Permian–Triassic extinction event

Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene
Extinction intensity.svg
Tr-J
P-T
Late D
O-S
Millions of years ago
Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene
Extinction intensity.svg
Marine extinction intensity through time. Blue graph shows apparent percentage (not absolute number) of marine animal genera becoming extinct during any given time interval. Does not represent all marine species, just those which are readily fossilized. The "Big Five" extinction events are linked; see Extinction event for more details. (source and image info)

The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 251.4 million years ago,[1][2] forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96 percent of all marine species[3] and 70 percent of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects.[4][5] 57% of all families and 83% of all genera were killed off. Because so much biodiversity was lost, the recovery of life on earth took significantly longer than after other extinction events.[3] This event has been described as the "mother of all mass extinctions".[6] The pattern of extinction is still disputed,[7] as different studies suggest one[1] to three[8] different pulses. There are several proposed mechanisms for the extinctions; the earlier peak was likely due to gradualistic environmental change, while the later was probably due to a catastrophic event. Possible mechanisms for the latter include large or multiple bolide impact events, increased volcanism, or sudden release of methane hydrates from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity,[9] and a shift in ocean circulation driven by climate change.

Contents

Dating the extinction

Until about 2000 it was thought that rock sequences spanning the Permian-Triassic boundary were too few and contained too many gaps for scientists to estimate reliably when the extinction occurred, how long it took or whether it happened at the same time all over the world.[10] However, a study of uranium/lead ratios of zircons from rock sequences near Meishan, Changxing, Zhejian Province, China[2] date the extinction to 251.4  ±.03 Ma, with an ongoing elevated extinction rate occurring for some time thereafter.[1] A large (-9‰[11]), abrupt global change in the ratio of 13C to 12C, denoted δ13C, coincides with this extinction,[12][13][14][15], and is sometimes used to identify the Permian-Triassic boundary in rocks that are unsuitable for radiometric dating.[16]

It has been suggested that the Permian-Triassic boundary is associated with a sharp increase in the abundance of marine and terrestrial fungi, and that this was caused by the sharp increase in the amount of dead plants and animals fed upon by the fungi,[17] For a while this "fungal spike" was used by some paleontologists to identify the boundary to define the Permian-Triassic boundary in rocks that are unsuitable for radiometric dating or lack suitable index fossils, but even the proposers of the fungal spike hypothesis pointed out that "fungal spikes" may have been a repeating phenomenon created by the post-extinction ecosystem in the earliest Triassic.[17] More recently the very idea of a fungal spike has been criticized on several grounds, including that: Reduviasporonites, the most common supposed "fungal spore", was actually a fossilized alga;[18][11] the spike did not appear world-wide; [19][20]; and in many places it did not fall on the Permian-Triassic boundary.[21] The algae which were mis-identified as fungal spores may even represent a transition to a lake-dominated Triassic world rather than an earliest Triassic zone of death and decay in some terrestrial fossil beds.[22]

There is still uncertainty about the duration of the overall extinction and about the timing and duration of various groups' extinctions within the greater process. Some evidence suggests that the extinction was spread out over a few million years, with a very sharp peak in the last 1 million years of the Permian.[21][23] Statistical analyses of some highly fossiliferous strata in Meishan, South China suggest that the main extinction was clustered around one peak.[1] Recent research shows that different groups went extinct at different times; for example, while difficult to date absolutely, ostracode and brachiopod extinctions were separated by between 0.72 and 1.22 million years.[24] In a well preserved sequence in east Greenland, the decline of animals is concentrated in a period 10 to 60 thousand years long, with plants taking several hundred thousand further years to show the full impact of the event.[25] An older theory, still supported in some recent papers,[26] is that there were two major extinction pulses 5 million years apart, separated by a period of extinctions well above the background level; and that the final extinction killed off "only" about 80% of marine species alive at that time while the other losses occurred during the first pulse or the interval between pulses. According to this theory the first of these extinction pulses occurred at the end of the Guadalupian epoch of the Permian.[27] For example, all but one of the surviving dinocephalian genera died out at the end of the Guadalupian,[28] as did the Verbeekinidae, a family of large-size fusuline foraminifera.[29] The impact of the end-Guadalupian extinction on marine organisms appears to have varied between locations and between taxonomic groups - brachiopods and corals had severe losses.[30][31]

Extinction patterns

The event had a profound effect on the terrestrial ecosystem, which is still being felt today, a quarter of a billion years later.[5] In the late Permian, there were many sorts of reptiles and amphibians on land, together with many plants, especially ferns but also conifers and gingkos. There were also complicated coral reef ecologies undersea.[32] By this time, Pangea was in existence, and animals could roam freely. There were lush jungles, oceans, and deserts. After the extinction, one genus of land vertebrate dominated: a medium-sized herbivore called Lystrosaurus. Only one genus of sea life is common after the extinction as well: a brachiopod called Lingula. Eventually other genera and species seem to reappear - the so-called "Lazarus taxa", named after the Biblical character who returned from the dead. Clearly they must have survived the extinction event, but in very low numbers.[32] Like the end-Ordovician event, it seems to have been composed of two bursts, separated by an interval of about 10 million years, the second being the larger of the two. Notable extinction happened again amongst brachiopods, ammonoids, and corals, as well as gastropods and, unusually, insects. It took about 50 million years for life on land to fully recover its biodiversity. Nothing resembling a coral reef shows up until 10 million years after the Permian extinction, and full recovery of marine life took about 100 million years.[32]

Marine extinctions Genera extinct Notes
Marine invertebrates

Foraminifera

 97% Fusulinids died out, but were almost extinct before the catastrophe

Radiolaria (plankton)

 99%[33]

Anthozoa (sea anemones, corals, etc.)

 96% Tabulate and rugose corals died out

Bryozoans

 79% Fenestrates, trepostomes, and cryptostomes died out

Brachiopods

 96% Orthids and productids died out

Bivalves

 59%  

Gastropods (snails)

 98%  

Ammonites (cephalopods)

 97%  

Crinoids (echinoderms)

 98% Inadunates and camerates died out

Blastoids (echinoderms)

 100% May have become extinct shortly before the P–Tr boundary

Trilobites

 100% In decline since the Devonian; only 2 genera living before the extinction

Eurypterids ("sea scorpions")

 100% May have become extinct shortly before the P–Tr boundary

Ostracods (small crustaceans)

 59%  

Graptolites

 100% In decline since the Devonian (may have living relatives amongst Pterobranchia)
Fish

Acanthodians

 100% In decline since the Devonian, with only one living family

Marine organisms

Marine invertebrates suffered the greatest losses during the P–Tr extinction. In the intensively-sampled south China sections at the P-Tr boundary, for instance, 280 out of 329 marine invertebrate genera disappear within the final 2 sedimentary zones containing conodonts from the Permian.[1]

Statistical analysis of marine losses at the end of the Permian suggests that the decrease in diversity was caused by a sharp increase in extinctions instead of a decrease in speciation.[34]

Among benthic organisms, the extinction event multiplied background extinction rates, and therefore caused most damage to taxa that had a high background extinction rate (by implication, taxa with a high turnover).[35][36] The extinction rate of marine organisms was catastrophic.[37][6][1][38]

Marine invertebrate groups which survived include: articulate brachiopods (those with a hinge), which have suffered a slow decline in numbers since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which very nearly became extinct but later became abundant and diverse.

