The endPermian mass extinction and subsequent recovery

The mass extinction at the end of the Permian was recognized already for a long time as the most severe of all the Phanerozoic perturbations (Phillips 1860). The radical faunal change associated with this biotic crisis was the reason to distinguish between the Paleozoic below and the Mesozoic above. More than 50% of marine and terrestrial families went extinct and an estimated 80-96% of all the species. Until recently (Erwin 1990, 1993), the end-Permian mass extinction was seen as a protracted crisis, which lasted for approximately 10 Ma. Newer research has shown that there were actually two discrete events (O Figure 16.9). The first occurred in the latest Guadalupian (terminal Middle Permian) and affected only some groups in a more gradual way (Hallam and Wignall 1997). The event at the very end of the Permian was apparently of quite short duration (probably less than 0.1 Ma; Erwin et al. 2002). It is this interval that has been called "the mother of all extinctions,'' "the great dying,'' or the "Paleozoic nemesis'' (Erwin 1996; Benton 2003). This massive crisis affected all groups of organisms, both in the seas and on land (Benton and Twitchett 2003).

In the Middle Permian, the seas were teeming with life and many different faunal provinces can be distinguished. Highly diverse stromatoporoid-coral reefs were widely distributed. On soft- and hard-bottoms, rich communities dominated by brachiopods and echinoderms flourished, and in the water-column, numerous ammonoids and various fish groups had attained a high diversity.

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On land, insects had reached their highest diversity in the Paleozoic and tetrapod communities were probably as complex as modern mammal communities (Benton 2003). Plants were also highly diverse and distributed in different biogeographical provinces.

During the first extinction event, which occurred at the end of the Middle Permian (Guadalupian), some groups were affected both on land and in the seas but none became entirely extinct (Hallam and Wignall 1997). The reasons for this first extinction pulse are not well understood, but global cooling has been cited as the underlying cause (Stanley 1988; Hallam and Wignall 1997).

The second and by far the more severe extinction pulse near the Permian-Triassic (P-Tr) boundary affected all the taxonomic and ecologic groups, both in the seas and in the terrestrial environment. Among the larger groups that completely went extinct in the seas were the rugose and tabulate corals, fenestrate bryozoans, and orthid brachiopods. On land, glossopterids and cordaitales were suddenly replaced by a low-diversity conifer-lycopod-fern assemblage with little provinciality. Palynological samples from immediately above the P-Tr boundary are dominated by fungal spores which normally account for only a small proportion of the pollen and spores. This "fungal spike'' (Eshet et al. 1995; Visscher et al. 1996), similar in its magnitude to the "fern spike'' at the Cretaceous-Tertiary boundary, might indicate vast areas of rotting plants which were decomposed by fungi (Hallam and Wignall 1997; Benton and Twitchett 2003). A wide range of tetrapods, among them the hitherto dominant pareiasaurs, went abruptly extinct, and the Early Triassic vertebrate faunas were completely dominated by the single genus Lystrosaurus (Hallam and Wignall 1997; Benton and Twitchett 2003; Ward et al. 2005). Both remarkable were the magnitude (up to an estimated 96% of the marine species) and the extraordinary long recovery interval. It took almost 100 Ma for family diversity to reach preextinction levels and almost 10 Ma for complex ecosystems like reefs to become established again (Benton and Twitchett 2003). The first communities that appeared during the recovery interval are composed of a remarkably cosmopolitan, opportunistic fauna of thin-shelled bivalves (e.g., Claraia) and lingulid brachiopods (Hallam and Wignall 1997). Burrowing organisms were almost completely absent, and disaster taxa like stromatolites became locally abundant (Schubert and Bottjer 1992). Lazarus taxa were especially common among the gastropods, and most of them were small ("Lilliput''-effect; Twitchett 2006). The long-term consequences for seafloor communities were the replacement of the hitherto dominant epibenthic sessile suspension feeders by a vagile, epi- and endobenthic, mol-lusk-dominated fauna (Hallam and Wignall 1997). The recovery interval after the end-Permian mass extinction is much longer than for any other extinction event and indicates that the ecosystems were almost completely devastated and severe environmental perturbations continued through the Lower Triassic (Twitchett 1999; Payne et al. 2004).

The scenario for this mass extinction and its likely causes have received much attention in the last decade. All the evidence indicates that the mass extinction occurred during a phase of marine transgression and severe global warming. The catastrophe probably started with the release of huge amounts of CO2 into the atmosphere, first through volcanic eruptions in South China (Emeishan flood basalt province; Lo et al. 2002) and perhaps also through coal oxidation (Hallam and Wignall 1997), later through vast eruptions in Siberia (Siberian traps; Courtillot 1999; Benton and Twitchett 2003; but see Wignall 2001a). This led to global warming and in the seas to decreased ocean circulation and oxygen depletion (Wignall and Twitchett 1996). With further increasing CO2 levels, methane hydrates began to melt and released large quantities of methane, which first acted as greenhouse gas and later was oxidized to CO2. Through this positive feedback, a "runaway greenhouse'' developed, which went out of control after some threshold was reached (Benton and Twitchett 2003). The seas flooding the shelves became anoxic, perhaps even sulfidic (Kump et al. 2005), and killed most of the benthic and pelagic organisms. On land, the vegetation suffered a severe deterioration with equally devastating consequences for the animals, which also experienced hypoxic stress (Huey and Ward 2005).

Inevitably, many additional or alternative explanations have been presented. A major global regression during the terminal Permian was long a popular explanation for the extinction but this is no longer tenable. There was clearly a transgression during the P-Tr boundary interval (Hallam and Wignall 1997). Darkening and global cooling with a collapse in photosynthesis was also proposed as extinction cause (Campbell et al. 1992), but all the evidence points to global warming at the end of the Permian. Older suggestions include brackish oceans and an increase in cosmic radiation. The claim for evidence of an impact is relatively recent (Becker et al. 2001,2004). However, the impact hypothesis is now generally considered invalid (Erwin 2003). The end-Permian mass extinction thus seems truly "home-made'' (Benton and Twitchett 2003).

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