PALEOSOL AND VERTEBRATE EXTINCTION ACROSS THE PERMIAN-TRIASSIC BOUNDARY IN THE KAROO BASIN, SOUTH AFRICA

Paleosols above and below the Permian-Triassic boundary in South Africa at Lootsberg Pass, Carlton Heights and Bethulie are evidence for ecosystem change across this greatest of all mass extinctions. The Permian-Triassic boundary is based on the last appearance of the therapsid Dicynodon. The boundary is marked by laminated shales, a shift from purple (10R) to brownish red (2.5YR) paleosols, and a thin claystone breccia of reworked soil clasts. Paleoclimatic change cannot be inferred from very weakly developed paleosols in the boundary laminites, but arid highly seasonal paleoclimate is evident from shallow and diffuse nodular horizons in late Permian paleosols, and semiarid, less seasonal paleoclimate from deeper and narrower calcic horizons in early Triassic paleosols. An earliest Triassic shift to warmer and wetter paleoclimate is also indicated by increased chemical weathering, abundance of lycopsids, and diversity of labyrinthodonts. Permian paleosols have root traces of open shrubland and riparian woodland, whereas Triassic paleosols have root traces and profile form of open woodland. This change is not as dramatic as would be expected from devastating extinctions of fossil vertebrates in this sequence (88% of genera were lost). Latest Permian therapsids were diverse and ecologically specialized, but the most common earliest Triassic therapsid, Lystrosaurus, was a burrower with little habitat preference. Its short internal nares, barrel chest and high neural spines would have given it greater aerobic scope than pre-existing therapsids, and may have been an advantage under conditions of hypercapnia and hypoxia. These adaptations and ecosystem changes are compatible with vertebrate mortality by acidosis and pulmonary edema in a post-apocalyptic low-oxygen (12%) greenhouse resulting from a massive release of methane from shallow marine and permafrost clathrates.