The groups with the highest survival rates generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification; more heavily calcified organisms with simpler breathing apparatus were the worst hit.[39][40] In the case of the brachiopods at least, surviving taxa were generally small, rare members of a diverse community.[41]

The ammonoids, which had been in a long-term decline for the 30 million years since the Roadian (middle Permian), suffered a selective end-Guadalupian extinction pulse. This extinction greatly reduced disparity, and suggests that environmental factors were responsible for this extinction. Diversity and disparity fell further until the P-T boundary; the extinction here was non-selective, consistent with a catastrophic initiator. During the Triassic, diversity rose rapidly, but disparity remained low.[42]

The range of morphospace occupied by the ammonoids became more restricted as the Permian progressed. Just a few million years into the Triassic, the original morphospace range was once again occupied, but shared differently between clades.[43]

Terrestrial invertebrates

The Permian had great diversity in insect and other invertebrate species, including the largest insects ever to have existed. The end-Permian is the only known mass extinction of insects,[4] with eight or nine insect orders becoming extinct and ten more greatly reduced in diversity. Palaeodictyopteroids (insects with piercing and sucking mouthparts) began to decline during the mid-Permian; these extinctions have been linked to a change in flora. The greatest decline, however, occurred in the Late Permian and were probably not directly caused by weather-related floral transitions.[6]

Most fossil insect groups which are found after the Permian–Triassic boundary differ significantly from those which lived prior to the P–Tr extinction. With the exception of the Glosselytrodea, Miomoptera, and Protorthoptera, Paleozoic insect groups have not been discovered in deposits dating to after the P–Tr boundary. The caloneurodeans, monurans, paleodictyopteroids, protelytropterans, and protodonates became extinct by the end of the Permian. In well-documented Late Triassic deposits, fossils overwhelmingly consist of modern fossil insect groups.[4]

Terrestrial plants

Plant ecosystem response

The geological record of terrestrial plants is sparse, and based mostly on pollen and spore studies. Interestingly, plants are relatively immune to mass extinction, with the impact of all the major mass extinctions "negligible" at a family level.[11] Even the reduction observed in species diversity (of 50%) may be mostly due to taphonomic processes.[11] However, a massive rearrangement of ecosystems does occur, with plant abundances and distributions changing profoundly.[11]

At the P–Tr boundary, the dominant floral groups changed, with many groups of land plants entering abrupt decline, such as Cordaites (gymnosperms) and Glossopteris (seed ferns).[44] Dominant gymnosperm genera were replaced post-boundary by lycophytes - extant lycophytes are recolonizers of disturbed areas.[45]

Palynological or pollen studies from East Greenland of sedimentary rock strata laid down during the extinction period indicate dense gymnosperm woodlands before the event. At the same time that marine invertebrate macrofauna are in decline these large woodlands die out and are followed by a rise in diversity of smaller herbaceous plants including Lycopodiophyta, both Selaginellales and Isoetales. Later on other groups of gymnosperms again become dominant but again suffer major die offs; these cyclical fauna shifts occur a few times over the course of the extinction period and afterwards. These fluctuations of the dominant flora between woody and herbaceous taxa indicate chronic environmental stress resulting in a loss of most large woodland plant species. The successions and extinctions of plant communities do not coincide with the shift in δ13C values, but occurs many years after. [46] The recovery of gymnosperm forests would take 4-5 million years.[11]

The Coal Gap

No coal deposits are known from the Early Triassic, and those in the Middle Triassic are thin and low-grade.[47] This "coal gap" has been explained in many ways. It has been suggested that new, more aggressive fungi, insects and vertebrates evolved, and killed vast amounts of trees. However these decomposers themselves suffered heavy losses of species during the extinction, and not considered a likely cause of the coal gap.[47] It could simply be that all coal forming plants were rendered extinct by the P/T extinction, and that it took 10 million years for a new suite of plants to adapt to the moist, acid conditions of peat bogs.[47] On the other hand abiotic factors (not caused by organisms), such as decreased rainfall or increased input of clastic sediments, may also be to blame.[11] Finally, it is also true that there are very few sediments of any type known from the Early Triassic, and the lack of coal may simply reflect this scarcity. This opens the possibility that coal-producing ecosystems may have responded to the changed conditions by relocating, perhaps to areas where we have no sedimentary record for the Early Triassic.[11] For example in eastern Australia a cold climate had been the norm for a long period of time, with a peat mire ecosystem specialising to these conditions. Approximately 95% of these peat-producing plants went locally extinct at the P-T boundary;[48] Interestingly, coal deposits in Australia and Antarctica disappear significantly before the P-Tr boundary.[11]

Terrestrial vertebrates

Even the groups that survived suffered extremely heavy losses of species, and some terrestrial vertebrate groups very nearly became extinct at the end-Permian. Some of the surviving groups did not persist for long past this period, while others that barely survived went on to produce diverse and long-lasting lineages. There is enough evidence to indicate that over two-thirds of terrestrial amphibian, sauropsid ("reptile") and therapsid ("mammal-like reptile") families became extinct. Large herbivores suffered the heaviest losses. All Permian anapsid reptiles died out except the procolophonids (testudines have anapsid skulls but are most often thought to have evolved later, from diapsid ancestors). Pelycosaurs died out before the end of the Permian. Too few Permian diapsid fossils have been found to support any conclusion about the effect of the Permian extinction on diapsids (the "reptile" group from which lizards, snakes, crocodilians, dinosaurs, and birds evolved).[49][50]

Possible explanations of these patterns

The most vulnerable marine organisms were those which produced calcareous hard parts (i.e. from calcium carbonate) and had low metabolic rates and weak respiratory systems - notably calcareous sponges, rugose and tabulate corals, calciate brachiopods, bryozoans, and echinoderms; about 81% of such genera became extinct. Close relatives which did not produce calcareous hard parts suffered only minor losses, for example sea anemones, from which modern corals later evolved. Animals which had high metabolic rates, well-developed respiratory systems and non-calcareous hard parts had negligible losses - except for conodonts, in which 33% of genera died out.[51]

This pattern is consistent with what is known about the effects of hypoxia (shortage but not total absence of oxygen). However hypoxia cannot have been the only killing mechanism for marine organisms: nearly all of the continental shelf waters would have had to become severely hypoxic to account for the magnitude of the extinction, but such a catastrophe would make it difficult to explain the very selective pattern of the extinction. Models of the Late Permian and Early Triassic atmospheres show a significant but protracted decline in atmospheric oxygen levels, with no acceleration near the P-Tr boundary and with minimum levels in the Early Triassic that are never less than present day levels - in other words, the decline in oxygen levels does not match the temporal pattern of the extinction.[51]

The observed pattern of marine extinctions is also consistent with hypercapnia (excessive levels of carbon dioxide). Carbon dioxide (CO2) is actively toxic at above-normal concentrations, as it: reduces the ability of respiratory pigments to oxygenate tissues; makes body fluids more acidic, which hampers the production of carbonate hard parts (shells, etc.) and, at high concentrations, causes narcosis ("intoxication"). In addition to these direct effects, it reduces the concentration of carbonates in water by "crowding them out", which further increases the difficulty of producing carbonate hard parts. Marine organisms are more sensitive to changes in CO2 levels than terrestrial ones are, because: CO2 is 28 times more soluble in water than oxygen is; marine animals normally function with lower concentrations of CO2 in their bodies than land animals, because in air-breathing animals the removal of CO2 is impeded by the need for the gas to pass through the membranes of their respiratory systems (lungs, tracheae, etc.). In marine organisms relatively modest but sustained increases in CO2 concentrations hamper the synthesis of proteins, reduce fertilization rates and produce deformities in calcareous hard parts.[51]

It is difficult to analyze extinction and survival rates of land organisms in such detail, because there are few terrestrial fossil beds that span across the Permian-Triassic boundary. Triassic insects are very different from those of the Permian, but there is a gap of about 15M years in the insect fossil record from the late Permian to early Triassic. The best known record of vertebrate changes across the Permian-Triassic boundary occurs in the Karoo Supergroup of South Africa; but statistical analyses have so far not produced clear conclusions.[51]

Biotic recovery

Earlier analyses indicated that life on Earth recovered quickly after the Permian extinctions, but this was mostly in the form of disaster taxa, such as the hardy Lystrosaurus. The most recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover. It is thought that this long recovery was due to the successive waves of extinction which inhibited recovery, as well as to prolonged environmental stress to organisms which continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction;[52] and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.[53]

A braided river - the Waimakariri River in the South Island of New Zealand.

