Permian–Triassic extinction event 24749 226124645 2008-07-16T22:41:26Z Philcha 2404663 /* Land vertebrates */ specific therocephalian survivors in Karoo {{annotated image/Extinction|caption=The Permian–Triassic extinction event, labelled "P-T" here, is the most significant extinction event in this plot for marine [[genus|genera]].}} The '''Permian–Triassic (P–Tr) extinction event''', informally known as the '''Great Dying''', was an [[extinction event]] that occurred {{Ma|251.4}},<ref name="Jin2000"> {{ cite journal|title=Pattern of Marine Mass Extinction Near the Permian–Triassic Boundary in South China|author=Jin YG, Wang Y, Wang W, Shang QH, Cao CQ, Erwin DH|date=2000|journal=Science|volume=289|issue=5478|pages=432–436|pmid=10903200|doi=10.1126/science.289.5478.432 }} </ref> <ref name="Bowring1998"/> forming the boundary between the [[Permian]] and [[Triassic]] [[geologic period]]s. It was the Earth's most severe extinction event, with up to 96 percent of all [[marine biology|marine]] [[species]]<ref name="Benton"> {{ cite book|author=Benton M J|date=2005|title=When Life Nearly Died: The Greatest Mass Extinction of All Time|publisher=Thames & Hudson|isbn=978-0500285732 }} </ref> and 70 percent of [[Terrestrial ecoregion|terrestrial]] [[vertebrate]] species becoming [[extinction|extinct]]; it is the only known mass extinction of [[insects]].<ref name="Labandeira" /> Because so much biodiversity was lost, the recovery of life on earth took significantly longer than after other extinction events.<ref name="Benton"/> This event has been described as the "mother of all mass extinctions".<ref name="Erwin1993"> {{ cite book|author=Erwin DH|date=1993|title=The great Paleozoic crisis; Life and death in the Permian|publisher=Columbia University Press|isbn=0231074670 }} </ref> The pattern of extinction is still disputed <ref name="YinGSSP">{{cite journal|title=The Global Stratotype Section and Point (GSSP) of the Permian-Triassic Boundary|author=Yin H, Zhang K, Tong J, Yang Z, Wu S|journal=Episodes|volume=24|issue=2|pages=102–114}}</ref>, as different studies suggest one<ref name="Jin2000"/> to three<ref name="Yin1992">{{cite journal|title=Permo-Triassic Events in the Eastern Tethys|author=Yin HF, Sweets WC, Yang ZY, Dickins JM,|journal=Cambridge Univ. Pres, Cambridge, 1992}}</ref> 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 event]]s, increased [[volcanoes|volcanism]], or sudden release of [[methane hydrates]] from the sea floor; gradual changes include sea-level change, [[anoxia]], and increasing [[arid]]ity.<ref name="TannerLucas"/>{{-}}{{include timeline}} ==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.<ref name="Erwin1993Paleozoic Crisis">{{ cite book | author=Erwin, D.H | date=1993 | title=The Great Paleozoic Crisis: Life and Death in the Permian | publisher=Columbia University Press | location= New York isbn=0231074670 }}</ref> However, a study of [[Uranium lead dating|uranium/lead]] ratios of [[zircon]]s from rock sequences near Meishan, Changxing, Zhejian Province, China<ref name="Bowring1998">{{cite journal|title=U/Pb Zircon Geochronology and Tempo of the End-Permian Mass Extinction|author=Bowring SA, Erwin DH, Jin YG, Martin MW, Davidek K, Wang W|date=1998|journal=Science|volume=280|issue=1039|pages=1039–1045|doi=10.1126/science.280.5366.1039 }}</ref> date the extinction to {{Ma|251.4|&nbsp;}}±.03 Ma, with an ongoing elevated extinction rate occurring for some time thereafter.<ref name="Jin2000"/> A large (-9‰<ref name=McElwain2007 />), abrupt global change in the [[Isotope analysis|ratio]] of [[carbon-13|<sup>13</sup>C]] to [[carbon-12|<sup>12</sup>C]], denoted {{delta|13|C}}, coincides with this extinction,<ref> {{ cite journal|author=Magaritz, M|date=1989|title=<sup>13</sup>C minima follow extinction events: a clue to faunal radiation|journal=Geology|volume=17|pages=337–340 }} </ref><ref> {{ cite journal|author=Krull, S.J., and Retallack, J.R.|date=2000|title=<sup>13</sup>C depth profiles from paleosols across the Permian–Triassic boundary: Evidence for methane release|journal=GSA Bulletin|volume=112|issue=9|pages=1459–1472|doi=10.1130/0016-7606(2000)112%3C1459:CDPFPA%3E2.0.CO;2 }} </ref><ref> {{ cite journal|author=Dolenec, T., Lojen, S., Ramovs, A.