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PALAEOZOIC | End Permian Extinctions
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End Permian Extinctions R J Twitchett, University of Plymouth, Plymouth, UK ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction The extinction event at the close of the Permian period was the largest of the Phanerozoic. Understanding this event is crucial to understanding the history of life on Earth, yet it is only since the late 1980s that scientists have begun to study this event in detail. This is partly because of the discovery of many new geological sections spanning the time of the extinction crisis, partly because of a renewed public and scientific interest in extinction, and partly because of increasing dialogue, travel, and exchanges between geologists and palaeontologists from the West (Europe and America) and those from Russia and China. Over the years, a bewildering number of hypotheses have been suggested to explain the end-Permian extinction crisis, and new data are constantly appearing. There is currently no consensus as to the precise cause, and the current leading contenders are considered below.
with far higher resolution and cementing 251 Ma in the subsequent literature. However, accurate dating is notoriously difficult, and the further back in time one goes the worse the resolution becomes and the greater is the potential error. In 2001, Roland Mundil and colleagues confirmed that there were problems with the Bowring dates. Mundil’s group reassigned the ash bed above the Permian–Triassic boundary an (older) age of 252.7 0.4 Ma. However, the ash bed below the Permian–Triassic boundary, approximating to the level of the mass extinction, proved far more difficult to date: that it is probably older than 254 Ma was the best that the geochronologists could achieve.
Definition and Dating The Permian–Triassic boundary is now defined as the point at which a specific species of fossil conodont (an extinct type of primitive vertebrate) called Hindeodus parvus first appears in a geological section at Meishan in South China (Figure 1). This definition was accepted only after many years of debate: its significance is immense. Palaeontologists from around the world are now able to correlate their local sections with this single reference section, thus unifying a confusing plethora of names that geologists working independently, in different countries, had erected to describe rocks of the same age. It also confirmed that the main extinction event occurred in the latest Permian, before the Permian–Triassic boundary (i.e. before the appearance of H. parvus), in the Changhsingian Stage (Figure 2). The presence of layers of volcanic ash interbedded with the fossiliferous sediments at Meishan have also allowed absolute dating of the boundary interval through the precise measurement of radiogenic elements, such as 206Pb and 238U, in minerals such as zircons. In 1991, Claoue-Long and colleagues dated the Permian–Triassic boundary at 251.2 3.4 Ma. Seven years later, Sam Bowring and his group reanalysed the same beds and arrived at a date of 251.4 0.3 Ma, confirming the earlier result but
Figure 1 Sketch of Permian–Triassic palaeogeography, showing the major regions discussed in the text: 1, South China; 2, Karoo Basin, South Africa; 3, eastern Greenland; 4, Siberia; 5, northern Italy; and 6, western USA.
Figure 2 Permian and Triassic stratigraphy. Arrows indicate the stratigraphical positions of the extinction events discussed in the text.
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Is this change in date and loss of resolution important? For many Permian–Triassic studies it is largely irrelevant. Correlation between marine sections relies on well-established methods of biostratigraphy and is unaffected by such results: the relative sequence of events can be deduced regardless. However, it is a significant blow for correlation between fossiliferous marine sediments and distant terrestrial sections, and for the hypothesis that the Siberian flood basalts were responsible for the extinction crisis (see below). It also means that absolute rates of change, both faunal and environmental, cannot be determined.
What Went Extinct? Estimating the severity of past extinction events is not easy. Owing to the vagaries of preservation and fossil recovery and the difficulty of recognizing biological species from fossils, palaeontologists investigating extinctions tend to work at higher taxonomic levels. Thus, estimates of extinction magnitude are derived from large databases of family-level diversity through time, which show that 49% of marine invertebrate families disappeared during the Permian–Triassic interval. Extinction at the species level is then estimated, rather than directly measured, using a statistical technique called reverse rarefaction. From this method, the often-quoted figure of a 96% loss of marine species is derived. However, such a calculation makes a number of assumptions, including, for example, the assumption that species extinction was random, with no selectivity against certain groups. The balance of evidence indicates that this assumption is incorrect, and, thus, the true magnitude of species loss may be closer to 80%. However, this still makes the end-Permian extinction event the most severe crisis of the Phanerozoic! Marine Extinctions
Despite these shortcomings, it is clear that several groups were severely diminished in number in the Late Permian, some groups went completely extinct, and not all groups were affected equally. For example, the diverse and successful fusulinid foraminifera disappear suddenly during the Changhsingian stage with the loss of some 18 families, whereas other benthic foraminifera suffered much lower levels of extinction. The diversity of the stenolaemate bryozoans also decreased markedly through the latter half of the Permian, but only the Order Fenestrata became extinct. By contrast, other bryozoan groups, such as the gymnolaemate order Ctenostomata, maintained their diversity at the family level.
