IS THE LYSTROSAURUS ASSEMBLAGE ZONE OF THE KAROO BASIN, SOUTH AFRICA, THE EXPRESSION OF THE POST PERMIAN EXTINCTION ON LAND?

The vertebrate biostratigraphy of the Beaufort Group, Karoo Basin, South Africa, plays a central role in our understanding of the global terrestrial response to the End-Permian extinction. Estimates of >60% loss in tetrapod-family biodiversity have been described across the boundary from the Dicynodon Assemblage Zone (AZ) to the overlying Lystosaurus AZ, which has been reported to represent the continental expression of the extinction event. In the marine realm, this event has been placed in a Normal polarity magnetozone above a reversal with an initiation date of 251.88 ± 0.03 Ma. Prior to the present study, no ages for volcanic beds have been published for latest Permian strata in the Karoo Basin. Only a single detrital zircon, dated at 252.5 ± 0.7 Ma, from a bed above a transition from a Reverse to Normal polarity magnetozone from the Commando Drift Dam site has been reported, which was interpreted to provide a maximum age for deposition.

We report new paleontologic data, including plant-and-vertebrate fossils, that demonstrate the continuation of Dicynodon AZ taxa–reported as extinct at the assemblage zone boundary–into stratigraphically higher positions. These fossils are placed in a new lithostratigraphic, magnetostratigraphic, and geochonometric context, that differ from those reported previously from the Lootsberg Pass locality. The stratigraphy is dominated by normal magnetic polarity, with a short reverse-polarity zone in wacke and siltstone exposed immediately above a porcellanite. A U-Pb ID-TIMS age from this latter unit provides an early Changhsingian maximum depositional age and, in conjunction with our other data, we suggest that the Dicynodon–Lystrosaurus Assemblage-Zone boundary is not only stratigraphically higher than currently reported, but more than 1 Ma older than the Permian–Triassic extinction event in the marine realm.