During the early Triassic (4-6M years after the P-Tr extinction), the plant biomass was insufficient to form coal deposits, which implies a limited food mass for herbivores.[47] River patterns in the Karoo changed from meandering to braided, indicating that vegetation there was very sparse for a long time.[54]

Each major segment of the early Triassic ecosystem — plant and animal, marine and terrestrial — was dominated by a small number of genera, which appeared virtually world-wide, for example: the herbivorous therapsid Lystrosaurus (which accounted for about 90% of early Triassic land vertebrates) and the bivalves Claraia, Eumorphotis, Unionites and Promylina. A healthy ecosystem has a much larger number of genera, each living in a few preferred types of habitat.[44][55]

Disaster taxa (opportunist organisms) took advantage of the devastated ecosystem and enjoyed a temporary population boom and increase in their territory. For example: Lingula (a brachiopod); stromatolites, which had been confined to marginal environments since the Ordovician; Pleuromeia (a small, weedy plant); Dicrodium (a seed fern).[56][9][57][55]

Sessile filter feeders like this Crinoid were significantly less abundant after the P-Tr extinction.

Changes in marine ecosystems

Prior to the extinction, approximately 67% of marine animals were sessile and attached to the sea floor, but during the Mesozoic only about half of the marine animals were sessile while the rest were free living. Analysis of marine fossils from the period indicated a decrease in the abundance of sessile epifaunal suspension feeders, such as brachiopods and sea lilies, and an increase in more complex mobile species such as snails, urchins and crabs.

Before the Permian mass extinction event, both complex and simple marine ecosystems were equally common; after the recovery from the mass extinction, the complex communities outnumbered the simple communities by nearly three to one,[58] and the increase in predation pressure led to the Mesozoic Marine Revolution.

Bivalves were fairly rare before the P–Tr extinction but became numerous and diverse in the Triassic and one group, the rudist clams, became the Mesozoic's main reef-builders. Some researchers think much of this change happened in the 5 million years between the two major extinction pulses.[59]

Crinoids ("sea lilies") suffered a selective extinction, resulting in a decrease in the variety of forms in which they grew.[60] Their ensuing adaptive radiation was brisk, and resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent.[61]

Land vertebrates

Lystrosaurus was by far the most abundant early Triassic land vertebrate.

Lystrosaurus, a pig-sized herbivorous dicynodont therapsid, constituted as much as 90% of some earliest Triassic land vertebrate faunas.[9] Smaller carnivorous cynodont therapsids also survived, including the ancestors of mammals. In the Karoo region of southern Africa the therocephalians Tetracynodon, Moschorhinus and Ictidosuchoides survived but do not appear to have been abundant in the Triassic.[62]

Archosaurs (which included the ancestors of crocodilians) were initially rarer than therapsids, but they began to displace therapsids in the mid-Triassic.[9] In the mid to late Triassic the dinosaurs evolved from one group of archosaurs, and went on to dominate terrestrial ecosystems for the rest of the Mesozoic.[63] This "Triassic Takeover" may have contributed to the evolution of mammals by forcing the surviving therapsids and their mammaliform successors to live as small, mainly nocturnal insectivores; nocturnal life probably forced at least the mammaliforms to develop fur and higher metabolic rates.[64]

Some temnospondyl amphibians made a relatively quick recovery, in spite of nearly becoming extinct. Mastodonsaurus and trematosaurians were the main aquatic and semi-aquatic predators during most of the Triassic, some preying on tetrapods and others on fish.[65]

Land vertebrates took an unusually long time to recover from the P-Tr extinction; one writer estimates that the recovery was not complete until 30 million years after the extinction, in other words not until the Late Triassic, in which dinosaurs, pterosaurs, crocodiles, archosaurs, amphibians and mammaliforms were abundant and diverse.[3]

Causes of extinction event

There are several proposed mechanisms for the extinction event, including both catastrophic and gradualistic processes, similar to those theorized for the Cretaceous–Tertiary extinction event. The former include large or multiple bolide impact events, increased volcanism, or sudden release of methane hydrates from the sea floor. The latter include sea-level change, anoxia, and increasing aridity.[9]

Impact event

Artist's impression of a major impact event. The collision between Earth and an asteroid a few kilometers in diameter will release as much energy as several million nuclear weapons detonating.

Evidence that an impact event caused the Cretaceous–Tertiary extinction event has led to speculation that similar impacts may have been the cause of other extinction events, including the P–Tr extinction, and therefore to a search for evidence of impacts at the times of other extinctions and for large impact craters of the appropriate age.

Reported evidence for an impact event from the P–Tr boundary level includes rare grains of shocked quartz in Australia and Antarctica;[66][67] fullerenes trapping extraterrestrial noble gases;[68] meteorite fragments in Antarctica;[69] and grains rich in iron, nickel and silicon, which may have been created by an impact.[70] However, the veracity of most of these claims has been challenged.[71][72][73][74] The shocked quartz from Graphite Peak in Antarctica has recently been reexamined by optical and transmission electron microscopy. It was concluded that the observed features were not due to shock, but rather to plastic deformation, consistent with formation in a tectonic environment such as volcanism.[75]

Several possible impact craters have been proposed as possible causes of the P–Tr extinction, including the Bedout structure off the northwest coast of Australia,[67] and the so-called Wilkes Land crater of East Antarctica.[76] In each of these cases the idea that an impact was responsible has not been proven, and has been widely criticized. In the case of Wilkes Land, the age of this sub-ice geophysical feature is very uncertain – it may be later than the Permian–Triassic extinction.

If impact is a major cause of the P–Tr extinction, it is likely that the crater would no longer exist. As 70% of the Earth's surface is sea, an asteroid or comet fragment is more than twice as likely to hit ocean as it is to hit land. However, Earth has no ocean-floor crust more than 200 million years old, because the "conveyor belt" process of sea-floor spreading and subduction destroys it within that time. It has also been speculated that craters produced by very large impacts may be masked by extensive lava flooding from below after the crust is punctured or weakened.[77]

One attraction of large impact theories is that theoretically they could trigger other cause-considered extinction-paralleling phenomena[78], such as the Siberian Traps eruptions (see below) as being either an impact site[79] or the antipode of an impact site.[80] [78] Subduction should not be taken as an excuse that no firm evidence can be found; much like the K-T event, an ejecta blanket stratum rich in siderophilic elements (e.g. iridium) would be found in a great many formations from the time. The abruptness of an impact would also explain why species did not rapidly evolve in adaptation to more slowly-manifesting and/or less than global-in-scope phenomena.

Volcanism

The world around the time of the P-Tr extinction. The Siberian Traps eruptions occurred on the eastern shore of the shallow sea (paler blue) at the north of the map. The earlier Emeishan eruptions occurred on the north edge of the almost enclosed shallow sea just north of the equator - at this time the blocks that currently form China and South-East Asia were just emerging.

