Permian tetrapod extinction events

Abstract

Four substantial tetrapod extinctions have been identified during the Permian, but only one of these is an apparent mass extinction. Analyses of global compilations of the family-level diversity of Permian tetrapods have been confounded by incorrect and compiled correlations. Instead, analyzing diversity patterns at the genus level in “best sections” identifies only one apparent mass extinction of Permian tetrapods. Much evolutionary turnover took place among tetrapods during the latter part of the early Permian and had been identified as a single mass extinction at the Artinskian-Kungurian boundary. However, the only stratigraphically dense tetrapod record of the late early Permian, from the southwestern USA, indicates a succession of extinctions spread out from Redtankian through Littlecrontonian (Kungurian) time, not a single mass extinction. Olson's gap remains a hiatus in the global record of Permian tetrapods equivalent to part of the Kungurian-Roadian. Across the gap, eupelycosaur-dominated assemblages were replaced by therapsid-dominated assemblages, but the claim that this is associated with a mass extinction (“Olson's extinction”) has been based on compressing all of the extinctions of the Redtankian-Littlecrotonian and Olson's gap into one event. Recognition of Olson's gap does not preclude the possibility of an extinction at the early-middle Permian boundary (“Olson's extinction”). However, the gap in the tetrapod fossil record makes it impossible to establish the magnitude, precise timing and structure of the extinctions that took place across Olson's gap.The only Permian mass extinction of tetrapods is the dinocephalian extinction event during the Gamkan (near the end of the Guadalupian), which saw the total extinction of dinocephalians and major diversity drop in therocephalians. In the Karoo basin of South Africa, this extinction is the loss of at least 64% of generic diversity. The changeover from dinocephalian assemblages to assemblages without dinocephalians in other parts of Permian Pangea suggests that the dinocephalian extinction event was a global event. The late Permian tetrapod extinctions are older than the end-Permian marine extinctions. Furthermore, the magnitude of the diversity drop and ecological severity of the end-Permian tetrapod extinctions have been greatly overstated. Best sections analysis in the Karoo basin indicates a stepwise late Permian tetrapod extinction during deposition of the upper Dicynodon Assemblage Zone that took at least 250,000 and perhaps more than a million years. The culmination of this stepwise extinction, across the highest occurrence of Dicynodon (= boundary of Platbergian and Lootsbergian land-vertebrate faunachrons) is a loss of genera not much above an inferred background extinction rate of Permian tetrapod genera and resembles the amount of turnover seen at several other boundaries of Permian and Triassic land-vertebrate faunachrons. The case for coeval land plant, insect and tetrapod extinctions during the Permian is a weak one. The first coeval marine and nonmarine mass extinctions of the Phanerozoic were likely the end-Guadalupian extinction. Climate change, notably greenhouse climates, may have driven Permian tetrapod extinctions, but that hypothesized relationship needs better documentation.