Uneven landscapes inhabited by middle Miocene mammals from La Pava and Collón Cura formations in North Patagonia

Abstract

The Collón Cura Formation (CCF) bears a diverse assemblage of middle Miocene terrestrial mammals (the Colloncuran fauna) known from theend of XIX century. The CCF along with the La Pava Formation (LPF) constitutes the main syntectonic infill of intermontane basins in theextra-Andean NW Patagonia. The study of paleosols and terrestrial ichnofacies of both units in these basins, particularly in the Jacobacci basin(Rio Negro Province, Argentina), document the progressive environmental change since the end of the Miocene Climatic Optimum, related tothe drying and cooling of southern South America. This change is recognized in the andic Mollisols and mollic Andisols bearing Coprinisphaeraichnofacies formed in subhumid grasslands or wooded-savannas (LPF), and in the xeric-vertic Andisols and andic Aridisols bearing Celliformaichnofacies developed in semiarid woodlands or bushlands (CCF). However, a new U–Pb date of 14.38 ± 0.19 Ma (late Langhian), for the LPFat the Huahuel Niyeu valley in the Jacobacci basin, indicates that both landscapes co-existed likely following a vertical rainfall gradient(orographic precipitation). In the present work, we report the first mammals from the LPF recovered from the Huahuel Niyeu River associatedwith the dated strata. They include Palaeothentes intermedius, Prozaedius sp., Proeutatus sp., Propalaehoplophorinae indet., Glossotheriopsispascuali, Massoiamys obliquus, Pliolagostomus friasensis, Eusigmomys cf. E. oppositus, Pachyrukhos moyani, Hemihegetotherium tantillum,Protypotherium australe, Protypotherium praerutilum, Palyeidodon obtusum, Hyperoxotodon sp. nov, Toxodontidae indet., andAstrapotheriidae indet. All these taxa (except P. intermedius) also occur in roughly coeval sediments of the CCF in Jacobacci and other basinsof NW Patagonia. We conclude that the heterogeneous landscape developed in Jacobacci basin was inhabited by diverse functional groups ofColloncuran mammals adapted to different climatic and floristic settings, and therefore they are not sensitive to small-scale environmentalheterogeneity. These palaeoecologic inferences could be extended to other Miocene basins and to other Cenozoic mammal faunas of Patagonia,which paleohabitats were generally deduced from feeding behaviors.