|date=2001|title=The Permian–Triassic boundary in Western Slovenia (Idrijca Valley section): magnetostratigraphy, stable isotopes, and elemental variations|journal=Chemical Geology|volume=175|issue=1|pages=175–190|doi=10.1016/S0009-2541(00)00368-5 }} </ref><ref> {{ cite journal|author=Musashi, M., Isozaki, Y., Koike, T. and Kreulen, R.|date=2001|title=Stable carbon isotope signature in mid-Panthalassa shallow-water carbonates across the Permo–Triassic boundary: evidence for <sup>13</sup>C-depleted ocean|journal=Earth Planet. Sci. Lett.|volume=193|pages=9–20|doi=10.1016/S0012-821X(01)00398-3 }} </ref>, and is sometimes used to identify the Permian-Triassic boundary in rocks that are unsuitable for radiometric dating.<ref name="DolenecLojenRamovs2001PermianTriassicBoundary"> {{ cite journal | date=2001 | title=The Permian-Triassic boundary in Western Slovenia (Idrijca Valley section): magnetostratigraphy, stable isotopes, and elemental variations | journal=Chemical Geology | volume=175 | pages=175–190 | author=Dolenec, T., Lojen, S., and Ramovs, A. | doi=10.1016/S0009-2541(00)00368-5 }}</ref><!--Commented out as per Talk, will delete in 2 weeks if no justification for inclusion is presented. Further evidence for environmental change around the P-T boundary suggests an 8°C rise in temperature,<ref name=McElwain2007/> and an increase in {{co2}} levels by 2000ppm (by contrast, the concentration immediately before the industrial revolution was 280ppm).<ref name=McElwain2007>{{cite journal | author = McElwain, J.C. | coauthors = Punyasena, S.W. | year = 2007 | title = Mass extinction events and the plant fossil record | journal = Trends in Ecology & Evolution | volume = 22 | issue = 10 | pages = 548–557 | doi=10.1016/j.tree.2007.09.003 }}</ref> There is also evidence of increased ultraviolet radiation reaching the earth, and causing the mutation of plant spores.<ref name=McElwain2007/>--> 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,<ref name="VisscherBrinkhuisEtAl1996TerminalPaleozoicFungalEvent"> {{ cite journal|author=H Visscher, H Brinkhuis, D L Dilcher, W C Elsik, Y Eshet, C V Looy, M R Rampino, and A Traverse|date=1996|title=The terminal Paleozoic fungal event: Evidence of terrestrial ecosystem destabilization and collapse|journal=Proceedings of the National Academy of Sciences|volume=93|issue=5|pages=2155–2158|url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=39926&blobtype=pdf|accessdate=2007-09-20 }} </ref> 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 fossil]]s, 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.<ref name="VisscherBrinkhuisEtAl1996TerminalPaleozoicFungalEvent" /> 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]];<ref name="FosterEtAl2002RevisionOfReduviasporonites">{{cite journal | author = Foster, C.B. | coauthors = Stephenson, M.H.; Marshall, C.; Logan, G.A.; Greenwood, P.F. | year = 2002 | title = A Revision Of Reduviasporonites Wilson 1962: Description, Illustration, Comparison And Biological Affinities | journal = Palynology | volume = 26 | issue = 1 | pages = 35–58 | doi = 10.2113/0260035 | url=http://palynology.geoscienceworld.org/cgi/content/abstract/26/1/35}}</ref><ref name="McElwain2007">{{cite journal | author = McElwain, J.C. | coauthors = Punyasena, S.W. | year = 2007 | title = Mass extinction events and the plant fossil record | journal = Trends in Ecology & Evolution | volume = 22 | issue = 10 | pages = 548–557 | doi=10.1016/j.tree.2007.09.003 | url=http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=17919771&cmd=showdetailview&indexed=google}}</ref> the spike did not appear world-wide; <ref>{{ cite journal | title=Permian-Triassic Transition in Spain: A multidisciplinary approach | journal=Palaeogeography, Palaeoclimatology, Palaeoecology | volume=229 | issue=1-2 | date=2005 | pages=1–2 | doi=10.1016/j.palaeo.2005.06.028 | url=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 | author=López-Gómez, J. and Taylor, E.L. }}</ref><ref name="LooyEtAl2005EndPermianDeadZone">{{cite journal | author = Looy, C.V. | coauthors = Twitchett, R.J.; Dilcher, D.L.; Van Konijnenburg-van Cittert, J.H.A.; Visscher, H. | year = 2005 | title = Life in the end-Permian dead zone | journal = Proceedings of the National Academy of Sciences | volume = 162 | issue = 4 | pages = 653–659 | doi = 10.1073/pnas.131218098 | quote = See image 2 | pmid = 11427710 }}</ref>; and in many places it did not fall on the Permian-Triassic boundary.