The final stages of the Permian saw two flood basalt events. A small one centered at Emeishan in China occurred at the same time as the end-Guadalupian extinction pulse, in an area which was close to the equator at the time.[81] The flood basalt eruptions which produced the Siberian Traps constituted one of the largest known volcanic events on Earth and covered over 200,000 square kilometers (77,220.4 sq mi) with lava. The Siberian Traps eruptions were formerly thought to have lasted for millions of years, but recent research dates them to 251.2 ± 0.3 Ma — immediately before the end of the Permian.[1][82]

The Emeishan and Siberian Traps eruptions may have caused dust clouds and acid aerosols which would have blocked out sunlight and thus disrupted photosynthesis both on land and in the upper layers of the seas, causing food chains to collapse. These eruptions may also have caused acid rain when the aerosols washed out of the atmosphere. This may have killed land plants and mollusks and planktonic organisms which build calcium carbonate shells. The eruptions would also have emitted carbon dioxide, causing global warming. When all of the dust clouds and aerosols washed out of the atmosphere, the excess carbon dioxide would have remained and the warming would have proceeded without any mitigating effects.[78]

The Siberian Traps had unusual features which made them even more dangerous. Pure flood basalts produce a lot of runny lava and do not hurl debris into the atmosphere. It appears, however, that 20% of the output of the Siberian Traps eruptions was pyroclastic, i.e. consisted of ash and other debris thrown high into the atmosphere, increasing the short-term cooling effect.[83] The basalt lava erupted or intruded into carbonate rocks and into sediments which were in the process of forming large coal beds, both of which would have emitted large amounts of carbon dioxide, leading to stronger global warming after the dust and aerosols settled.[78]

There is doubt, however, about whether these eruptions were enough on their own to cause a mass extinction as severe as the end-Permian. Equatorial eruptions are necessary to produce sufficient dust and aerosols to affect life worldwide, whereas the much larger Siberian Traps eruptions were inside or near the Arctic Circle. Furthermore, if the Siberian Traps eruptions occurred within a period of 200,000 years, the atmosphere's carbon dioxide content would have doubled. Recent climate models suggest that such a rise in CO2 would have raised global temperatures by 1.5 °C (2.7 °F) to 4.5 °C (8.1 °F), which is bad but unlikely to cause a catastrophe as great as the P-Tr extinction.[78]

However, one theory, popularized by the documentary Miracle Planet, is that the slight volcanic warming caused a melting of methane hydrate, and this created a positive-feedback warming loop, as methane is 45 times more efficient than CO2 at exacerbating global warming.

Methane hydrate gasification

Scientists have found worldwide evidence of a swift decrease of about 10 ‰ (parts per thousand) in the 13C/12C isotope ratio in carbonate rocks from the end-Permian (δ13Ccarbonate of -10 ‰).[84][38] This is the first, largest and most rapid of a series of negative and positive excursions (decreases and increases in 13C/12C ratio) that continues until the isotope ratio abruptly stabilises in the middle Triassic, followed soon afterwards by the recovery of calcifying life forms (organisms that use calcium carbonate to build hard parts such as shells).[39]

A variety of factors may have contributed to this drop in the 13C/ 12C ratio, but most turn out to be insufficient to account fully for it:[85]

Other hypotheses include mass oceanic poisoning releasing vast amounts of CO2[88] and a long-term reorganisation of the global carbon cycle.[85]

However, only one sufficiently powerful cause has been proposed for the global 10 ‰ reduction in the 13C/12C ratio: the release of methane from methane clathrates;[6] and carbon-cycle models confirm that it would have been sufficient to produce the observed reduction.[85] [88] Methane clathrates, also known as methane hydrates, consist of methane molecules trapped in cages of water molecules. The methane is produced by methanogens (microscopic single-celled organisms) and has a 13C/12C ratio about 60 ‰ below normal (δ13C -60  ‰). At the right combination of pressure and temperature it gets trapped in clathrates fairly close to the surface of permafrost and in much larger quantities at continental margins (continental shelves and the deeper seabed close to them). Oceanic methane hydrates are usually found buried in sediments where the seawater is at least 300 meters (984 ft) deep. They can be found up to about 2,000 meters (6,562 ft) below the sea floor, but usually only about 1,100 meters (3,609 ft) below the sea floor.[89]

The area covered by lava from the Siberian Traps eruptions is about twice as large as was originally thought, and most of the additional area was shallow sea at the time. It is very likely that the seabed contained methane hydrate deposits and that the lava caused the deposits to dissociate, releasing vast quantities of methane.[90]

One would expect a vast release of methane to cause significant global warming, since methane is a very powerful greenhouse gas. A "methane burp" could have released 10,000 billion tons of carbon dioxide equivalent - twice as much as in all the fossil fuels on Earth. [32] There is strong evidence that global temperatures increased by about 6 °C (10.8 °F) near the equator and therefore by more at higher latitudes: a sharp decrease in oxygen isotope ratios (18O/16O);[91] the extinction of Glossopteris flora (Glossopteris and plants which grew in the same areas), which needed a cold climate, and its replacement by floras typical of lower paleolatitudes.[92][9]

However, the pattern of isotope shifts expected to result from a massive relase of methane do not match the patterns seen throughout the early Triassic. Not only would a methane cause require the release of five times as much methane as postulated for the PETM,[39] but it would also have to be re-buried at an unrealistically high rate to account for the rapid increases in the 13C/12C ratio (episodes of high positive δ13C) throughout the early Triassic, before being released again several times.[39]

Sea level fluctuations

Marine regression occurs when areas of submerged seafloor are exposed above sea level. This lowering of sea level causes a reduction in shallow marine habitats, leading to biotic turnover. Shallow marine habitats are productive areas for organisms at the bottom of the food chain, their loss increasing competition for food sources.[93] There is some correlation between incidents of pronounced sea level regression and mass extinctions, but other evidence indicates there is no relationship and that regression may itself create new habitats.[9] It has also been suggested that sea-level changes result in changes in sediment deposition rates and effects water temperature and salinity, resulting in a decline in marine diversity.[94]

Anoxia

There is evidence that the oceans became anoxic (severely deficient in oxygen) towards the end of the Permian. There was a noticeable and rapid onset of anoxic deposition in marine sediments around East Greenland near the end of the Permian.[95] The uranium/thorium ratios of several late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction.[96]

This would have been devastating for marine life, producing massive die-offs except for anaerobic bacteria inhabiting the sea-bottom mud. There is also evidence that anoxic events can cause catastrophic hydrogen sulfide emissions from the sea floor (see below).

The possible sequence of events leading to anoxic oceans might have involved a period of global warming that reduced the temperature gradient between the equator and the poles which slowed or perhaps even stopped the thermohaline circulation. The slow-down or stoppage of the thermohaline circulation could have reduced the mixing of oxygen in the ocean.[96]

However, one research article suggests that the types of oceanic thermohaline circulation which may have existed at the end of the Permian are not likely to have supported deep-sea anoxia.[97]

Hydrogen sulfide emissions

A severe anoxic event at the end of the Permian could have made sulfate-reducing bacteria the dominant force in oceanic ecosystems, causing massive emissions of hydrogen sulfide which poisoned plant and animal life on both land and sea, as well as severely weakening the ozone layer, exposing much of the life that remained to fatal levels of UV radiation.[98] Indeed, anaerobic photosynthesis by Chlorobiaceae (green sulfur bacteria), and its accompanying hydrogen sulfide emissions, occurred from the end-Permian into the early Triassic. The fact that this anaerobic photosynthesis persisted into the early Triassic is consistent with fossil evidence that the recovery from the Permian–Triassic extinction was remarkably slow.[99]

This theory has the advantage of explaining the mass extinction of plants, which ought otherwise to have thrived in an atmosphere with a high level of carbon dioxide. Fossil spores from the end-Permian further support the theory: many show deformities that could have been caused by ultraviolet radiation, which would have been more intense after hydrogen sulfide emissions weakened the ozone layer.

The supercontinent Pangaea

Map of Pangaea showing where today's continents were at the Permian–Triassic boundary

About half way through the Permian (in the Kungurian age of the Permian's Cisuralian epoch) all the continents joined to form the supercontinent Pangaea, surrounded by the superocean Panthalassa, although blocks which are now parts of Asia did not join the supercontinent until very late in the Permian.[100] This configuration severely decreased the extent of shallow aquatic environments, the most productive part of the seas, and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. Pangaea's formation would also have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons near the coasts and an arid climate in the vast continental interior.