A pattern of gradual decline throughout the Permian, particularly during the latter half, followed by final extinction of the last few remaining taxa in the Changhsingian, is typical of many groups. Examples include the Palaeozoic corals (Rugosa and Tabulata), trilobites, and goniatites, all of which became extinct, and the articulate brachiopods, which were reduced to a handful of surviving genera. This pattern implies that longer-term (possibly climate driven) changes in the marine realm may have been largely to blame for the loss of diversity. A few groups, such as the echinoderms, suffered dramatic losses prior to the end-Permian. All of the extant echinoderm groups experienced severe bottlenecks during the Permian–Triassic interval: only a couple of lineages of crinoids and echinoids survived into the Mesozoic. However, the major crisis interval appears to have been in the Late Guadalupian, when crinoids experienced over 90% loss and other groups of echinoderms, such as the Blastoidea, became extinct. Terrestrial Extinctions
The extinction on land was just as severe as in the oceans, with some 77% of vertebrate families becoming extinct. In the Karoo Basin of South Africa (Figure 1), only seven out of 44 tetrapod genera (16%) cross the Permian–Triassic boundary. When all fossil terrestrial organisms are considered together, the Permian–Triassic event is the single largest extinction episode in the otherwise exponential rise of terrestrial diversity through the Phanerozoic. A long-term (more than 20 Ma) change in terrestrial vegetation occurred during the Permian–Triassic interval. Termed the Palaeophytic–Mesophytic transition, it probably reflects gradual shifts in climate and palaeogeography. However, superimposed on this longer-term trend is a catastrophic collapse of the dominant gymnosperm forests in the latest Permian, as evidenced by the disappearance of pollen taxa such as Vittatina, Weylandites, and Lueckisporites. Studies in eastern Greenland (Figure 1) have shown that this ecological crisis occurred simultaneously with the marine extinction event. Some workers have suggested that a peak in the abundance of Reduviasporonites (also called Tympanicysta), observed in some sections, marks the sudden destruction of these forests. They have interpreted this taxon as a saprophytic fungus, which thrived on the piles of dead and dying vegetation. Unfortunately, this attractive scenario must now be rejected. Recent geochemical, structural, and biomarker studies have shown that Reduviasporonites is definitely not a fungus and is probably a photosynthetic alga.
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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 3 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 Guadalupian, the number of Lazarus taxa increases and so the completeness of the fossil record decreases. Stratigraphy from Figure 2. Data from The Fossil Record 2 (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
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and that their samples came from below the extinction level. Later in 2001, Kaiho and colleagues described Permian–Triassic sediment grains that were supposedly formed by impact, as well as geochemical shifts that they interpreted as indicative of a huge impact. However, their data are far from conclusive and were rapidly and severely criticized by other geochemists. In 2003, Asish Basu and colleagues revived interest by claiming to have found 40 tiny (50–400 mm) unaltered fragments of meteorite in a sediment sample from a terrestrial Permian–Triassic boundary section in Antarctica. Although Basu and colleagues were quick to dismiss contamination as a source of these grains, meteorite experts were immediately sceptical of the findings, as meteoritic metals are highly reactive and, in terrestrial settings, oxidize extremely quickly. Doubtless scientists will continue to produce evidence of extraterrestrial impact at or near the Permian–Triassic boundary, if only because such hypotheses readily appeal to the wider public and journal editors alike. However, current data are highly controversial and lack the key criteria of major impact that have been recorded time and again in Cretaceous–Tertiary sections the world over. Independent replication of results is crucial for scientific acceptance, especially when the data are unusual and controversial. So far, all the evidence proposed for an impact at the Permian–Triassic boundary has failed this necessary test. Eruption
The largest outpouring of continental flood basalts in the Phanerozoic occurred in Siberia during the Permian–Triassic interval. Including both the Siberian Platform basalts and the newly discovered coeval deposits buried in the West Siberia Basin, described by Marc Reichow and colleagues, the flood basalts covered an area of 1.6 106 km2 to maximum depths of 3.5 km (Figure 1). If all other igneous rocks, such as pyroclastic flows, are included, then this coverage increases to 3.9 106 km2. Dating the top and bottom of the lava pile shows that the eruptions occurred over a relatively short period of time, maybe just 600 000 years. Was this huge volcanic event a cause of the Permian–Triassic extinction crisis? Radiometric dating is the only way to answer this question because no fossils have yet been collected from sediments interbedded with the basalts that provide correlation with other regions. Early efforts at dating the Siberian Traps produced an array of ages, from 160 Ma to 280 Ma. By contrast,
more recent results, by different scientists using a variety of methods, cluster around 250 1 Ma. In the late 1990s, this was considered to be exactly the date required, and the flood basalts were thought to be the primary trigger for the catastrophic extinction. However, the recent redating of the Meishan beds now implies that the Permian–Triassic event occurred before 253 Ma, and the Siberian Traps are therefore several million years too young. On a Phanerozoic time-scale, the excellent correlation between extinction episodes and flood-basalt provinces means that it is difficult to accept the possibility that the Siberian Traps played no role in the end-Permian extinction event. There is still a chance. The oldest date that Reichow’s team have for the onset of volcanic activity (dating the emplacement of intrusive gabbros) is 253.4 0.8 Ma. Another intriguing consequence of the 253 Ma age for the Permian–Triassic boundary is that the Emeishan flood basalts of western South China now need to be considered. Dating by Ching-Hua Lo and colleagues (published in 2002) shows that the main eruptions, which were largely marginal marine events, occurred around 251–253 Ma, with an initial phase of activity at around 255 Ma; thus, they predate the Siberian Traps and are dated close to the new dates for the Permian–Triassic crisis. Interestingly, Ching-Hua Lo’s new dates preclude the possibility that the Emeishan flood basalts played a role in the end-Guadalupian crisis (at about 256–259 Ma; see above). Mei-Fu Zhou and colleagues had championed this particular correlation following their dating of the Emeishan basalts to 259 3 Ma, which was published just a few months before Ching-Hua Lo’s results in the same journal! Absolute dating is a difficult business (see Analytical Methods: Geochronological Techniques). Global Warming