<ref name="wardetal"/> 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.<ref name="RetallackEtAl2003VertebrateExtinctionInKaroo">{{cite journal | author = Retallack, G.J. | coauthors = Smith, R.M.H.; Ward, P.D. | year = 2003 | title = Vertebrate extinction across Permian-Triassic boundary in Karoo Basin, South Africa | journal = Bulletin of the Geological Society of America | volume = 115 | issue = 9 | pages = 1133–1152 | url = http://intl-bulletin.geoscienceworld.org/cgi/content/abstract/115/9/1133 | doi=10.1130/B25215.1 }}</ref> 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.<ref name="wardetal"> {{ cite journal|author=Ward PD, Botha J, Buick R, De Kock MO, Erwin DH, Garrison GH, Kirschvink JL & Smith R|date=2005|title=Abrupt and Gradual Extinction Among Late Permian Land Vertebrates in the Karoo Basin, South Africa|journal=Science|volume=307|issue=5710|pages=709–714|doi=10.1126/science.1107068 }} </ref><ref> {{ cite journal|author=Rampino MR, Prokoph A & Adler A|date=2000|doi=10.1130%2F0091-7613%282000%2928%3C643%3ATOTEEH%3E2.0.CO%3B2|title=Tempo of the end-Permian event: High-resolution cyclostratigraphy at the Permian–Triassic boundary|journal=Geology|volume=28|issue=7|pages=643–646 }} </ref> Statistical analyses of some highly fossiliferous strata in Meishan, South China suggest that the main extinction was clustered around one peak.<!--, with tentative support for a second peak {{Ma|250.6}}.<ref name="Benton"/><!--The second reference rejects this second peak - I'd be interested to know what Benton says, but can't access the book.--><ref name="Jin2000"/> Recent research shows that different groups went extinct at different times; for example, while difficult to date absolutely, [[ostracod]]e and [[brachiopod]] extinctions were separated by between 0.72 and 1.22&nbsp;million years.<ref name=Wang2007>{{cite journal | author = Wang, S.C. | coauthors = Everson, P.J. | year = 2007 | title = Confidence intervals for pulsed mass extinction events | journal = Paleobiology | volume = 33 | issue = 2 | pages = 324–336 | doi = 10.1666/06056.1 }}</ref> 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.<ref>{{cite journal|author=Twitchett RJ Looy CV Morante R Visscher H & Wignall PB|title=Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis |journal=Geology|date=2001|volume=29|issue=4|pages=351–354|doi=10.1130/0091-7613(2001)029<0351:RASCOM>2.0.CO;2|year=2001}}</ref> An older theory, still supported in some recent papers,<ref name=Retallack2006>{{cite journal | author = Retallack, G.J. | coauthors = Metzger, C.A.; Greaver, T.; Jahren, A.H.; Smith, R.M.H.; Sheldon, N.D. | year = 2006 | title = Middle-Late Permian mass extinction on land | journal = Bulletin of the Geological Society of America | volume = 118 | issue = 11-12 | pages = 1398–1411 | doi=10.1130/B26011.1 }}</ref> 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]] [[Geologic time scale|epoch]] of the [[Permian]].<ref>{{cite journal|author=Stanley SM & Yang X|date=1994|title=A Double Mass Extinction at the End of the Paleozoic Era|journal=Science|volume=266|issue=5189|pages=1340–1344|doi=10.1126/science.266.5189.1340|pmid=17772839}}</ref> For example, all but one of the surviving [[dinocephalia| dinocephalian]] genera died out at the end of the Guadalupian,<ref>{{cite journal | author=Retallack, G.J., Metzger, C.A., Jahren, A.H., Greaver, T., Smith, R.M.H., and Sheldon, N.D | title=Middle-Late Permian mass extinction on land | journal=GSA Bulletin | date=November/December 2006 | volume=118 | issue=11/12 | pages=1398–1411 | doi = 10.1130/B26011.1 }}</ref> as did the Verbeekinidae, a family of large-size [[fusulinid|fusuline]] [[foraminifera]].