PERMIAN TETRAPOD EXTINCTION EVENTS

Three or four substantial tetrapod extinctions have been identified during the Permian, but only one of these is an apparent mass extinction. Much evolutionary turnover took place among tetrapods during the latter part of the early Permian and has been identified as a single mass extinction at either the Sakmarian-Artinskian boundary or the Artinskian-Kungurian boundary. However, the only stratigraphically dense tetrapod record of the late early Permian, from Texas-Oklahoma, USA, indicates a succession of extinctions spread out from late Mitchellcreekian through Littlecrontonian (late Artinskian-Kungurian) time, not a single extinction at any stage boundary. 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”) is based on compressing all the extinctions of the Mitchellcreekian-Littlecrotonian and Olson’s gap into one event. No reliable data support recognition of “Olson’s extinction” as a single mass extinction event. The only Permian mass extinction of tetrapods is the dinocephalian extinction event during the Gamkan (late Capitanian), which saw the total extinction of dinocephalians and major diversity drops in gorgonopsians and therocephalians. In the Karoo section, by a recent estimate this extinction is a 74-80% loss of generic diversity. The end-Permian tetrapod extinctions are older than the end-Permian marine extinctions. Furthermore, the magnitude of the diversity drop and ecological severity of the extinctions have been greatly overstated. Thus, by generous estimates, less than 10 tetrapod genera went extinct at the end of the Permian, and the pre-extinction vertebrate ecosystem was reorganized after the extinctions in mostly the same clades. The tetrapod extinctions that did occur across the Permo-Triassic boundary are part of a prolonged and complex replacement of therapsids by archosaurs that began during the late Permian and was not complete until the Late Triassic.