Survivorship and recovery of early triassic tetrapods following the permo-triassic extinction event

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Of the five known mass extinction events, the Permo-Triassic extinction event is regarded unanimously as the most severe in Earth history. The consensus view has been that 70-90% of species became extinct 251 million years ago. Such estimates of high species mortality were based on ranks-based studies of low taxonomic resolution (family-level groups). My own investigations, using phylogenetic studies of higher resolution (species and genera), suggest that terrestrial vertebrates (tetrapods) experienced low extinction pressure during the Permo-Triassic extinction event.     To date I have examined the Permo-Triassic boundary survivorship of amniotes (mammals, reptiles, birds, and their fossil relatives). The two reptile groups examined, procolophonoid parareptiles and archosauromorph diapsids, exhibit roughly 29% and 14-17% extinction, respectively. Current work on therocephalians, a group of synapsid amniotes, suggests that they experienced 43% extinction. Judging from published phylogenies, cynodont synapsids suffered 40-50% extinction. The evidence so far suggests that the amniote extinctions were not as severe as commonly believed, and that the effects of extinction were not even handed among the groups.     Procolophonoids and therocephalians are the most diverse and, after lystrosaurids, the most commonly encountered amniotes in the Karoo Basin of South Africa, the country with the best fossil record for studying the effect of this extinction event on tetrapods. I propose to continue my phylogenetically-based research on the survivorship of the above mentioned amniote groups through the Permo-Triassic boundary. I will expand my program by looking at the post-extinction survivorship of amniotes in the Karoo, in light of a recent suggestion that a "survivor fauna" succumbed to a second pulse of extinctions that followed the main extinctions at the Permo-Triassic boundary. The results of the study will help to elucidate the differences of effect of the Permo-Triassic extinctions on tetrapods with respect to that on plants and marine invertebrates, which experienced a single extinction pulse followed by a recovery period of 4-5 million years.