Animals and plants do not curl up and die just because a volcano is erupting on the other side of the world. Species become extinct when their population falls below viable levels. Population decline in natural systems is a response to local changes, such as loss of habitat, drought, and temperature change, many of which are ultimately driven by climate. What climate changes are recorded around the extinction interval and how might they be related to the flood-basalt volcanism? Global cooling was proposed early on as a cause of the Permian–Triassic event; this largely hinged on the identification of latest-Permian glacial deposits in Siberia and eastern Australia. However, reanalysis of the biostratigraphy proved that these deposits are,
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in fact, Middle Permian in age. With evidence of extensive ice sheets confined to the Early Permian and the last vestiges of glaciation now dated as Middle Permian, it is evident that the Permian as a whole records a warming trend. At the culmination of this long-term warming trend, there is also evidence for an additional rapid greenhouse episode at the time of the extinction crisis. A large drop in the proportion of heavy oxygen isotopes (18O) in carbonates from the Gartnerkofel-1 core of southern Austria has been interpreted as representing an increase in temperature of 6 C. However, interpretation of the oxygen isotope record is problematic as it is very sensitive to alteration during burial and diagenesis. The limestones of Gartnerkofel-1 have been largely recrystallized, especially around the boundary interval, and thus the oxygen isotope data are surely suspect. However, there is growing independent support for global warming at the boundary. Gregory Retallack has demonstrated, from studies of the stomatal index of fossil leaves, that peaks in atmospheric CO2 (a proven greenhouse gas) occur at both the endPermian and end-Guadalupian boundaries. His studies of fossil soils also indicate a change to humid greenhouse conditions in the Early Triassic. Finally, there is a large (usually 3–6%) increase in the proportion of light 12C, relative to heavier 13C, in carbonates deposited at this time. This shift is interpreted by most geochemists to be the result of methane (CH4) release, as this is the only mechanism currently known that could rapidly deliver the required amount of light 12C to the atmosphere and oceans. This methane, which is also a potent greenhouse gas, is assumed to have been produced by the melting of methane hydrate deposits in shallow marine shelves and polar tundra. Thus, a runaway greenhouse scenario is envisaged, whereby warming led to melting of gas hydrates and release of methane, which fuelled further warming and further methane release, in a positive-feedback loop that presumably continued until the reserves of gas hydrate were depleted (Figure 4). Flood basalts are assigned a triggering role in this runaway greenhouse model, by venting enough initial CO2 to drive temperatures above the threshold for gas-hydrate breakdown. However, volcanic eruptions also reduce global temperatures by releasing particulates and SO2. Some authors have incorporated a cooling episode into their extinction models, but regard it as a brief event, which was subsequently eclipsed by the later warming. There is currently no geological evidence to support an episode of global cooling at the onset of the Permian–Triassic extinction crisis.
Figure 4 The runaway greenhouse model for the end-Permian extinction event. See text for details.
Post-Extinction Recovery Douglas Erwin has remarked that the duration of the post-Permian recovery interval is the greatest of any Phanerozoic mass extinction and is proportionally longer than would be expected given the magnitude of the species loss. Certainly, a literal reading of the fossil record shows that it took some 100 Ma (until the mid-Jurassic) for marine biodiversity at the family level to return to pre-extinction levels. However, ecological recovery was somewhat quicker, with complex communities such as reefs being reestablished some 10 Ma after the Permian–Triassic boundary. Thus, how one views the duration of the recovery interval, measured on a local or global scale, depends on how one defines ‘recovery’. Details of the marine recovery are best known from the low palaeolatitudes of Panthalassa (the presentday western USA) and Palaeotethys (e.g. northern Italy) (Figure 1). Epifaunal benthic communities in the immediate aftermath of the event (the Early Induan) comprise low-diversity assemblages of small suspension feeders (typified by the bivalves Promyalina and Claraia and often the inarticulate brachiopod Lingula). Bedding planes are dominated by a single taxon, which may occur in prodigious numbers. These taxa are considered by many, with varying degrees of evidence, to be pioneering r-selected opportunists (i.e. animals that mature early and produce many offspring). Stromatolites and other microbial mats are commonly encountered and may have provided grazing opportunities for the depauperate microgastropod community. Occasional horizons of mm-diameter Planolites burrows attest to temporary colonization events by a scarce infauna
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of small deposit feeders living just 1–2 cm below the sediment surface. In the higher palaeolatitude regions of the Boreal Ocean (Greenland, Spitsbergen) and NeoTethys (Madagascar, western Australia) a fairly diverse but small-sized nekton of fishes and ammonoids is recorded. Sedimentary and geochemical data indicate that environmental conditions were particularly harsh. Most recent palaeoenvironmental studies have concluded that the Early Triassic oceans contained very low concentrations of dissolved oxygen and may have been anoxic or even euxinic for considerable periods of time. Low oxygen levels were probably the result of a combination of factors. First, levels of atmospheric oxygen had fallen steadily from 30% in the Early Permian to approximately 15% (i.e. 70% of present-day values) by the Permian–Triassic boundary. Second, global warming in the latest Permian (see above) would have increased sea-surface temperatures, and warm water holds less dissolved oxygen than cool water. Finally, climate models indicate that global warming would also lead to a reduction in the temperature gradient between the poles and the equator, thus slowing down thermohaline circulation. In present-day oceans, the temperature difference between the warm equator and the cold poles drives a relatively vigorous thermohaline circulation, which maintains oxygenation of the ocean floors. In the sluggish earliest Triassic oceans, oxygen would have been used up faster than it could be replaced, resulting in stagnation and anoxia. The ecology of the post-extinction survivors and the long interval of low diversity are interpreted as being the result of this anoxic event. In the few places where the oceans were well-oxygenated in the immediate aftermath of the extinction (e.g. Oman), a far more diverse benthic community, comprising upwards of 20 genera, was soon established. Unfortunately, these localized experiments in rapid post-extinction recovery were snuffed out by the arrival of anoxia in the later Induan. With the disappearance of benthic oxygen restriction later in the Early Triassic, larger organisms and more diverse communities reappeared. The ecological complexity of infaunal communities began to increase with the reappearance of deeper-burrowing suspension feeders (producing Arenicolites, Skolithos, and Diplocraterion burrows). Burrowing crustaceans (producing traces such as Thalassinoides) are the last significant component of the infauna to reappear. In the palaeotropics, Thalassinoides first reappears in the Middle Triassic. In higher palaeolatitudes (East Greenland, Spitsbergen), small Thalassinoides reappear in the later Induan, suggesting faster recovery in the Boreal Realm.
