. The “Pinturan” rodent assemblage of the lower and middle sequences suggests the presence of humid forests, in accordance with other faunal components and palynological data. On the contrary, sedimentologic, paleopedologic and ichnologic evidences suggest environments dominated by herbaceous vegetation. This seeming contradiction is interpreted as the result of a marked environmental gradient due to the paleotopography and/or climatic fluctuations. The mammal record would correspond to the more humid intervals, which are the ones that have lesser representation in the sedimentary record. The Pinturas case reveals that more detailed analysis are necessary to understand the relationships between the stressed abiotic Patagonian scenarios and the evolution of their Tertiary vertebrate communities.