Assessing the Impact of the End-Permian Mass-Extinction on the Vertebrate Faunas of the Karoo

The end-Permian mass-extinction, the biggest of all Phanerozoic extinctions, is known almost exclusively from the marine record, with the only well-studied terrestrial sequence being the rocks in the Karoo Basin of South Africa. The latter is the largest of several sedimentary basins that formed in southern Gondwana during the late Paleozoic to mid Mesozoic and contains an especially rich vertebrate fossil fauna which allowed for the recognition of eight vertebrate biozones.

Traditional placement of the Permo-Triassic boundary (PTB) in the Karoo coincides with the contact between Dicynodon (Permian) and Lystrosaurus (Triassic) assemblage zones. A dramatic faunal turn-over characterize this contact and the post-extinction fauna display a general decrease in diversity, as well as average body size. This has been has been attributed to a decline in primary production which reflect a stressed paleoenvironment – conditions which persisted throughout the entire Lower Triassic. It is only with the Middle Triassic Cynognathus Subzone B fauna that the diversity and average size of vertebrate fossils returned to pre-extinction levels

Over the past decade several studies attempted to constrain and define the end-Permian event in the Karoo, with various sources of data being used to infer pre- and post-extinction paleoenvironmental conditions. High resolution biostratigraphic studies were used to propose biodiversity trends, while paleoenvironmental models are based on fluvial sedimentological, pedogenic and isotopic data sources. However, conflicting conclusions were reached by discrete studies using some of these data sources, while new sedimentological findings suggest a reassessment of fluvial depositional models. Also, deposition in the Karoo Basin was controlled by a complex interplay of both tectonic and environmental factors which, in addition to regional variation, needs to be taken into account in all extinction studies. These developments necessitate an urgent review and consolidation of the existing body of PTB research.