In most parts of the world, epifaunal complexity did not significantly recover until the Late Olenekian, with an increase in tiering (vertical stratification of organisms above the sediment surface) as crinoids and bryozoans returned. However, the most complex ecological structures – metazoan reefs – do not return until the Middle Triassic, accompanied by a significant increase in faunal diversity and the reappearance of many Lazarus taxa. Our understanding of the recovery of terrestrial ecosystems is less refined, with most data deriving from the Karoo Basin of South Africa. Like the marine survivors, terrestrial vertebrate survivors tended to be small: in the Karoo Basin five small carnivorous therocephalians and one small anapsid (Procolophon) survived the Permian–Triassic crisis. The other survivor, and the dominant terrestrial vertebrate for several million years, was the herbivore Lystrosaurus, which, like Claraia, was globally widespread. More is known of the recovery of the terrestrial vegetation. The most detailed studies are those of Cindy Looy and colleagues, who have analysed fossil spores and pollen from Europe and eastern Greenland. Following the collapse of the Late Permian gymnosperm forests, open herbaceous vegetation rapidly took over, with short-lived blooms of pioneering opportunistic lycopsids, ferns, and bryophytes – stress-tolerant forms that were subordinate members of the pre-crisis vegetation. Pollen from woody gymnosperms seems to indicate that a few surviving elements of the Permian forests lingered on for a while, but equally these records could just represent reworking of the pollen. Certainly, by the earliest Triassic no tree-like gymnosperms remained, and a stable low-diversity open-shrubland vegetation of cycads and lycopsids was established. Complex and diverse forest communities did not reappear, at least in Europe, until the latest Olenekian and early Middle Triassic. Thus, the return of ecological complexity on land closely mirrors that in epifaunal marine communities.
Conclusions The greatest mass-extinction event of the Phanerozoic is receiving an unprecedented level of scientific interest. New sections are being discovered and described, allowing new hypotheses of causes and consequences to be tested and old ones to be modified or rejected. The 1990s witnessed a number of major advances, culminating in the acceptance of Meishan, China, as the appropriate locality to define the Permian–Triassic boundary and the subsequent formalization of Permian–Triassic stratigraphy. The
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picture presented here is a current (2004) view of this crisis: a snapshot that doubtless will be refined in the decade to come.
See Also Analytical Methods: Geochronological Techniques. Carbon Cycle. Impact Structures. Large Igneous Provinces. Mesozoic: Triassic; End Cretaceous Extinctions. Palaeoclimates. Palaeozoic: Permian. Sedimentary Environments: Anoxic Environments.
Further Reading Benton MJ (ed.) (1993) The Fossil Record 2. London: Chapman & Hall.
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Benton MJ (2003) When Life Nearly Died: The Greatest Mass Extinction of all Time. London: Thames and Hudson. Benton MJ and Twitchett RJ (2003) How to kill all life: the end-Permian extinction event. Trends in Ecology and Evolution 18: 358–365. Erwin DH (1993) The Great Paleozoic Crisis: Life and Death in the Permian. New York: Columbia University Press. Erwin DH (1994) The Permo-Triassic extinction. Nature 367: 231–236. Erwin DH (1996) Understanding biotic recoveries: extinction, survival and preservation during the end-Permian mass extinction. In: Jablonski D, Erwin DH, and Lipps JH (eds.) Evolutionary Paleobiology, pp. 398–418. Chicago: University of Chicago Press. Hallam A and Wignall PB (1997) Mass Extinctions and Their Aftermath. Oxford: Oxford University Press.
PANGAEA S G Lucas, New Mexico Museum of Natural History, Albuquerque, NM, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Geologists apply the term Pangaea (from the Greek words meaning ‘all Earth’, and usually pronounced pan-JEE-uh) to the supercontinent that existed during the Late Palaeozoic and Early Mesozoic, about 300–200 Ma ago. Pangaea was an amalgamation of continental blocks that were the precursors of the existing continents, though much of eastern Asia was an archipelago of islands loosely connected to eastern Pangaea.
Components of Pangaea Pangaea (Figure 1) was accreted from continental blocks that differed from today’s continents. The southern supercontinent of Gondwana (also called Gondwanaland) (see Gondwanaland and Gondwana) was the Palaeozoic amalgamation of South America, Africa, Antarctica, Australia, and the IndoPakistani subcontinent, as well as several smaller terranes. The northern supercontinent of Laurussia (or Laurasia) consisted of North America and most of Europe. Asia, however, did not exist as a single block. The continental nucleus of present-day Asia was the Siberian block, but other blocks included Kazakhstan,
Tarim, and a loose archipelago of blocks that were to become much of China and South-east Asia. Many of these eastern Asian blocks originated as slivers of northern Gondwana and drifted northwards during the Late Palaeozoic and earliest Mesozoic to accrete to the larger Asian blocks during the Late Triassic–Jurassic. Pangaea was surrounded by a universal ocean called Panthalassa (from the Greek words meaning ‘all sea’), which was the ancestor of today’s Pacific Ocean. The Atlantic Ocean did not exist even in ancestral form, because of the fusions of North America to Europe and of South America to Africa. An arm of Panthalassa formed a deep east–west embayment in the eastern edge of Pangaea (Figure 1). This was the Tethys Sea, which is named after the sister and consort of Oceanus in Greek mythology. Tethys was the ancestor of the present Mediterranean Sea. Successive ocean basins within it are termed Palaeotethys (Devonian–Permian) and Neotethys; the latter opened during the Late Permian as a result of rifting between Gondwana and the smaller central and southern Asian terranes.
