The largest mass extinction of all time took place at the end of the Permian period 250 mya (Table 3). Of 45 families of animals present during the last 5 my of the Permian, no less than 36 - a loss of 75% - died out. These included 10 families of basal tetrapods (mostly anthracosaurs) and 17 families of therapsids including the gorgonopsians, the last dinocephalians and most of the dicyno-donts. Only 12 families of tetrapods survived. Those that disappeared included a broad array of ecological types from large to small, herbivores and carnivores. At the same time, 50% of marine invertebrate families died out (Benton 2003,2004).
Not surprisingly, the possible cause or causes of the mass extinction are numerous and have been much disputed. Asteroids and meteorites have been blamed, but there was no evidence of impact, and little shocked quartz (Sect. 12.4.1) was found before 2001. In that year, Luann Becker et al. reported that they had measured quantities of helium and argon trapped in large molecules of carbon (known as fullerenes) in geological samples from Hungary, China and Japan. These gases were identical to those previously derived from meteorites, and the authors argued that the Permo-Triassic boundary fullerenes must have come from impact with a meteorite - a claim that has been hotly disputed. Union of the elements of the supercontinent Pangaea (Sect. 2.2) had begun to occur, however, so there must have been considerable tectonic activity at the time. Flood basalt eruptions were undoubtedly taking place in Siberia in a region known as the Siberian Traps. During the course of 1 my, ca. 3 million km3 of lava erupted over a large area, so huge amounts of smoke, carbon dioxide and sulphur dioxide would have been discharged into the atmosphere. The initial reduction in light intensity engendered a drop in temperature, which was followed as the darkness cleared by a rebound to ca. 5 °C above the original ambient temperature. This in itself would probably have been insufficient to explain the mass extinction, but it could have triggered a further rise of ca. 4-5 °C through the release of methane gas from the methane hydrate that accumulated on the continental margins (Hallam and Wignall 1997; Benton 2003). The same argument applies to the volcanic activity that preceded the Cretaceous extinction (Sect. 12.2). In each case, impact with a meteorite or bolide might perhaps have provided the coup de grâce for certain taxa.
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