GLOBAL MIDDLE PERMIAN REPTILE MASS EXTINCTION: THE DINOCEPHALIAN EXTINCTION EVENT

A profound extinction of reptiles took place during the Middle Permian, here termed the dinocephalian extinction event, and it was at least as significant of a tetrapod extinction as the one that took place close to the end-Permian. This Middle Permian (Gamkan land-vertebrate faunachron [lvf]) extinction is marked by the total disappearance of the dinocephalians, the first significant evolutionary diversification of the therapsids, a group of carni vores and herbivores with some very large specialized forms. Furthermore, pelycosaurs also disappeared at (or just before) this extinction, large drops in the diversity of gorgonopsians and therocephalians took place, and these groups did not recover diversity till substantially much later in the Permian (during the Platbergian lvf). In the Karoo basin of South Africa, where the extinction is best documented, it encompasses the total extinction of dinocephalians (loss of 18 genera) and the decimation of most therocephalians (loss of 6 of 8 genera) and gorgonospians (loss of 7 of 8 genera). Dinocephalian-dominated tetrapod assemblages are known from the Karoo basin, the Madumabisa strata in Zimbabwe, the Ural foreland basin in Russia (Isheevo assemblage=Zone II), the Ordos basin of China and the Paraná basin in Brazil. Their cross-correlation to the Permian marine timescale is established in the Russian section, where they predate the Illawara magnetic reversal, which indicates that they are of early Capitanian or late Wordian age. Thus, the dinocephalian extinction event is older than the end-Guadalupian marine extinction. Younger (latest Gamkan-Hoedmakeran lvfs) post-extinction tetrapod assemblages lack dinocephalians and are initially characterized in the Karoo basin by a low diversity assemblage numerically dominated (~ 85% of all specimens) by the dicynodont Diictodon. Indeed, the dinocephalian extinction event sets the stage for the takeover of the Permian tetrapod herbivore niche by dicynodonts. High extinction rates and low origination rates characterize the dinocephalian extinction event, and these rates are comparable to those of the end-Permian tetrapod extinction. Previously suggested causes of the dinocephalian extinction event include marine regression and greenhouse climates, but the cause remains uncertain.