WHERE IN THE KAROO IS CARMEN SANDIEGO’S LYSTROSAURUS DECLIVIS ASSEMBLAGE-ZONE BOUNDARY?

Over the past quarter century, the prevailing model for the response of terrestrial ecosystems to the end-Permian crisis has centered on the vertebrate-fossil record of the Karoo Basin, South Africa, extrapolated to other southern and northern hemisphere regions. During the intervening years, this extinction model has been modified from that of where a distinctive and unique horizon marks the event, to one of a protracted response preserved over several tens of meters of section. Regardless of the version, one set of criteria has been used to recognized the extinction turnover, in the face of hypothesized extreme vegetational perturbation as a consequence of increased aridity. These are the Last Appearance Datums of Daptocephalus Assemblage Zone (AZ) taxa and the First Appearance Datum (FAD) of two Lystrosaurus taxa in overlying the L. declivis AZ. The boundary between AZs is reported to be isochronous across the basin and within ~300 ka of the marine extinction event.

We present a new Karoo Basin model based on a multidisciplinary approach incorporating geochronology, magnetostratigraphy, palynostratigraphy and paleobotany into a stratigraphic framework spanning classic AZ-boundary localities from the Eastern Cape to the Free State provinces. We demonstrate that the diagnostic taxa used to distinguish pre-extinction and post-extinction vertebrate AZs are coeval, and coexisted on the same Changhsingian land surfaces. Diagnostic taxa of the Daptocephalus AZ are preserved in green mudstone whereas L. declivis AZ taxa are preserved in laterally correlatable reddish-gray mudstone. Our data show that both assemblages lived in a Glossopteris-dominated landscape. A new U-Pb CA-ID-TIMS age date constrains all of these elements to the latest Permian, ~0.3 Ma older than the marine event. We also demonstrate that the currently defined AZ boundary occurs in a normal polarity magnetozone in the Eastern Cape with an early Changhsingian age estimate (Lootsberg Pass) and in a reverse polarity magnetozone in the Free State (Bethel) province with a late Changhsingian age estimate. Our data negate the utilization of an isochronous horizon and vertebrate turnover in the basin and, by extension, across Gondwana and elsewhere.