Marine life suffered very high but not catastrophic rates of extinction after the formation of Pangaea (see the diagram "Marine genus biodiversity" at the top of this article) - almost as high as in some of the "Big Five" mass extinctions. The formation of Pangaea seems not to have caused a significant rise in extinction levels on land, and in fact most of the advance of the Therapsids and increase in their diversity seems to have occurred in the late Permian, after Pangaea was almost complete. So it seems likely that Pangaea initiated a long period of increased marine extinctions but was not directly responsible for the "Great Dying" and the end of the Permian.

Combination of causes

The possible causes which are supported by strong evidence (see above) appear to describe a sequence of catastrophes, each one worse than the previous: the Siberian Traps eruptions were bad enough in their own right, but because they occurred near coal beds and the continental shelf, they also triggered very large releases of carbon dioxide and methane. The resultant global warming may have caused perhaps the most severe anoxic event in the oceans' history: according to this theory, the oceans became so anoxic that anaerobic sulfur-reducing organisms dominated the chemistry of the oceans and caused massive emissions of toxic hydrogen sulfide.

However, there may be some weak links in this chain of events: the changes in the 13C/12C ratio expected to result from a massive release of methane do not match the patterns seen throughout the early Triassic;[39] and the types of oceanic thermohaline circulation which may have existed at the end of the Permian are not likely to have supported deep-sea anoxia.[97]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Jin YG, Wang Y, Wang W, Shang QH, Cao CQ, Erwin DH (2000). "Pattern of Marine Mass Extinction Near the Permian–Triassic Boundary in South China". Science 289 (5478): 432–436. doi:10.1126/science.289.5478.432. PMID 10903200. 
  2. ↑ 2.0 2.1 Bowring SA, Erwin DH, Jin YG, Martin MW, Davidek K, Wang W (1998). "U/Pb Zircon Geochronology and Tempo of the End-Permian Mass Extinction". Science 280 (1039): 1039–1045. doi:10.1126/science.280.5366.1039. 
  3. ↑ 3.0 3.1 3.2 Benton M J (2005). When Life Nearly Died: The Greatest Mass Extinction of All Time. Thames & Hudson. ISBN 978-0500285732. 
  4. ↑ 4.0 4.1 4.2 Labandeira CC, Sepkoski JJ (1993). "Insect diversity in the fossil record". Science 261 (5119): 310–5. PMID 11536548. 
  5. ↑ 5.0 5.1 Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record - Volume Two, The earth system: biological and ecological dimensions of global environment change" pp. 297-391, Encyclopedia of Global Enviromental Change John Wilely & Sons.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Erwin DH (1993). The great Paleozoic crisis; Life and death in the Permian. Columbia University Press. ISBN 0231074670. 
  7. ↑ Yin H, Zhang K, Tong J, Yang Z, Wu S. "The Global Stratotype Section and Point (GSSP) of the Permian-Triassic Boundary". Episodes 24 (2): 102–114. 
  8. ↑ Yin HF, Sweets WC, Yang ZY, Dickins JM,. "Permo-Triassic Events in the Eastern Tethys". Cambridge Univ. Pres, Cambridge, 1992. 
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 Tanner LH, Lucas SG & Chapman MG (2004). "Assessing the record and causes of Late Triassic extinctions" (PDF). Earth-Science Reviews 65 (1-2): 103-139. doi:10.1016/S0012-8252(03)00082-5. http://nmnaturalhistory.org/pdf_files/TJB.pdf. Retrieved on 2007-10-22. 
  10. ↑ Erwin, D.H (1993). The Great Paleozoic Crisis: Life and Death in the Permian. New York isbn=0231074670: Columbia University Press. 
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 McElwain, J.C.; Punyasena, S.W. (2007). "Mass extinction events and the plant fossil record". Trends in Ecology & Evolution 22 (10): 548–557. doi:10.1016/j.tree.2007.09.003. http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=17919771&cmd=showdetailview&indexed=google. 
  12. ↑ Magaritz, M (1989). "13C minima follow extinction events: a clue to faunal radiation". Geology 17: 337–340. 
  13. ↑ Krull, S.J., and Retallack, J.R. (2000). "13C depth profiles from paleosols across the Permian–Triassic boundary: Evidence for methane release". GSA Bulletin 112 (9): 1459–1472. doi:10.1130/0016-7606(2000)112%3C1459:CDPFPA%3E2.0.CO;2. 
  14. ↑ Dolenec, T., Lojen, S., Ramovs, A. (2001). "The Permian–Triassic boundary in Western Slovenia (Idrijca Valley section): magnetostratigraphy, stable isotopes, and elemental variations". Chemical Geology 175 (1): 175–190. doi:10.1016/S0009-2541(00)00368-5. 
  15. ↑ Musashi, M., Isozaki, Y., Koike, T. and Kreulen, R. (2001). "Stable carbon isotope signature in mid-Panthalassa shallow-water carbonates across the Permo–Triassic boundary: evidence for 13C-depleted ocean". Earth Planet. Sci. Lett. 193: 9–20. doi:10.1016/S0012-821X(01)00398-3. 
  16. ↑ Dolenec, T., Lojen, S., and Ramovs, A. (2001). "The Permian-Triassic boundary in Western Slovenia (Idrijca Valley section): magnetostratigraphy, stable isotopes, and elemental variations". Chemical Geology 175: 175–190. doi:10.1016/S0009-2541(00)00368-5. 
  17. ↑ 17.0 17.1 H Visscher, H Brinkhuis, D L Dilcher, W C Elsik, Y Eshet, C V Looy, M R Rampino, and A Traverse (1996). "The terminal Paleozoic fungal event: Evidence of terrestrial ecosystem destabilization and collapse". Proceedings of the National Academy of Sciences 93 (5): 2155–2158. http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=39926&blobtype=pdf. Retrieved on 2007-09-20. 
  18. ↑ Foster, C.B.; Stephenson, M.H.; Marshall, C.; Logan, G.A.; Greenwood, P.F. (2002). "A Revision Of Reduviasporonites Wilson 1962: Description, Illustration, Comparison And Biological Affinities". Palynology 26 (1): 35–58. doi:10.2113/0260035. http://palynology.geoscienceworld.org/cgi/content/abstract/26/1/35. 
  19. ↑ López-Gómez, J. and Taylor, E.L. (2005). "Permian-Triassic Transition in Spain: A multidisciplinary approach". Palaeogeography, Palaeoclimatology, Palaeoecology 229 (1-2): 1–2. doi:10.1016/j.palaeo.2005.06.028. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6R-4GR8RWF-5&_user=1495569&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000053194&_version=1&_urlVersion=0&_userid=1495569&md5=537a1a5b0a8e04cca2221ecb12afb1e9. 