Late Carboniferous Accretion of Pangaea The Pangaean supercontinent came together (accreted) at the end of the Carboniferous (see Palaeozoic: Carboniferous), when Laurussia and Gondwana were sutured along what has been termed the Hercynian megasuture. Very old mountain ranges mark the
End Permian Extinctions R J Twitchett, University of Plymouth, Plymouth, UK ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction The extinction event at the close of the Permian period was the largest of the Phanerozoic. Understanding this event is crucial to understanding the history of life on Earth, yet it is only since the late 1980s that scientists have begun to study this event in detail. This is partly because of the discovery of many new geological sections spanning the time of the extinction crisis, partly because of a renewed public and scientific interest in extinction, and partly because of increasing dialogue, travel, and exchanges between geologists and palaeontologists from the West (Europe and America) and those from Russia and China. Over the years, a bewildering number of hypotheses have been suggested to explain the end-Permian extinction crisis, and new data are constantly appearing. There is currently no consensus as to the precise cause, and the current leading contenders are considered below.
with far higher resolution and cementing 251 Ma in the subsequent literature. However, accurate dating is notoriously difficult, and the further back in time one goes the worse the resolution becomes and the greater is the potential error. In 2001, Roland Mundil and colleagues confirmed that there were problems with the Bowring dates. Mundil’s group reassigned the ash bed above the Permian–Triassic boundary an (older) age of 252.7 0.4 Ma. However, the ash bed below the Permian–Triassic boundary, approximating to the level of the mass extinction, proved far more difficult to date: that it is probably older than 254 Ma was the best that the geochronologists could achieve.
Definition and Dating The Permian–Triassic boundary is now defined as the point at which a specific species of fossil conodont (an extinct type of primitive vertebrate) called Hindeodus parvus first appears in a geological section at Meishan in South China (Figure 1). This definition was accepted only after many years of debate: its significance is immense. Palaeontologists from around the world are now able to correlate their local sections with this single reference section, thus unifying a confusing plethora of names that geologists working independently, in different countries, had erected to describe rocks of the same age. It also confirmed that the main extinction event occurred in the latest Permian, before the Permian–Triassic boundary (i.e. before the appearance of H. parvus), in the Changhsingian Stage (Figure 2). The presence of layers of volcanic ash interbedded with the fossiliferous sediments at Meishan have also allowed absolute dating of the boundary interval through the precise measurement of radiogenic elements, such as 206Pb and 238U, in minerals such as zircons. In 1991, Claoue-Long and colleagues dated the Permian–Triassic boundary at 251.2 3.4 Ma. Seven years later, Sam Bowring and his group reanalysed the same beds and arrived at a date of 251.4 0.3 Ma, confirming the earlier result but
Figure 1 Sketch of Permian–Triassic palaeogeography, showing the major regions discussed in the text: 1, South China; 2, Karoo Basin, South Africa; 3, eastern Greenland; 4, Siberia; 5, northern Italy; and 6, western USA.
Figure 2 Permian and Triassic stratigraphy. Arrows indicate the stratigraphical positions of the extinction events discussed in the text.
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Is this change in date and loss of resolution important? For many Permian–Triassic studies it is largely irrelevant. Correlation between marine sections relies on well-established methods of biostratigraphy and is unaffected by such results: the relative sequence of events can be deduced regardless. However, it is a significant blow for correlation between fossiliferous marine sediments and distant terrestrial sections, and for the hypothesis that the Siberian flood basalts were responsible for the extinction crisis (see below). It also means that absolute rates of change, both faunal and environmental, cannot be determined.
What Went Extinct? Estimating the severity of past extinction events is not easy. Owing to the vagaries of preservation and fossil recovery and the difficulty of recognizing biological species from fossils, palaeontologists investigating extinctions tend to work at higher taxonomic levels. Thus, estimates of extinction magnitude are derived from large databases of family-level diversity through time, which show that 49% of marine invertebrate families disappeared during the Permian–Triassic interval. Extinction at the species level is then estimated, rather than directly measured, using a statistical technique called reverse rarefaction. From this method, the often-quoted figure of a 96% loss of marine species is derived. However, such a calculation makes a number of assumptions, including, for example, the assumption that species extinction was random, with no selectivity against certain groups. The balance of evidence indicates that this assumption is incorrect, and, thus, the true magnitude of species loss may be closer to 80%. However, this still makes the end-Permian extinction event the most severe crisis of the Phanerozoic! Marine Extinctions
Despite these shortcomings, it is clear that several groups were severely diminished in number in the Late Permian, some groups went completely extinct, and not all groups were affected equally. For example, the diverse and successful fusulinid foraminifera disappear suddenly during the Changhsingian stage with the loss of some 18 families, whereas other benthic foraminifera suffered much lower levels of extinction. The diversity of the stenolaemate bryozoans also decreased markedly through the latter half of the Permian, but only the Order Fenestrata became extinct. By contrast, other bryozoan groups, such as the gymnolaemate order Ctenostomata, maintained their diversity at the family level.