<ref>{{ cite journal | author=Ota, A, and Isozaki, Y. | title=Fusuline biotic turnover across the Guadalupian–Lopingian (Middle–Upper Permian) boundary in mid-oceanic carbonate buildups: Biostratigraphy of accreted limestone in Japan | journal=Journal of Asian Earth Sciences | volume=26 | issue=3-4 | date=March 2006 | pages=353-368 }}</ref> 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.<ref>{{ cite journal | author=Shen, S., and Shi, G.R. | title=Paleobiogeographical extinction patterns of Permian brachiopods in the Asian-western Pacific region | journal=Paleobiology | date=2002 | volume=28 | pages=449–463 | doi=10.1666/0094-8373(2002)028<0449:PEPOPB>2.0.CO;2 | year=2002 }}</ref><ref>{{ cite journal | author=Wang, X-D, and Sugiyama, T. | title=Diversity and extinction patterns of Permian coral faunas of China | journal=Lethaia | volume=33 | issue=4 | pages=285–294 | date=December 2000 | doi=10.1080/002411600750053853 | url=http://www.blackwell-synergy.com/doi/abs/10.1080/002411600750053853 }}</ref> ==Extinction patterns== {| style="margin: 0 0 0.5em 1em; float:right; width:350px; text-align:left; font-size:90%;" class="wikitable" |- ! style="background:#aaeecc; text-align:center" | Marine extinctions | style="background:#aaeecc; text-align:center" | [[genus|Genera]] extinct | style="background:#aaeecc; text-align:center" | Notes |- | colspan="3" style="background:#bfd" | '''[[Marine invertebrates]]''' |- | [[Foraminifera]] | align="center" | 97% || Fusulinids died out, but were almost extinct before the catastrophe |- | [[Radiolaria]] ([[plankton]]) | align="center" | 99%<ref> {{ cite journal|author=Racki G|date=1999|title=Silica-secreting biota and mass extinctions: survival processes and patterns|volume=Palaeogeography Palaeoclimatology Palaeoecology|volume=154|issues=1–2|pages=107–132|doi=10.1016/S0031-0182(99)00089-9 }} </ref> || |- | [[Anthozoa]] ([[sea anemones]], [[corals]], etc.) | align="center" | 96% || [[tabulate coral|Tabulate]] and [[rugosa|rugose]] corals died out |- | [[Bryozoans]] | align="center" | 79% || Fenestrates, trepostomes, and cryptostomes died out |- | [[Brachiopods]] | align="center" | 96% || [[orthida|Orthid]]s and [[Productida|productid]]s died out |- | [[Bivalves]] | align="center" | 59% || &nbsp; |- | [[Gastropods]] (snails) | align="center" | 98% || &nbsp; |- | [[Ammonites]] ([[cephalopods]]) | align="center" | 97% || &nbsp; |- | [[Crinoids]] ([[echinoderms]]) | align="center" | 98% || Inadunates and camerates died out |- | [[Blastoids]] ([[echinoderms]]) | align="center" | '''100%''' || May have become extinct shortly before the P–Tr boundary |- | [[Trilobites]] | align="center" | '''100%''' || In decline since the Devonian; only 2 genera living before the extinction |- | [[Eurypterids]] ("sea scorpions") | align="center" | '''100%''' || May have become extinct shortly before the P–Tr boundary |- | [[Ostracods]] (small [[crustaceans]]) | align="center" | 59% || &nbsp; |- | [[Graptolites]] | align="center" | '''100%''' || In decline since the [[Devonian]] (may have living relatives amongst [[Pterobranchia]]) |- | colspan="4" style="background:#bfd" | '''Fish''' |- | [[Acanthodians]] | align="center" | '''100%''' || &nbsp; |- |- | [[Placoderms]] | align="center" | '''100%''' || &nbsp; |- |} ===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 [[conodont]]s from the Permian.<ref name="Jin2000"/> 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]].<ref> {{ Citation | last=Bambach | first=R.K. | last2=Knoll | first2=A.H. | last3=Wang | first3=S.C. | title=Origination, extinction, and mass depletions of marine diversity | journal=Paleobiology | volume=30 | issue=4 | pages=522–542 |date=December 2004 | url=http://www.bioone.org/perlserv/?request=get-document&issn=0094-8373&volume=30&page=522 }} </ref> 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).