Bone diagenesis of tetrapods from the Middle Triassic Tarjados Formation: implication for depositional environment and palaeoclimate

An understanding of the palaeoenvironmental and palaeoclimatic conditions that affects the preserved fossil record is a powerful tool aiding the reconstruction of past ecosystems. In this study, bone diagenesis is used as a sensitive indicator of the palaeoenvironmental and palaeoclimatic conditions during deposition of the Middle Triassic Tarjados Formation of western Argentina. The results provide new information that will improve palaeoecosystem reconstructions throughout western Gondwana. The tetrapod fossil record of the Tarjados Formation is scarce, mainly represented by isolated and dispersed bones. Bones from several stratigraphic levels were analysed. Different mineral infillings are recognised, such as sediment, iron oxides, and calcite. Based on the sequential precipitation of mineral infillings documented in the samples, we identified three diagenetic pathways. The stratigraphic distribution of the diagenetic pathways was analysed to evaluate the depositional and palaeoclimatic conditions through the Tarjados Formation. The integration of the sedimentology and stacking pattern with bone diagenesis suggests changes in the sediment supply and the strongly seasonal soil moisture fluctuations characterised by alternating short wet and long dry seasons, under an overall semi-arid climatic regime. This climatic evidence agrees with a palaeogeographic configuration with a local rain-shadow effect of highlands in contrast with the documented by Cuyana Basin, where the palaeogeographic configuration suggests without local rain-shadow effect with the wet period dominance over drier ones.