A pattern of gradual decline throughout the Permian, particularly during the latter half, followed by final extinction of the last few remaining taxa in the Changhsingian, is typical of many groups. Examples include the Palaeozoic corals (Rugosa and Tabulata), trilobites, and goniatites, all of which became extinct, and the articulate brachiopods, which were reduced to a handful of surviving genera. This pattern implies that longer-term (possibly climate driven) changes in the marine realm may have been largely to blame for the loss of diversity. A few groups, such as the echinoderms, suffered dramatic losses prior to the end-Permian. All of the extant echinoderm groups experienced severe bottlenecks during the Permian–Triassic interval: only a couple of lineages of crinoids and echinoids survived into the Mesozoic. However, the major crisis interval appears to have been in the Late Guadalupian, when crinoids experienced over 90% loss and other groups of echinoderms, such as the Blastoidea, became extinct. Terrestrial Extinctions
The extinction on land was just as severe as in the oceans, with some 77% of vertebrate families becoming extinct. In the Karoo Basin of South Africa (Figure 1), only seven out of 44 tetrapod genera (16%) cross the Permian–Triassic boundary. When all fossil terrestrial organisms are considered together, the Permian–Triassic event is the single largest extinction episode in the otherwise exponential rise of terrestrial diversity through the Phanerozoic. A long-term (more than 20 Ma) change in terrestrial vegetation occurred during the Permian–Triassic interval. Termed the Palaeophytic–Mesophytic transition, it probably reflects gradual shifts in climate and palaeogeography. However, superimposed on this longer-term trend is a catastrophic collapse of the dominant gymnosperm forests in the latest Permian, as evidenced by the disappearance of pollen taxa such as Vittatina, Weylandites, and Lueckisporites. Studies in eastern Greenland (Figure 1) have shown that this ecological crisis occurred simultaneously with the marine extinction event. Some workers have suggested that a peak in the abundance of Reduviasporonites (also called Tympanicysta), observed in some sections, marks the sudden destruction of these forests. They have interpreted this taxon as a saprophytic fungus, which thrived on the piles of dead and dying vegetation. Unfortunately, this attractive scenario must now be rejected. Recent geochemical, structural, and biomarker studies have shown that Reduviasporonites is definitely not a fungus and is probably a photosynthetic alga.
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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 3 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 Guadalupian, the number of Lazarus taxa increases and so the completeness of the fossil record decreases. Stratigraphy from Figure 2. Data from The Fossil Record 2 (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
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and that their samples came from below the extinction level. Later in 2001, Kaiho and colleagues described Permian–Triassic sediment grains that were supposedly formed by impact, as well as geochemical shifts that they interpreted as indicative of a huge impact. However, their data are far from conclusive and were rapidly and severely criticized by other geochemists. In 2003, Asish Basu and colleagues revived interest by claiming to have found 40 tiny (50–400 mm) unaltered fragments of meteorite in a sediment sample from a terrestrial Permian–Triassic boundary section in Antarctica. Although Basu and colleagues were quick to dismiss contamination as a source of these grains, meteorite experts were immediately sceptical of the findings, as meteoritic metals are highly reactive and, in terrestrial settings, oxidize extremely quickly. Doubtless scientists will continue to produce evidence of extraterrestrial impact at or near the Permian–Triassic boundary, if only because such hypotheses readily appeal to the wider public and journal editors alike. However, current data are highly controversial and lack the key criteria of major impact that have been recorded time and again in Cretaceous–Tertiary sections the world over. Independent replication of results is crucial for scientific acceptance, especially when the data are unusual and controversial. So far, all the evidence proposed for an impact at the Permian–Triassic boundary has failed this necessary test. Eruption
The largest outpouring of continental flood basalts in the Phanerozoic occurred in Siberia during the Permian–Triassic interval. Including both the Siberian Platform basalts and the newly discovered coeval deposits buried in the West Siberia Basin, described by Marc Reichow and colleagues, the flood basalts covered an area of 1.6 106 km2 to maximum depths of 3.5 km (Figure 1). If all other igneous rocks, such as pyroclastic flows, are included, then this coverage increases to 3.9 106 km2. Dating the top and bottom of the lava pile shows that the eruptions occurred over a relatively short period of time, maybe just 600 000 years. Was this huge volcanic event a cause of the Permian–Triassic extinction crisis? Radiometric dating is the only way to answer this question because no fossils have yet been collected from sediments interbedded with the basalts that provide correlation with other regions. Early efforts at dating the Siberian Traps produced an array of ages, from 160 Ma to 280 Ma. By contrast,
more recent results, by different scientists using a variety of methods, cluster around 250 1 Ma. In the late 1990s, this was considered to be exactly the date required, and the flood basalts were thought to be the primary trigger for the catastrophic extinction. However, the recent redating of the Meishan beds now implies that the Permian–Triassic event occurred before 253 Ma, and the Siberian Traps are therefore several million years too young. On a Phanerozoic time-scale, the excellent correlation between extinction episodes and flood-basalt provinces means that it is difficult to accept the possibility that the Siberian Traps played no role in the end-Permian extinction event. There is still a chance. The oldest date that Reichow’s team have for the onset of volcanic activity (dating the emplacement of intrusive gabbros) is 253.4 0.8 Ma. Another intriguing consequence of the 253 Ma age for the Permian–Triassic boundary is that the Emeishan flood basalts of western South China now need to be considered. Dating by Ching-Hua Lo and colleagues (published in 2002) shows that the main eruptions, which were largely marginal marine events, occurred around 251–253 Ma, with an initial phase of activity at around 255 Ma; thus, they predate the Siberian Traps and are dated close to the new dates for the Permian–Triassic crisis. Interestingly, Ching-Hua Lo’s new dates preclude the possibility that the Emeishan flood basalts played a role in the end-Guadalupian crisis (at about 256–259 Ma; see above). Mei-Fu Zhou and colleagues had championed this particular correlation following their dating of the Emeishan basalts to 259 3 Ma, which was published just a few months before Ching-Hua Lo’s results in the same journal! Absolute dating is a difficult business (see Analytical Methods: Geochronological Techniques). Global Warming