  20. ↑ Looy, C.V.; Twitchett, R.J.; Dilcher, D.L.; Van Konijnenburg-van Cittert, J.H.A.; Visscher, H. (2005). "Life in the end-Permian dead zone". Proceedings of the National Academy of Sciences 162 (4): 653–659. doi:10.1073/pnas.131218098. PMID 11427710. "See image 2". 
  21. ↑ 21.0 21.1 Ward PD, Botha J, Buick R, De Kock MO, Erwin DH, Garrison GH, Kirschvink JL & Smith R (2005). "Abrupt and Gradual Extinction Among Late Permian Land Vertebrates in the Karoo Basin, South Africa". Science 307 (5710): 709–714. doi:10.1126/science.1107068. 
  22. ↑ Retallack, G.J.; Smith, R.M.H.; Ward, P.D. (2003). "Vertebrate extinction across Permian-Triassic boundary in Karoo Basin, South Africa". Bulletin of the Geological Society of America 115 (9): 1133–1152. doi:10.1130/B25215.1. http://intl-bulletin.geoscienceworld.org/cgi/content/abstract/115/9/1133. 
  23. ↑ Rampino MR, Prokoph A & Adler A (2000). "Tempo of the end-Permian event: High-resolution cyclostratigraphy at the Permian–Triassic boundary". Geology 28 (7): 643–646. doi:10.1130%2F0091-7613%282000%2928%3C643%3ATOTEEH%3E2.0.CO%3B2. 
  24. ↑ Wang, S.C.; Everson, P.J. (2007). "Confidence intervals for pulsed mass extinction events". Paleobiology 33 (2): 324–336. doi:10.1666/06056.1. 
  25. ↑ Twitchett RJ Looy CV Morante R Visscher H & Wignall PB (2001). "Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis". Geology 29 (4): 351–354. doi:10.1130/0091-7613(2001)029<0351:RASCOM>2.0.CO;2. 
  26. ↑ Retallack, G.J.; Metzger, C.A.; Greaver, T.; Jahren, A.H.; Smith, R.M.H.; Sheldon, N.D. (2006). "Middle-Late Permian mass extinction on land". Bulletin of the Geological Society of America 118 (11-12): 1398–1411. doi:10.1130/B26011.1. 
  27. ↑ Stanley SM & Yang X (1994). "A Double Mass Extinction at the End of the Paleozoic Era". Science 266 (5189): 1340–1344. doi:10.1126/science.266.5189.1340. PMID 17772839. 
  28. ↑ Retallack, G.J., Metzger, C.A., Jahren, A.H., Greaver, T., Smith, R.M.H., and Sheldon, N.D (November/December 2006). "Middle-Late Permian mass extinction on land". GSA Bulletin 118 (11/12): 1398–1411. doi:10.1130/B26011.1. 
  29. ↑ Ota, A, and Isozaki, Y. (March 2006). "Fusuline biotic turnover across the Guadalupian–Lopingian (Middle–Upper Permian) boundary in mid-oceanic carbonate buildups: Biostratigraphy of accreted limestone in Japan". Journal of Asian Earth Sciences 26 (3-4): 353-368. 
  30. ↑ Shen, S., and Shi, G.R. (2002). "Paleobiogeographical extinction patterns of Permian brachiopods in the Asian-western Pacific region". Paleobiology 28: 449–463. doi:10.1666/0094-8373(2002)028<0449:PEPOPB>2.0.CO;2. 
  31. ↑ Wang, X-D, and Sugiyama, T. (December 2000). "Diversity and extinction patterns of Permian coral faunas of China". Lethaia 33 (4): 285–294. doi:10.1080/002411600750053853. http://www.blackwell-synergy.com/doi/abs/10.1080/002411600750053853. 
  32. ↑ 32.0 32.1 32.2 32.3 [http://math.ucr.edu/home/baez/extinction extinction
  33. ↑ Racki G (1999). Silica-secreting biota and mass extinctions: survival processes and patterns. 154. pp. 107–132. doi:10.1016/S0031-0182(99)00089-9. 
  34. ↑ Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004), "Origination, extinction, and mass depletions of marine diversity", Paleobiology 30 (4): 522–542, http://www.bioone.org/perlserv/?request=get-document&issn=0094-8373&volume=30&page=522 
  35. ↑ Stanley, S.M. (2008). "Predation defeats competition on the seafloor". Paleobiology 34 (1): 1–21. doi:10.1666/07026.1. http://paleobiol.geoscienceworld.org/cgi/content/short/34/1/1. Retrieved on 2008-05-13. 
  36. ↑ Stanley, S.M. (2007). "An Analysis of the History of Marine Animal Diversity". Paleobiology 33 (sp6): 1–55. doi:10.1666/06020.1. http://paleobiol.geoscienceworld.org/cgi/content/abstract/33/4_Suppl/1. 
  37. ↑ McKinney, M.L. (1987). "Taxonomic selectivity and continuous variation in mass and background extinctions of marine taxa". Nature 325 (6100): 143–145. doi:10.1038/325143a0. 
  38. ↑ 38.0 38.1 Twitchett RJ, Looy CV, Morante R, Visscher H, Wignall PB (2001). "Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis". Geology 29 (4): 351–354. doi:10.1130/0091-7613(2001)029%3C0351:RASCOM%3E2.0.CO;2. 
  39. ↑ 39.0 39.1 39.2 39.3 39.4 39.5 39.6 Payne, J.L.; Lehrmann, D.J.; Wei, J.; Orchard, M.J.; Schrag, D.P.; Knoll, A.H. (2004). "Large Perturbations of the Carbon Cycle During Recovery from the End-Permian Extinction". Science 305 (5683): 506. doi:10.1126/science.1097023. PMID 15273391. http://www.sciencemag.org/cgi/content/abstract/305/5683/506. 
  40. ↑ Knoll, A.H.; Bambach, R.K.; Canfield, D.E.; Grotzinger, J.P. (1996). "Comparative Earth history and Late Permian mass extinction". Science(Washington) 273 (5274): 452–456. doi:10.1126/science.273.5274.452. PMID 8662528. 
  41. ↑ Leighton, L.R.; Schneider, C.L. (2008). "Taxon characteristics that promote survivorship through the Permian–Triassic interval: transition from the Paleozoic to the Mesozoic brachiopod fauna". Paleobiology 34 (1): 65–79. doi:10.1666/06082.1. 
  42. ↑ doi:10.1126/science.1102127
    Incomplete citation. Click here to automatically expand (or here to expand by hand)
  43. ↑ doi:10.1666/07053.1
    Incomplete citation. Click here to automatically expand (or here to expand by hand)
  44. ↑ 44.0 44.1 Retallack, GJ (1995). "Permian–Triassic life crisis on land". Science 267 (5194): 77–80. doi:10.1126/science.267.5194.77. 
  45. ↑ Looy, CV Brugman WA Dilcher DL & Visscher H (1999). "The delayed resurgence of equatorial forests after the Permian–Triassic ecologic crisis". Proceedings National Academy of Sciences 96: 13857–13862. PMID 10570163. 
  46. ↑ Looy, CV; Twitchett RJ, Dilcher DL, &Van Konijnenburg-Van Cittert JHA and Henk Visscher. (July 3, 2001). "Life in the end-Permian dead zone". Proceedings of the National Academy of Sciences 14 (98): 7879–7883. doi:10.1073/pnas.131218098. 
  47. ↑ 47.0 47.1 47.2 47.3 Retallack GJ Veevers JJ & Morante R (1996). "Global coal gap between Permian–Triassic extinctions and middle Triassic recovery of peat forming plants". GSA Bulletin 108 (2): 195–207. http://bulletin.geoscienceworld.org/cgi/content/abstract/108/2/195. Retrieved on 2007-09-29. 
  48. ↑ Michaelsen P (2002). "Mass extinction of peat-forming plants and the effect on fluvial styles across the Permian–Triassic boundary, northern Bowen Basin, Australia". Palaeogeography, Palaeoclimatology, Palaeoecology 179 (3–4): 173–188. doi:10.1016/S0031-0182(01)00413-8. 
