Biotic recovery in the aftermath of the Late Permian mass extinction event : new data from the oceanic seamounts of SW Japan
The Permian–Triassic interval witnessed the most severe mass extinction of the Phanerozoic. Despite its significance for the evolution of the marine biosphere, the subsequent recovery has attracted relatively little research effort. Recent work has demonstrated that the most rapid post-Permian recovery occurred in shallow, seamount settings of Neotethys (Twitchett et al., 2004, Geology, v. 32, pp. 805–808). Is rapid recovery typical of all Early Triassic seamounts, or only those in Neotethys? To address this question we undertook palaeoecological analyses of limestones of the Lower-Middle Triassic Taho Formation of SW Japan, which were deposited in an oceanic seamount setting in Panthalassa. The fauna comprises gastropods, bivalves, brachiopods, echinoderms, foraminifera, ostracods and serpulids. Firstly, the abundance of the different taxa, from thin sections and acetate peels, was assessed. Secondly, the palaeoecological parameters of dominance, evenness and diversity were calculated.From the Griesbachian to the Anisian, dominance decreases slightly (from 1.0 to 0.8) as diversity increases (from 0.4 to 1.0). This contrasts with results from a shallow seamount in Neotethys, where high diversity and low dominance assemblages occur in the middle and late Griesbachian. The Taho Formation seamount limestones show no rapid recovery in the Griesbachian, but, instead, much slower recovery typical of shelf settings worldwide (e.g. western USA, northern Italy). Levels of ecological recovery recorded in the Griesbachian seamount limestones of Neotethys are not recorded in the Taho Formation limestones until the Anisian.