Evidence of torpor in the tusks of Lystrosaurus from the Early Triassic of Antarctica.

Antarctica has hosted a wide range of ecosystems over the past 500-million years. Early in the Mesozoic, the Antarctic portion of southern Pangaea had a more habitable climate, but its position within the polar circle imposed extreme photoperiod seasonality on its resident flora and fauna. It remains unclear to what degree physiological adaptations underpinned the ability of tetrapods to establish the terrestrial communities captured in the fossil record. Here we use regular and stressful growth marks preserved in the dentine of ever-growing tusks of the Early Triassic mammalian predecessor, Lystrosaurus, to test for adaptations specific to this polar inhabitant. We find evidence of prolonged stress indicative of torpor when compared to tusk samples from non-polar populations of Lystrosaurus. These preliminary findings are to our knowledge the oldest instance of torpor yet reported in the fossil record and demonstrate unexpected physiological flexibility in Lystrosaurus that may have contributed its survivorship through the Permo-Triassic mass extinction.