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31 2 Therapsid Biodiversity Patterns and Paleoenvironments of the Karoo Basin, South Africa Roger Smith, Bruce Rubidge, and Merrill van der Walt Therapsids lived in Pangaean times and their fossils have been found on every continent (see Plate 1A); however, there is no doubt that the most abundant and diverse collecting grounds are in the main Karoo Basin of South Africa. This 10-km-thick sedimentary succession accumulated in a large intracratonic, retro-arc, foreland basin (Johnson 1991; Catuneanu et al. 2005) in front of the rising Cape Fold Belt portion of the Gondwanide Mountain range that fringed the southern margin of Gondwana. Today, rocks of the Karoo Supergroup have a spatial distribution of some 300,000 km2 (Smith 1990b), which is more than one-half the land surface of South Africa. They were deposited under environments ranging from glacial through intracontinental shallow marine, fluvio-lacustrine, fluvial, and aeolian and provide an almost continuous 110-My record of continental sedimentation from the Late Carboniferous (300 Ma) to the Early Jurassic (190 Ma) (Smith 1995). Fossils from the Karoo, especially the therapsids, provide the best record of continental biodiversity from the Middle Permian to the Early Jurassic and have been crucial to studies of the global Permo-Triassic mass extinction, as well as giving insight into other extinction events. Recent research has demonstrated the importance of Karoo fossils for global stratigraphic correlation and for the conceptualization of basin development models (e.g., Catuneanu, Hancox, and Rubidge 1998; Hancox 1998; Neveling 2002; Rubidge 2005). The discovery of datable ash beds that can be linked to biozone-defining fossils has considerably enhanced research possibilities by providing radiometric ages for vertebrate biozones (Rubidge et al. 2010), and thus opening up a way to ascertain rates of evolution in fossil tetrapod lineages, as well as the timing and duration of extinction events. The recent development of a geographic information system–based database of all the fossil tetrapods from the Beaufort Group, which are housed in South African museum collections, has facilitated research on tetrapod biodiversity and biogeographic changes during the Permian and Triassic (Nicolas 2007; Nicolas and Rubidge 2009) and has endless possibilities for future research. Introduction Smith et al. 32 Andrew Geddes Bain was the first to draw attention to the rocks of the Karoo and named these strata the Karoo “Series” (Bain 1856). Later, Jones (1867 cited in North 1878) introduced the nomenclature Beaufort “Beds” for sedimentary rocks within the lower part of the Karoo Series around Beaufort West. In 1873, Dunn proposed the term Stormberg “Beds” for the uppermost part of the succession and then subdivided the sequence into the “Coal Measures, Red Beds, Cave Sandstone and Volcanic Beds”(Dunn 1878). Rogers (1903) lithostratigraphically subdivided what he termed the Karoo “System,” into the Dwyka, Ecca, Beaufort, and Stormberg groups, and in 1905, he and Broom (1906b) further subdivided the Beaufort Group into a Lower, Middle, and Upper Series. This stratigraphic subdivision remained until Johnson (1976) erected a new lithostratigraphic subdivision for the Beaufort Group in the Eastern Cape. He established the Adelaide Subgroup to broadly correspond to the Lower Beaufort Series of Rogers (1905) and Broom (1906b) and the Tarkastad Subgroup to their Middle and Upper Series. Johnson’s nomenclature was accepted by the Karoo Working Group of the South African Council for Stratigraphy (SACS 1980). For some 80 My, from the Early Permian to Early Jurassic, the alluvial plains of the intracontinental Karoo Basin were continually adjusting to base level movement caused by subduction-driven uplift along the southern margin of the basin and resultant reciprocal flexure in the foreland sag as well as experiencing periods of rapid climate change. The constantly changing base level had a major influence over the drainage networks, topography, groundwater levels, vegetation types, and the terrestrial and aquatic communities that lived there. For most of this time, Gondwanan tetrapod communities were dominated by therapsids, although after the End-Permian mass extinction, therapsid dominance was progressively challenged by archosauromorphs. Continuous environmental change kept these communities in a state of flux. Migrations, invasions, extinctions, and radiations were happening all the time and form the basis of the current biostratigraphic scheme. The extended and almost continuous depositional history of the Karoo Supergroup affords a unique opportunity to study changes in ecological representation from the Middle Permian (Wordian) to Early Jurassic (Pliensbachian). A therapsid-based biostratigraphic scheme (Kitching 1977; Kitching and Raath 1984; Keyser and Smith 1978; Rubidge 1995a) has facilitated division of the vertebrate-bearing sequences of the Karoo...

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