Animals and plants do not curl up and die just because a volcano is erupting on the other side of the world. Species become extinct when their population falls below viable levels. Population decline in natural systems is a response to local changes, such as loss of habitat, drought, and temperature change, many of which are ultimately driven by climate. What climate changes are recorded around the extinction interval and how might they be related to the flood-basalt volcanism? Global cooling was proposed early on as a cause of the Permian–Triassic event; this largely hinged on the identification of latest-Permian glacial deposits in Siberia and eastern Australia. However, reanalysis of the biostratigraphy proved that these deposits are,
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in fact, Middle Permian in age. With evidence of extensive ice sheets confined to the Early Permian and the last vestiges of glaciation now dated as Middle Permian, it is evident that the Permian as a whole records a warming trend. At the culmination of this long-term warming trend, there is also evidence for an additional rapid greenhouse episode at the time of the extinction crisis. A large drop in the proportion of heavy oxygen isotopes (18O) in carbonates from the Gartnerkofel-1 core of southern Austria has been interpreted as representing an increase in temperature of 6 C. However, interpretation of the oxygen isotope record is problematic as it is very sensitive to alteration during burial and diagenesis. The limestones of Gartnerkofel-1 have been largely recrystallized, especially around the boundary interval, and thus the oxygen isotope data are surely suspect. However, there is growing independent support for global warming at the boundary. Gregory Retallack has demonstrated, from studies of the stomatal index of fossil leaves, that peaks in atmospheric CO2 (a proven greenhouse gas) occur at both the endPermian and end-Guadalupian boundaries. His studies of fossil soils also indicate a change to humid greenhouse conditions in the Early Triassic. Finally, there is a large (usually 3–6%) increase in the proportion of light 12C, relative to heavier 13C, in carbonates deposited at this time. This shift is interpreted by most geochemists to be the result of methane (CH4) release, as this is the only mechanism currently known that could rapidly deliver the required amount of light 12C to the atmosphere and oceans. This methane, which is also a potent greenhouse gas, is assumed to have been produced by the melting of methane hydrate deposits in shallow marine shelves and polar tundra. Thus, a runaway greenhouse scenario is envisaged, whereby warming led to melting of gas hydrates and release of methane, which fuelled further warming and further methane release, in a positive-feedback loop that presumably continued until the reserves of gas hydrate were depleted (Figure 4). Flood basalts are assigned a triggering role in this runaway greenhouse model, by venting enough initial CO2 to drive temperatures above the threshold for gas-hydrate breakdown. However, volcanic eruptions also reduce global temperatures by releasing particulates and SO2. Some authors have incorporated a cooling episode into their extinction models, but regard it as a brief event, which was subsequently eclipsed by the later warming. There is currently no geological evidence to support an episode of global cooling at the onset of the Permian–Triassic extinction crisis.
Figure 4 The runaway greenhouse model for the end-Permian extinction event. See text for details.
Post-Extinction Recovery Douglas Erwin has remarked that the duration of the post-Permian recovery interval is the greatest of any Phanerozoic mass extinction and is proportionally longer than would be expected given the magnitude of the species loss. Certainly, a literal reading of the fossil record shows that it took some 100 Ma (until the mid-Jurassic) for marine biodiversity at the family level to return to pre-extinction levels. However, ecological recovery was somewhat quicker, with complex communities such as reefs being reestablished some 10 Ma after the Permian–Triassic boundary. Thus, how one views the duration of the recovery interval, measured on a local or global scale, depends on how one defines ‘recovery’. Details of the marine recovery are best known from the low palaeolatitudes of Panthalassa (the presentday western USA) and Palaeotethys (e.g. northern Italy) (Figure 1). Epifaunal benthic communities in the immediate aftermath of the event (the Early Induan) comprise low-diversity assemblages of small suspension feeders (typified by the bivalves Promyalina and Claraia and often the inarticulate brachiopod Lingula). Bedding planes are dominated by a single taxon, which may occur in prodigious numbers. These taxa are considered by many, with varying degrees of evidence, to be pioneering r-selected opportunists (i.e. animals that mature early and produce many offspring). Stromatolites and other microbial mats are commonly encountered and may have provided grazing opportunities for the depauperate microgastropod community. Occasional horizons of mm-diameter Planolites burrows attest to temporary colonization events by a scarce infauna
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of small deposit feeders living just 1–2 cm below the sediment surface. In the higher palaeolatitude regions of the Boreal Ocean (Greenland, Spitsbergen) and NeoTethys (Madagascar, western Australia) a fairly diverse but small-sized nekton of fishes and ammonoids is recorded. Sedimentary and geochemical data indicate that environmental conditions were particularly harsh. Most recent palaeoenvironmental studies have concluded that the Early Triassic oceans contained very low concentrations of dissolved oxygen and may have been anoxic or even euxinic for considerable periods of time. Low oxygen levels were probably the result of a combination of factors. First, levels of atmospheric oxygen had fallen steadily from 30% in the Early Permian to approximately 15% (i.e. 70% of present-day values) by the Permian–Triassic boundary. Second, global warming in the latest Permian (see above) would have increased sea-surface temperatures, and warm water holds less dissolved oxygen than cool water. Finally, climate models indicate that global warming would also lead to a reduction in the temperature gradient between the poles and the equator, thus slowing down thermohaline circulation. In present-day oceans, the temperature difference between the warm equator and the cold poles drives a relatively vigorous thermohaline circulation, which maintains oxygenation of the ocean floors. In the sluggish earliest Triassic oceans, oxygen would have been used up faster than it could be replaced, resulting in stagnation and anoxia. The ecology of the post-extinction survivors and the long interval of low diversity are interpreted as being the result of this anoxic event. In the few places where the oceans were well-oxygenated in the immediate aftermath of the extinction (e.g. Oman), a far more diverse benthic community, comprising upwards of 20 genera, was soon established. Unfortunately, these localized experiments in rapid post-extinction recovery were snuffed out by the arrival of anoxia in the later Induan. With the disappearance of benthic oxygen restriction later in the Early Triassic, larger organisms and more diverse communities reappeared. The ecological complexity of infaunal communities began to increase with the reappearance of deeper-burrowing suspension feeders (producing Arenicolites, Skolithos, and Diplocraterion burrows). Burrowing crustaceans (producing traces such as Thalassinoides) are the last significant component of the infauna to reappear. In the palaeotropics, Thalassinoides first reappears in the Middle Triassic. In higher palaeolatitudes (East Greenland, Spitsbergen), small Thalassinoides reappear in the later Induan, suggesting faster recovery in the Boreal Realm.
