Biostratigraphy of the Lystrosaurus declivis Assemblage Zone (Beaufort Group, Karoo Supergroup), South Africa

Abstract

Abstract The earliest Triassic (Induan) Lystrosaurus Assemblage Zone has long been recognised as a particularly significant biozone in palaeoecological and palaeoenvironmental studies as it contains a community assemblage that records the survival and recovery from the end-Permian mass extinction (EPME). Here renamed the Lystrosaurus declivis Assemblage Zone it represents the best record globally of the ecological changes in terrestrial community structure and stability during this time period. The assemblage is dominated by two species of small to medium-sized herbivorous dicynodonts L. declivis and L. murrayi that co-occur in equal abundance, along with a range of smaller and less common faunivorous and insectivorous taxa. The latter comprise cynodonts (Thrinaxodon, Galesaurus, and Platycraniellus), therocephalians (Olivierosuchus, Regisaurus and Promoschorhynchus), the diminutive parareptiles (Saurodektes, Sauropareion, Colleta, Phonodus and Procolophon) and eureptilian arrivals possibly representing immigrant taxa (Prolacerta, Heleosuchus and Noteosuchus), among others. The attendant large carnivores were sabre-toothed Moschorhinus and the long-snouted archosauromorph Proterosuchus. In the aftermath of the mass extinction, new small temnospondyl taxa established their first occurrences i.e. Broomistega, Lydekkerina, and Micropholis, and have relatively high abundances compared to earlier temnospondyl records in the Karoo. Lithostratigraphically, the biozone for the most part spans the upper Palingkloof Member of the Balfour Formation and the overlying Katberg Formation in the western part of the basin, and the Normandien Formation in the east. The Lystrosaurus declivis Assemblage Zone of the main Karoo Basin hosts the type locality of the global Lootsbergian land-vertebrate faunachron. The biozone is one of the most widespread terrestrial faunal assemblages of western Gondwana with closely related species occurring in India and Antarctica. Similar, but much more distantly related taxa, occur in Induan-aged strata of Russia, China and Brazil.