  49. ↑ Maxwell, W. D. (1992). ""Permian and Early Triassic extinction of non-marine tetrapods"". Palaeontology 35: 571–583. 
  50. ↑ Erwin DH (1990). "The End-Permian Mass Extinction". Annual Review of Ecology and Systematics 21: 69–91. doi:10.1146/annurev.es.21.110190.000441. 
  51. ↑ 51.0 51.1 51.2 51.3 Knoll, A.H., Bambach, R.K., Payne, J.L., Pruss, S., and Fischer, W.W. (2007). "Paleophysiology and end-Permian mass extinction". Earth and Planetary Science Letters 256: 295–313. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4N1JRRY-B&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=e1b8f12423387baa895e6e5325aa345a. Retrieved on 2008-07-04.  Full contents may be available online at "Paleophysiology and end-Permian mass extinction" (PDF). Retrieved on 2008-07-04.
  52. ↑ Lehrmann, D.J., Ramezan, J., Bowring, S.A., et al (December 2006). "Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China". Geology 34 (12): 1053–1056. doi:10.1130/G22827A.1. http://geology.geoscienceworld.org/cgi/content/abstract/34/12/1053. 
  53. ↑ Sahney, S. and Benton, M.J. (2008). "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society: Biological 275: 759. doi:10.1098/rspb.2007.1370. http://journals.royalsociety.org/content/qq5un1810k7605h5/fulltext.pdf. 
  54. ↑ Ward PD, Montgomery DR, & Smith R (2000). "Altered river morphology in South Africa related to the Permian–Triassic extinction". Science 289 (5485): 1740–1743. doi:10.1126/science.289.5485.1740. 
  55. ↑ 55.0 55.1 Hallam A & Wignall PB (1997). Mass Extinctions and their Aftermath. Oxford University Press. ISBN 978-0198549161. 
  56. ↑ Rodland, DL & Bottjer, DJ (2001). "Biotic Recovery from the End-Permian Mass Extinction: Behavior of the Inarticulate Brachiopod Lingula as a Disaster Taxon". Palaios 16 (1): 95–101. doi:10.1669/0883-1351(2001)0163%C0095:BRFTEP3%E2.0.CO;2. 
  57. ↑ Zi-qiang W (1996). "Recovery of vegetation from the terminal Permian mass extinction in North China". Review of Palaeobotany and Palynology 91: 121–142. doi:10.1016/0034-6667(95)00069-0. 
  58. ↑ Wagner PJ, Kosnik MA, & Lidgard S (2006). "Abundance Distributions Imply Elevated Complexity of Post-Paleozoic Marine Ecosystems". Science 314 (5803): 1289–1292. doi:10.1126/science.1133795. 
  59. ↑ Clapham, M.E., Bottjer, D.J. and Shen, S. (2006). "Decoupled diversity and ecology during the end-Guadalupian extinction (late Permian)". Geological Society of America Abstracts with Programs 38 (7): 117. http://gsa.confex.com/gsa/2006AM/finalprogram/abstract_111312.htm. Retrieved on 2008-03-28. 
  60. ↑ Foote, M. (1999). "Morphological diversity in the evolutionary radiation of Paleozoic and post-Paleozoic crinoids" (PDF). Paleobiology 25 (sp1): 1–116. doi:10.1666/0094-8373(1999)25[1:MDITER]2.0.CO;2. http://www.jstor.org/stable/pdfplus/2666042.pdf. Retrieved on 2008-05-12. 
  61. ↑ doi:10.1146/annurev.earth.36.031207.124116
    Incomplete citation. Click here to automatically expand (or here to expand by hand)
  62. ↑ Botha, J., and Smith, R.M.H. (2007). "Lystrosaurus species composition across the Permo–Triassic boundary in the Karoo Basin of South Africa". Lethaia 40: 125–137. doi:10.1111/j.1502-3931.2007.00011.x. http://www3.interscience.wiley.com/journal/117996985/abstract?CRETRY=1&SRETRY=0. Retrieved on 2008-07-02.  Full version online at "Lystrosaurus species composition across the Permo–Triassic boundary in the Karoo Basin of South Africa" (PDF). Retrieved on 2008-07-02.
  63. ↑ Benton, M.J. (2004). Vertebrate Paleontology. Blackwell Publishers. xii-452. ISBN 0-632-05614-2. 
  64. ↑ Ruben, J.A., and Jones, T.D. (2000). "Selective Factors Associated with the Origin of Fur and Feathers". American Zoologist 40 (4): 585–596. doi:10.1093/icb/40.4.585. http://icb.oxfordjournals.org/cgi/content/full/40/4/585. 
  65. ↑ Yates AM & Warren AA (2000). "The phylogeny of the 'higher' temnospondyls (Vertebrata: Choanata) and its implications for the monophyly and origins of the Stereospondyli". Zoological Journal of the Linnean Society 128 (1): 77-121. http://www.ingentaconnect.com/content/ap/zj/2000/00000128/00000001/art00184;jsessionid=f6bl337idrkcp.alice?format=print. Retrieved on 2008-01-18. 
  66. ↑ Retallack GJ, Seyedolali A, Krull ES, Holser WT, Ambers CP, Kyte FT (1998). "Search for evidence of impact at the Permian–Triassic boundary in Antarctica and Australia". Geology 26 (11): 979–982. http://geology.geoscienceworld.org/cgi/content/abstract/26/11/979. 
  67. ↑ 67.0 67.1 Becker L, Poreda RJ, Basu AR, Pope KO, Harrison TM, Nicholson C, Iasky R (2004). "Bedout: a possible end-Permian impact crater offshore of northwestern Australia". Science 304 (5676): 1469–1476. doi:10.1126/science.1093925. 
  68. ↑ Becker L, Poreda RJ, Hunt AG, Bunch TE, Rampino M (2001). "Impact event at the Permian–Triassic boundary: Evidence from extraterrestrial noble gases in fullerenes". Science 291 (5508): 1530–1533. doi:10.1126/science.1057243. 
  69. ↑ Basu AR, Petaev MI, Poreda RJ, Jacobsen SB, Becker L (2003). "Chondritic meteorite fragments associated with the Permian–Triassic boundary in Antarctica". Science 302 (5649): 1388–1392. doi:10.1126/science.1090852. 
  70. ↑ Kaiho K, Kajiwara Y, Nakano T, Miura Y, Kawahata H, Tazaki K, Ueshima M, Chen Z, Shi GR (2001). "End-Permian catastrophe by a bolide impact: Evidence of a gigantic release of sulfur from the mantle". Geology 29 (9): 815–818. http://geology.geoscienceworld.org/cgi/content/abstract/26/11/979. Retrieved on 2007-10-22. 
  71. ↑ Farley KA, Mukhopadhyay S, Isozaki Y, Becker L, Poreda RJ (2001). "An extraterrestrial impact at the Permian–Triassic boundary?". Science 293 (5539): 2343. doi:10.1126/science.293.5539.2343a. 
  72. ↑ Koeberl C, Gilmour I, Reimold WU, Philippe Claeys P, Ivanov B (2002). "End-Permian catastrophe by bolide impact: Evidence of a gigantic release of sulfur from the mantle: Comment and Reply". Geology 30 (9): 855–856. doi:10.1130/0091-7613(2002)030%3C0855:EPCBBI%3E2.0.CO;2. 
  73. ↑ Isbell JL, Askin RA, Retallack GR (1999). "Search for evidence of impact at the Permian–Triassic boundary in Antarctica and Australia; discussion and reply". Geology 27 (9): 859–860. doi:10.1130/0091-7613(1999)027%3C0859:SFEOIA%3E2.3.CO;2. 
  74. ↑ Koeberl K, Farley KA, Peucker-Ehrenbrink B, Sephton MA (2004). "Geochemistry of the end-Permian extinction event in Austria and Italy: No evidence for an extraterrestrial component". Geology 32 (12): 1053–1056. doi:10.1130/G20907.1. 