In most parts of the world, epifaunal complexity did not significantly recover until the Late Olenekian, with an increase in tiering (vertical stratification of organisms above the sediment surface) as crinoids and bryozoans returned. However, the most complex ecological structures – metazoan reefs – do not return until the Middle Triassic, accompanied by a significant increase in faunal diversity and the reappearance of many Lazarus taxa. Our understanding of the recovery of terrestrial ecosystems is less refined, with most data deriving from the Karoo Basin of South Africa. Like the marine survivors, terrestrial vertebrate survivors tended to be small: in the Karoo Basin five small carnivorous therocephalians and one small anapsid (Procolophon) survived the Permian–Triassic crisis. The other survivor, and the dominant terrestrial vertebrate for several million years, was the herbivore Lystrosaurus, which, like Claraia, was globally widespread. More is known of the recovery of the terrestrial vegetation. The most detailed studies are those of Cindy Looy and colleagues, who have analysed fossil spores and pollen from Europe and eastern Greenland. Following the collapse of the Late Permian gymnosperm forests, open herbaceous vegetation rapidly took over, with short-lived blooms of pioneering opportunistic lycopsids, ferns, and bryophytes – stress-tolerant forms that were subordinate members of the pre-crisis vegetation. Pollen from woody gymnosperms seems to indicate that a few surviving elements of the Permian forests lingered on for a while, but equally these records could just represent reworking of the pollen. Certainly, by the earliest Triassic no tree-like gymnosperms remained, and a stable low-diversity open-shrubland vegetation of cycads and lycopsids was established. Complex and diverse forest communities did not reappear, at least in Europe, until the latest Olenekian and early Middle Triassic. Thus, the return of ecological complexity on land closely mirrors that in epifaunal marine communities.
Conclusions The greatest mass-extinction event of the Phanerozoic is receiving an unprecedented level of scientific interest. New sections are being discovered and described, allowing new hypotheses of causes and consequences to be tested and old ones to be modified or rejected. The 1990s witnessed a number of major advances, culminating in the acceptance of Meishan, China, as the appropriate locality to define the Permian–Triassic boundary and the subsequent formalization of Permian–Triassic stratigraphy. The
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picture presented here is a current (2004) view of this crisis: a snapshot that doubtless will be refined in the decade to come.
See Also Analytical Methods: Geochronological Techniques. Carbon Cycle. Impact Structures. Large Igneous Provinces. Mesozoic: Triassic; End Cretaceous Extinctions. Palaeoclimates. Palaeozoic: Permian. Sedimentary Environments: Anoxic Environments.
Further Reading Benton MJ (ed.) (1993) The Fossil Record 2. London: Chapman & Hall.
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Benton MJ (2003) When Life Nearly Died: The Greatest Mass Extinction of all Time. London: Thames and Hudson. Benton MJ and Twitchett RJ (2003) How to kill all life: the end-Permian extinction event. Trends in Ecology and Evolution 18: 358–365. Erwin DH (1993) The Great Paleozoic Crisis: Life and Death in the Permian. New York: Columbia University Press. Erwin DH (1994) The Permo-Triassic extinction. Nature 367: 231–236. Erwin DH (1996) Understanding biotic recoveries: extinction, survival and preservation during the end-Permian mass extinction. In: Jablonski D, Erwin DH, and Lipps JH (eds.) Evolutionary Paleobiology, pp. 398–418. Chicago: University of Chicago Press. Hallam A and Wignall PB (1997) Mass Extinctions and Their Aftermath. Oxford: Oxford University Press.
PANGAEA S G Lucas, New Mexico Museum of Natural History, Albuquerque, NM, USA ß 2005, Elsevier Ltd. All Rights Reserved.
Introduction Geologists apply the term Pangaea (from the Greek words meaning ‘all Earth’, and usually pronounced pan-JEE-uh) to the supercontinent that existed during the Late Palaeozoic and Early Mesozoic, about 300–200 Ma ago. Pangaea was an amalgamation of continental blocks that were the precursors of the existing continents, though much of eastern Asia was an archipelago of islands loosely connected to eastern Pangaea.
Components of Pangaea Pangaea (Figure 1) was accreted from continental blocks that differed from today’s continents. The southern supercontinent of Gondwana (also called Gondwanaland) (see Gondwanaland and Gondwana) was the Palaeozoic amalgamation of South America, Africa, Antarctica, Australia, and the IndoPakistani subcontinent, as well as several smaller terranes. The northern supercontinent of Laurussia (or Laurasia) consisted of North America and most of Europe. Asia, however, did not exist as a single block. The continental nucleus of present-day Asia was the Siberian block, but other blocks included Kazakhstan,
Tarim, and a loose archipelago of blocks that were to become much of China and South-east Asia. Many of these eastern Asian blocks originated as slivers of northern Gondwana and drifted northwards during the Late Palaeozoic and earliest Mesozoic to accrete to the larger Asian blocks during the Late Triassic–Jurassic. Pangaea was surrounded by a universal ocean called Panthalassa (from the Greek words meaning ‘all sea’), which was the ancestor of today’s Pacific Ocean. The Atlantic Ocean did not exist even in ancestral form, because of the fusions of North America to Europe and of South America to Africa. An arm of Panthalassa formed a deep east–west embayment in the eastern edge of Pangaea (Figure 1). This was the Tethys Sea, which is named after the sister and consort of Oceanus in Greek mythology. Tethys was the ancestor of the present Mediterranean Sea. Successive ocean basins within it are termed Palaeotethys (Devonian–Permian) and Neotethys; the latter opened during the Late Permian as a result of rifting between Gondwana and the smaller central and southern Asian terranes.
Late Carboniferous Accretion of Pangaea The Pangaean supercontinent came together (accreted) at the end of the Carboniferous (see Palaeozoic: Carboniferous), when Laurussia and Gondwana were sutured along what has been termed the Hercynian megasuture. Very old mountain ranges mark the