<ref name=Stanley2008>{{cite journal | author = Stanley, S.M. | year = 2008 | title = Predation defeats competition on the seafloor | journal = Paleobiology | volume = 34 | issue = 1 | pages = 1–21 | url = http://paleobiol.geoscienceworld.org/cgi/content/short/34/1/1 | accessdate = 2008-05-13 | doi = 10.1666/07026.1 }}</ref><ref name=Stanley2007>{{cite journal | author = Stanley, S.M. | year = 2007 | title = An Analysis of the History of Marine Animal Diversity | journal = Paleobiology | volume = 33 | issue = sp6 | pages = 1–55 | url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/33/4_Suppl/1 | doi = 10.1666/06020.1 }}</ref> The extinction rate of marine organisms was catastrophic.<ref> {{ cite journal|author=McKinney, M.L.|date=1987|title=Taxonomic selectivity and continuous variation in mass and background extinctions of marine taxa|journal=Nature|volume=325|issue=6100|pages=143–145|doi=10.1038/325143a0 }} </ref><ref name="Erwin1993"/><ref name="Jin2000"/><ref name="Twitchett"> {{ cite journal|title=Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis|author=Twitchett RJ, Looy CV, Morante R, Visscher H, Wignall PB|journal=Geology|date=2001|volume=29|issue=4|pages=351–354|doi=10.1130/0091-7613(2001)029%3C0351:RASCOM%3E2.0.CO;2 }} </ref> 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 [[ammonite]]s; and [[crinoid]]s ("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.<ref name=Payne2004/><ref name=Knoll1996>{{cite journal | author = Knoll, A.H. | coauthors = Bambach, R.K.; Canfield, D.E.; Grotzinger, J.P. | year = 1996 | title = Comparative Earth history and Late Permian mass extinction | journal = Science(Washington) | volume = 273 | issue = 5274 | pages = 452–456 | doi = 10.1126/science.273.5274.452 | pmid = 8662528 }}</ref> In the case of the brachiopods at least, surviving taxa were generally small, rare members of a diverse community.<ref name=Leighton2008>{{cite journal | author = Leighton, L.R. | coauthors = Schneider, C.L. | year = 2008 | title = Taxon characteristics that promote survivorship through the Permian–Triassic interval: transition from the Paleozoic to the Mesozoic brachiopod fauna | journal = Paleobiology | volume = 34 | issue = 1 | pages = 65–79 | doi = 10.1666/06082.1 }}</ref> The [[ammonoids]], which had been in a long-term decline for the 30 million years since the Roadian (middle Permian), suffered a selective extinction 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.<ref>{{cite doi|10.1126/science.1102127}}</ref> 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.<ref>{{cite doi|10.1666/07053.1}}</ref> ===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,<ref name="Labandeira"> {{ cite journal |author=Labandeira CC, Sepkoski JJ |title=Insect diversity in the fossil record |journal=Science |volume=261 |issue=5119 |pages=310–5 |year=1993 |pmid=11536548 |doi= |accessdate=2008-01-08 }} </ref> with eight or nine insect orders becoming extinct and ten more greatly reduced in diversity. [[Palaeodictyopteroidea|Palaeodictyopteroid]]s (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.<ref name="Erwin1993"/> 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 [[Exopterygota|caloneurodeans]], [[monuran]]s, paleodictyopteroids, [[protelytroptera]]ns, and [[protodonata|protodonate]]s became extinct by the end of the Permian. In well-documented Late Triassic deposits, fossils overwhelmingly consist of modern fossil insect groups.<ref name="Labandeira"/> ===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.<ref name=McElwain2007/> Even the reduction observed in species diversity (of 50%) may be mostly due to [[Taphonomy|taphonomic]] processes.<ref name=McElwain2007/> However, a massive rearrangement of ecosystems does occur, with plant abundances and distributions changing profoundly.<ref name=McElwain2007/> At the P–Tr boundary, the dominant floral groups changed, with many groups of land plants entering abrupt decline, such as ''[[Cordaites]]'' ([[gymnosperm]]s) and ''[[Glossopteris]]'' ([[Pteridospermatophyta|seed ferns]]).<ref name=Retallack1995> {{ cite journal|author=Retallack, GJ|date=1995|title=Permian–Triassic life crisis on land|journal=Science|volume=267|pages=77–80|issue=5194|doi=10.1126/science.267.5194.77 }} </ref> Dominant [[gymnosperm]] genera were replaced post-boundary by [[Lycopodiophyta|lycophytes]] - extant lycophytes are recolonizers of disturbed areas.