  75. ↑ Langenhorst F, Kyte FT & Retallack GJ (2005). "Reexamination of quartz grains from the Permian–Triassic boundary section at Graphite Peak, Antarctica" (PDF). Lunar and Planetary Science Conference XXXVI. Retrieved on 2007-07-13. 
  76. ↑ von Frese RR, Potts L, Gaya-Pique L, Golynsky AV, Hernandez O, Kim J, Kim H & Hwang J (2006). "Abstract Permian–Triassic mascon in Antarctica". Eos Trans. AGU, Jt. Assem. Suppl. 87 (36): Abstract T41A-08. http://www.agu.org/cgi-bin/SFgate/SFgate?language=English&verbose=0&listenv=table&application=sm06&convert=&converthl=&refinequery=&formintern=&formextern=&transquery=von%20frese&_lines=&multiple=0&descriptor=%2fdata%2fepubs%2fwais%2findexes%2fsm06%2fsm06%7c789%7c3849%7cPermian-Triassic%20Mascon%20in%20Antarctica%7cHTML%7clocalhost:0%7c%2fdata%2fepubs%2fwais%2findexes%2fsm06%2fsm06%7c6292543%206296392%20%2fdata2%2fepubs%2fwais%2fdata%2fsm06%2fsm06.txt Abstract. Retrieved on 2007-10-22. 
  77. ↑ Jones AP, Price GD, Price NJ, DeCarli PS, Clegg RA (2002). "Impact induced melting and the development of large igneous provinces". Earth and Planetary Science Letters 202 (3): 551–561. doi:10.1016/S0012-821X(02)00824-5. 
  78. ↑ 78.0 78.1 78.2 78.3 78.4 White RV (2002). "Earth’s biggest 'whodunnit': unravelling the clues in the case of the end-Permian mass extinction" (PDF). Phil. Trans. Royal Society of London 360: 2963–2985. doi:10.1098/rsta.2002.1097. http://www.le.ac.uk/gl/ads/SiberianTraps/Documents/White2002-P-Tr-whodunit.pdf. Retrieved on 2008-01-12. 
  79. ↑ AHager, Bradford H (2001). "Giant Impact Craters Lead To Flood Basalts: A Viable Model". CCNet 33/2001: Abstract 50470. 
  80. ↑ Hagstrum, Jonathan T (2001). "Large Oceanic Impacts As The Cause Of Antipodal Hotspots And Global Mass Extinctions". CCNet 33/2001: Abstract 50288. 
  81. ↑ Zhou, M-F., Malpas, J, Song, X-Y, Robinson, PT, Sun, M, Kennedy, AK, Lesher, CM & Keays, RR (2002). "A temporal link between the Emeishan large igneous province (SW China) and the end-Guadalupian mass extinction". Earth and Planetary Science Letters 196 (3–4): 113–122. doi:10.1016/S0012-821X(01)00608-2. 
  82. ↑ Mundil, R., Ludwig, K.R., Metcalfe, I. & Renne, P.R (2004). "Age and Timing of the Permian Mass Extinctions: U/Pb Dating of Closed-System Zircons". Science 305 (5691): 1760–1763. doi:10.1126/science.1101012. 
  83. ↑ "Permian–Triassic Extinction - Volcanism"
  84. ↑ Palfy J, Demeny A, Haas J, Htenyi M, Orchard MJ, & Veto I (2001). "Carbon isotope anomaly at the Triassic– Jurassic boundary from a marine section in Hungary". Geology 29 (11): 1047–1050. doi:10.1130/0091-7613(2001)029%3C1047:CIAAOG%3E2.0.CO;2. 
  85. ↑ 85.0 85.1 85.2 Berner, R.A. (2002). "Examination of hypotheses for the Permo-Triassic boundary extinction by carbon cycle modeling". Proceedings of the National Academy of Sciences 99: 32095199. doi:10.1073/pnas.032095199. PMID 11917102. 
  86. ↑ 86.0 86.1 Dickens GR, O'Neil JR, Rea DK & Owen RM (1995). "Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene". Paleoceanography 10 (6): 965–71. doi:10.1029/95PA02087. 
  87. ↑ Schrag, D.P., Berner, R.A., Hoffman, P.F., and Halverson, G.P. (2002). "On the initiation of a snowball Earth". Geochemistry Geophysics Geosystems 3 (6 doi=10.1029/2001GC000219): 1036. doi:10.1029/2001GC000219. http://www.agu.org/pubs/crossref/2002/2001GC000219.shtml.  Preliminary abstract at Schrag, D.P. (June 2001). "On the initiation of a snowball Earth". Geological Society of America.
  88. ↑ 88.0 88.1 Benton, M.J.; Twitchett, R.J. (2003). "How to kill (almost) all life: the end-Permian extinction event". Trends in Ecology & Evolution 18 (7): 358–365. doi:10.1016/S0169-5347(03)00093-4. 
  89. ↑ Dickens GR (2001). "The potential volume of oceanic methane hydrates with variable external conditions". Organic Geochemistry 32 (10): 1179–1193. doi:10.1016/S0146-6380(01)00086-9. 
  90. ↑ Reichow MK, Saunders AD, White RV, Pringle MS, Al'Muhkhamedov AI, Medvedev AI & Kirda NP (2002). "40Ar/39Ar Dates from the West Siberian Basin: Siberian Flood Basalt Province Doubled". Science 296 (5574): 1846–1849. doi:10.1126/science.1071671. 
  91. ↑ Holser WT, Schoenlaub H-P, Attrep Jr M, Boeckelmann K, Klein P, Magaritz M, Orth CJ, Fenninger A, Jenny C, Kralik M, Mauritsch H, Pak E, Schramm J-F, Stattegger K & Schmoeller R (1989). "A unique geochemical record at the Permian/Triassic boundary". Nature 337 (6202): 39–44. doi:10.1038/337039a0. 
  92. ↑ Dobruskina IA (1987). "Phytogeography of Eurasia during the early Triassic". Palaeogeography, Palaeoclimatology, Palaeoecology 58 (1-2): 75–86. doi:10.1016/0031-0182(87)90007-1. 
  93. ↑ Newell ND (1971). "An Outline History of Tropical Organic Reefs" (PDF). American Museum novitates 2465: 1-37. http://digitallibrary.amnh.org/dspace/bitstream/2246/2673/1/N2465.pdf. Retrieved on 2007-11-03. 
  94. ↑ McRoberts, C.A., Furrer, H., Jones, D.S. (1997). "Palaeoenvironmental interpretation of a Triassic– Jurassic boundary section from western Austria based on palaeoecological and geochemical data.". Palaeogeography Palaeoclimatology Palaeoecology 136 (1-4): 79– 95. doi:10.1016/S0031-0182(97)00074-6. 
  95. ↑ Wignall PB & Twitchett RJ (2002). "Permian–Triassic sedimentology of Jameson Land, East Greenland: Incised submarine channels in an anoxic basin". Journal of the Geological Society 159 (6): 691-703. doi:110.1144/0016-764900-120. 
  96. ↑ 96.0 96.1 Monastersky, R. (May 25, 1996), "Oxygen starvation decimated Permian oceans", Science News, http://findarticles.com/p/articles/mi_m1200/is_n21_v149/ai_18351222 
  97. ↑ 97.0 97.1 Zhang R, Follows, MJ, Grotzinger, JP, & Marshall J (2001). "Could the Late Permian deep ocean have been anoxic?". Paleoceanography 16 (3): 317–329. doi:10.1029/2000PA000522. http://www.agu.org/pubs/crossref/2001/2000PA000522.shtml. 
  98. ↑ Kump LR, Pavlov A, & Arthur MA (2005). "Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia". Geology 33 (5): 397–400. doi:10.1130/G21295.1. 
  99. ↑ Grice K, Cao C, Love GD, Bottcher ME, Twitchett RJ, Grosjean E, Summons RE, Turgeon SC, Dunning W & Yugan J (2005). "Photic Zone Euxinia During the Permian–Triassic Superanoxic Event". Science 307 (5710): 706-709. doi:10.1126/science.110432. 
  100. ↑ The Permian - Palaeos

Further reading

External links