A baseline paleoecological study for the Santa Cruz Formation (late–early Miocene) at the Atlantic coast of Patagonia, Argentina

Coastal exposures of the Santa Cruz Formation (late–early Miocene, southern Patagonia, Argentina) between the Coyle and Gallegos rivers have been a fertile ground for recovery of Miocene vertebrates for more than 100 years. The formation contains an exceptionally rich mammal fauna, which documents a vertebrate assemblage very different from any living community, even at the ordinal level. Intensive fieldwork performed since 2003 (nearly 1200 specimens have been collected, including marsupials, xenarthrans, notoungulates, litopterns astrapotheres, rodents, and primates) document this assertion. The goal of this study is to attempt to reconstruct the trophic structure of the Santacrucian mammalian community with precise stratigraphic control. Particularly, we evaluate the depauperate carnivoran paleoguild and identify new working hypotheses about this community. A database has been built from about 390 specimens from two localities: Campo Barranca (CB) and Puesto Estancia La Costa (PLC). All species have been classified as herbivore or carnivore, their body masses estimated, and the following parameters estimated: population density, on-crop biomass, metabolic rates, and the primary and secondary productivity. According to our results, this model predicts an imbalance in both CB and PLC faunas which can be seen by comparing the secondary productivity of the ecosystem and the energetic requirements of the carnivores in it. While in CB, the difference between carnivores and herbivores is six-fold, in PLC this difference is smaller, the secondary productivity is still around three times that of the carnivore to herbivore ratio seen today. If both localities are combined, the difference rises to around four-fold in favour of secondary productivity. Finally, several working hypotheses about the Santacrucian mammalian community and the main lineages of herbivores and carnivores are offered.