TETRAPOD DISPERSAL DURING THE PERMIAN-TRIASSIC MASS EXTINCTION

The Permian-Triassic mass extinction (PTME) had an impact on life in two ways, by substantially reducing diversity, and by expelling life from the tropics following episodes of intense global warming. Both dynamics are explained by a killing model linked to release of volcanic gases and methane stores, so producing sharp episodes of global warming, with ocean-atmospheric temperatures rising to above 40^oC in the tropics. Key evidence comes from carbon and oxygen isotopes, but the geographic extent of the warming was initially assessed (Sun et al. 2012) based on the geographic distribution of fishes and tetrapods, which were absent in the tropical belt. In this presentation we consider all skeletal and footprint records of tetrapods, and analyse their latitudinal distribution across the Permian-Triassic boundary (PTB) from the Middle Permian (Guadalupian) to the Middle Triassic with the aim of exploring the signature left by the PTME on the palaeobiogeography of land vertebrates. A key benefit of this global-scale study was in mitigating the massive dominance of earlier analyses by two key regions, Russia and South Africa, although a problem is that absences could reflect gaps in the fossil record rather than real absences. We show that tetrapods were distributed both at high and low latitudes across the PTB (contra Sun et al., 2012), that there was a clear geographic disjunction across the PTB, with tetrapod distribution shifting 10-15° northward and, during the PTME, we highlight a rapid dispersal phase across the whole of Pangea supported by both footprint and skeletal data, considered with respect to available rock formations. These changes are consistent with a model of generalised migration of tetrapods to higher latitudinal, cooler regions, to escape from the superhot equatorial climate in the earliest Triassic, but the effect is shorter in timescale, and not as pronounced as had been proposed. In the recovery phase following the PTME, this episode of forced dispersal also appears to have promoted the radiation of entirely new groups, such as the archosaurs.