<ref> {{ cite journal|author=Looy, CV Brugman WA Dilcher DL & Visscher H|date=1999|title=The delayed resurgence of equatorial forests after the Permian–Triassic ecologic crisis|journal=Proceedings National Academy of Sciences|volume=96|pages=13857–13862|pmid=10570163 }} </ref> Palynological or pollen studies from East [[Greenland]] of sedimentary rock strata laid down during the extinction period indicate dense gymnosperm [[woodland]]s 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 {{delta|13|C}} values, but occurs many years after. <ref> {{ cite journal | last = Looy | first = CV | coauthors = Twitchett RJ, Dilcher DL, &Van Konijnenburg-Van Cittert JHA and Henk Visscher. | title = Life in the end-Permian dead zone | journal = Proceedings of the National Academy of Sciences | volume = 14 | issue = 98 | pages = 7879–7883 |date=July 3, 2001 | doi =10.1073/pnas.131218098 }} </ref> The recovery of gymnosperm forests would take 4-5 million years.<ref name=McElwain2007/> ====The Coal Gap==== No [[coal]] deposits are known from the Early Triassic, and those in the Middle Triassic are thin and low-grade.<ref name=Retallack1996/> 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.<ref name="Retallack1996"> {{ cite journal|author=Retallack GJ Veevers JJ & Morante R|date=1996|title=Global coal gap between Permian–Triassic extinctions and middle Triassic recovery of peat forming plants|journal=GSA Bulletin|volume=108|issue=2|pages=195–207|url=http://bulletin.geoscienceworld.org/cgi/content/abstract/108/2/195|accessdate=2007-09-29 }} </ref> 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.<ref name=Retallack1996/> 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.<ref name=McElwain2007/> 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 [[ecosystem]]s may have responded to the changed conditions by relocating, perhaps to areas where we have no sedimentary record for the Early Triassic.<ref name=McElwain2007/> 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;<ref> {{ cite journal|author=Michaelsen P|title=Mass extinction of peat-forming plants and the effect on fluvial styles across the Permian–Triassic boundary, northern Bowen Basin, Australia|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=179|issue=3–4|date=2002|pages=173–188|doi=10.1016/S0031-0182(01)00413-8 }} </ref> Interestingly, coal deposits in Australia and Antarctica disappear significantly ''before'' the P-Tr boundary.<ref name=McElwain2007/> ===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") [[Family (biology)|families]] became extinct. Large herbivores suffered the heaviest losses. All Permian [[anapsid]] reptiles died out except the [[Procolophonidae|procolophonid]]s ([[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).<ref>{{cite journal | author=Maxwell, W. D. | date=1992 | title="Permian and Early Triassic extinction of non-marine tetrapods" | journal=Palaeontology | volume=35 | pages=571–583}}</ref><ref>{{cite journal | author=Erwin DH | title=The End-Permian Mass Extinction | journal=Annual Review of Ecology and Systematics | volume=21 | pages=69–91 | date=1990 | doi=10.1146/annurev.es.21.110190.000441 }} </ref><!--<ref name="TannerLucas"/> ? irrelevant: about extinctions within and at end of Tr; focuses more on geological phenomena rather than patterns of extinction (see Talk) --> ===Possible explanations of these patterns=== <!-- An analysis of marine fossils from the Permian's final [[Changhsingian]] stage found that marine organisms with low tolerance for [[hypercapnia]] (high concentration of [[carbon dioxide]]) had high extinction rates, while the most tolerant organisms had very slight losses. --> The most vulnerable marine organisms were those which produced calcareous hard parts (i.e. from [[calcium carbonate]]) and had low [[metabolic rate]]s and weak respiratory systems - notably calcareous sponges, rugose and tabulate corals, calciate brachiopods, bryozoans, and echinoderms; about 81% of such [[genus| genera]] became extinct. Close relatives which did not produce calcareous hard parts suffered only minor losses, for example [[sea anemone]]s, 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.