PALAEOZOIC/End Permian Extinctions 221 One Event or Two? Early databases of global marine biodiversity recorded a broad peak of elevated extinction rates spanning the entire Late Permian As the data improved, two distinct peaks were resolved: one in the latest Permian (the major event) and one at the end of the Guadalupian This latter event marks the point at which many diverse groups (such as sponges and stenolaemate bryozoans) begin to decline in diversity For the brachiopods and echinoderms, especially the crinoids, it marks the main episode of diversity loss While there is some evidence of climate change at this time, there are also good reasons why the end-Guadalupian event might not be a ‘real’ biotic extinction event The Middle Permian was a time of incredible biodiversity, but the majority of the fossil taxa are endemic to the comprehensively monographed and exquisitely preserved silicified faunas of the Guadalupian reef belts of the southern USA, particularly western Texas At the close of the Guadalupian, sea-level fell across that region, and the overlying sediments are unfossiliferous evaporites The disappearance of the fossils is therefore a result of facies change and not real extinction Globally, Upper Permian fossiliferous marine rocks are relatively poorly known, and the quality (i.e completeness) of the Late Permian fossil record is correspondingly poor Many taxa that were common in the Guadalupian have not been found in Upper Permian rocks but must have been living at that time because they reappear in the Triassic These missing taxa are called Lazarus taxa The presence of many Lazarus taxa indicates that the completeness of the fossil record is low (Figure 3) The reality of the end-Guadalupian event is still debated If it is simply the result of bias in the rock record, then newly described sections should contain many of the taxa previously supposed to have become extinct Certainly this is true of the brachiopods: newly described Upper Permian sections from Tibet contain some taxa that were thought to have vanished earlier Further detailed analysis of this interval is clearly needed Possible Causes Extraterrestrial Impact Initial attempts to find evidence of a Permian–Triassic impact were made in the early 1980s, following the proposal that a massive meteorite impact had caused the end-Cretaceous extinction event (see Mesozoic: End Cretaceous Extinctions) Three pieces of evidence are considered crucial in identifying a meteorite impact event: a crater, an enrichment (‘spike’) of the rare metallic element iridium, and a layer of impact Figure Fluctuations in fossil diversity and the quality of the fossil record of marine sponges through the Permian Triassic extinction and recovery interval: solid line, families of sponges represented by actual fossils; dotted line, number of Lazarus taxa; dashed line, total diversity (sum of fossil taxa and Lazarus taxa) Grey bars show the simple completeness metric (SCM), which is the number of actual fossil taxa as a percentage of the total diversity The SCM gives an estimate of the completeness of the fossil record Note that, as diversity declines after the Gua dalupian, the number of Lazarus taxa increases and so the com pleteness of the fossil record decreases Stratigraphy from Figure Data from The Fossil Record (Benton, 1993) debris containing shocked quartz and tektites However, the few reports of these pieces of evidence in latest Permian rocks are considered to be, at best, equivocal Certainly, there is no Permian–Triassic iridium spike comparable to that found at the Cretaceous–Tertiary boundary, no widespread impact layer, and no large crater that can be accurately correlated with the extinction horizon For an impact event to be a possible cause of the extinction crisis, it must also be demonstrated that the extinctions were unequivocally instantaneous and coincident with the impact itself In 2001, Luann Becker and colleagues reported the presence of helium and argon trapped in the cage-like molecular structure of fullerenes from the Permian– Triassic boundary in China and Japan Isotopic profiles indicated that the helium and argon, and hence the fullerenes, had to have come from an extraterrestrial source However, there are persistent problems with the acceptance of Becker’s data, as other scientists have consistently failed to replicate her results, despite using samples from exactly the same sites and exactly the same laboratory procedures A Japanese expert, Yukio Isozaki, has also argued that the Permian–Triassic boundary is missing from the Japanese section studied by Becker and colleagues