CONICET | Buscador de Institutos y Recursos Humanos

The first report of tetrapod burrows from the early Late Triassic (early Carnian) Chañares Formation, La Rioja (Northwestern Argentina), is presented here. The burrows were found in different localities of the unit, being abundant in the first 15 meters of the lower member. They occur associated with a fauna dominated by therapsids, rhynchosaurids, and basal pseudosuchians at the first ~8 meters, which is quite distinctive from that represented by the typical Chañares fauna (i.e., some groups) upwards of the sequence. The burrows were found within paleosol profile that developed over ephemeral channelized flows, deposited by meandering ephemeral rivers under semi-arid climate conditions. The burrow systems are characterized by long, multiple branching tunnels that meander horizontally forming a complex network. The burrows display passive fill of a previously open burrow; they are cylindrical to sub-cylindrical in shape measuring between 10 to 20 cm in diameter and several meters of lateral extension. In general, these biogenic structures have a similar morphology and 3D organization to those described for the late Carnian-early Norian Ischigualasto Formation. Based on the occurrence of burrow systems within paleosols formed in semiarid environment and the presence of large suchians (e.g., Luperosuchus) in the same levels lend support to the hypothesis that small tetrapods (e.g., Chiniquodon, Probainognathus) from the Chañares Formation would excavate burrows to avoid climate-stress conditions and/or predation. This new finding complete the burrows record found within the units of the Ischigualasto-Villa Union Basin allowing understands the fossorial habits evolution of small vertebrates from Gondwana during the late Triassic.