<ref name="KNollBambach2007Paleophysiology"> {{ cite journal | author=Knoll, A.H., Bambach, R.K., Payne, J.L., Pruss, S., and Fischer, W.W. | title=Paleophysiology and end-Permian mass extinction | journal=Earth and Planetary Science Letters | volume=256 | date=2007 | pages=295–313 | dio=10.1016/j.epsl.2007.02.018 | url=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 | accessdate=2008-07-04 }} Full contents may be available online at {{ cite web | url=http://pangea.stanford.edu/~jlpayne/Knoll%20et%20al%202007%20EPSL%20Permian%20Triassic%20paleophysiology.pdf | title=Paleophysiology and end-Permian mass extinction | accessdate=2008-07-04 }}</ref> 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. [[Mathematical model | 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.<ref name="KNollBambach2007Paleophysiology" /> 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 pigment]]s to oxygenate tissues; makes body fluids more [[acid]]ic, 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, [[Invertebrate trachea | trachea]]e, etc.). In marine organisms relatively modest but sustained increases in {{CO2}} concentrations hamper the synthesis of [[protein]]s, reduce fertilization rates and produce deformities in calcareous hard parts.<ref name="KNollBambach2007Paleophysiology" /> 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.<ref name="KNollBambach2007Paleophysiology" /> ==Biotic recovery== Earlier analyses indicated that life on Earth recovered quickly after the Permian extinctions, but this was mostly in the form of [[pioneer organism|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;<ref name="LehrmannRamezanBowring2006TimingOfRecovery">{{ cite journal | title=Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China | author=Lehrmann, D.J., Ramezan, J., Bowring, S.A., ''et al'' | journal=Geology | date=December 2006 | volume=34 | number=12 | url=http://geology.geoscienceworld.org/cgi/content/abstract/34/12/1053 | pages=1053–1056 | doi=10.1130/G22827A.1 }}</ref> and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.<ref name="SahneyBenton2008RecoveryFromProfoundExtinction">{{ cite journal | url=http://journals.royalsociety.org/content/qq5un1810k7605h5/fulltext.pdf | author=Sahney, S. and Benton, M.J. | date=2008 | title=Recovery from the most profound mass extinction of all time | journal=Proceedings of the Royal Society: Biological | doi=10.1098/rspb.2007.1370 | volume = 275 | pages = 759}}</ref> [[Image:Waimakariri01 gobeirne.jpg|thumb|200px|right| 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.<ref name="Retallack1996"/> River patterns in the [[Karoo]] changed from [[Meandering river|meandering]] to [[Braided river|braided]], indicating that vegetation there was very sparse for a long time.<ref> {{ cite journal|author=Ward PD, Montgomery DR, & Smith R|date=2000|title=Altered river morphology in South Africa related to the Permian–Triassic extinction|journal=Science|volume=289|issue=5485|pages=1740–1743 |doi=10.1126/science.289.5485.1740 }} </ref> Each major segment of the early Triassic ecosystem &mdash; plant and animal, marine and terrestrial &mdash; 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 [[bivalve]]s ''Claraia'', ''Eumorphotis'', ''Unionites'' and ''Promylina''. A healthy [[ecosystem]] has a much larger number of genera, each living in a few preferred types of habitat.<ref name=Retallack1995/><ref name="HallamWignall"/> Disaster taxa (opportunist organisms) took advantage of the devastated ecosystem and enjoyed a temporary population boom and increase in their territory. For example: ''[[Lingula (genus)|Lingula]]'' (a [[brachiopod]]); [[stromatolites]], which had been confined to marginal environments since the [[Ordovician]]; ''[[Pleuromeia]]'' (a small, weedy plant); ''[[Dicrodium]]'' (a [[seed fern]]).<ref> {{ cite journal|author=Rodland, DL & Bottjer, DJ|date=2001|title=Biotic Recovery from the End-Permian Mass Extinction: Behavior of the Inarticulate Brachiopod ''Lingula'' as a Disaster Taxon |journal=Palaios|volume=16|issue=1|pages=95–101|doi=10.1669/0883-1351(2001)0163%C0095:BRFTEP3%E2.0.CO;2 }} </ref><ref name="TannerLucas"/><ref> {{ cite journal|author=Zi-qiang W|title=Recovery of vegetation from the terminal Permian mass extinction in North China|date=1996|journal=Review of Palaeobotany and Palynology|volume=91|issues=1–4|pages=121–142|doi=10.1016/0034-6667(95)00069-0 }} </ref><ref name="HallamWignall"> {{ cite book|author=Hallam A & Wignall PB|date=1997|title=Mass Extinctions and their Aftermath|publisher=Oxford University Press|isbn=978-0198549161 }} </ref> [[Image:Crinoïde Carbonifère 8127.jpg| thumb | right | 200px | Sessile filter feeders like this [[sea lily]] 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 feeder]]s, such as [[brachiopod]]s and [[sea lily|sea lilies]], and an increase in more complex mobile species such as [[snail]]s, [[sea urchin|urchins]] and [[crab]]s. 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,<ref> {{ cite journal|title=Abundance Distributions Imply Elevated Complexity of Post-Paleozoic Marine Ecosystems|author=Wagner PJ, Kosnik MA, & Lidgard S|journal=Science|date=2006|volume=314|issue=5803|pages=1289–1292|doi=10.1126/science.1133795 }} </ref> 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.<ref>{{cite journal|url=http://gsa.confex.com/gsa/2006AM/finalprogram/abstract_111312.htm|author=Clapham, M.E., Bottjer, D.J. and Shen, S. |date=2006|title=Decoupled diversity and ecology during the end-Guadalupian extinction (late Permian)|journal=Geological Society of America Abstracts with Programs|volume=38|issue=7|accessdate=2008-03-28|pages=117}}</ref> [[Crinoid]]s ("sea lilies") suffered a selective extinction, resulting in a decrease in the variety of forms in which they grew.<ref name=Foote1999>{{cite journal | author = Foote, M. | year = 1999 | title = Morphological diversity in the evolutionary radiation of Paleozoic and post-Paleozoic crinoids | journal = Paleobiology | volume = 25 | issue = sp1 | pages = 1–116 | url = http://www.jstor.org/stable/pdfplus/2666042.pdf | accessdate = 2008-05-12 | doi = 10.1666/0094-8373(1999)25[1:MDITER]2.0.CO;2 }}</ref> 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.<ref name=Baumiller2008>{{cite doi|10.1146/annurev.earth.36.031207.124116}}</ref> ===Land vertebrates=== <!-- Image with unknown copyright status removed: [[Image:Diversity-GlobalAlpha-Tetrapods.jpg|thumb|Figure 1. Global diversity (dashed line) and mean alpha diversity (solid line) of Permo-Triassic tetrapod families. Extinctions are labelled as 1 = Olson’s Extinction, 2 = End-Guadalupian Extinction and 3 = PTB Extinction. Geological stages are from Gradstein & Ogg 2004 <ref>Sahney, S. and Benton, M.J. 2007. Recovery from the most profound mass extinction of all time. Proceedings of the Royal Society B. In Press.</ref>.]] --> [[Image:Lystrosaurus BW.jpg| thumb | right | 200px | ''[[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.<ref name="TannerLucas"/> Smaller carnivorous [[cynodont]] [[therapsids]] also survived, including the ancestors of [[mammals]]. In the [[Karoo]] region of southern [[Africa]] the [[therocephalia]]ns ''[[Tetracynodon]]'', ''[[Moschorhinus]]'' and ''[[Ictidosuchoides]]'' survived but do not appear to have been abundant in the Triassic.<ref name="BothaSmith2007LystrosaurusSpeciesComposition">{{ cite journal | author=Botha, J., and Smith, R.M.H. | date=2007 | title=Lystrosaurus species composition across the Permo–Triassic boundary in the Karoo Basin of South Africa | journal=Lethaia | volume=40 | pages=125-137 | url=http://www3.interscience.wiley.com/journal/117996985/abstract?CRETRY=1&SRETRY=0 | accessdate=2008-07-02 | doi=10.1111/j.1502-3931.2007.00011.x }} Full version online at {{ cite web | url=http://www.nasmus.co.za/PALAEO/jbotha/pdfs/Botha%20and%20Smith%202007.pdf | title=Lystrosaurus species composition across the Permo–Triassic boundary in the Karoo Basin of South Africa | accessdate=2008-07-02 }}</ref> [[Archosaurs]] (which included the ancestors of [[crocodilians]]) were initially rarer than therapsids, but they began to displace therapsids in the mid-Triassic.<ref name="TannerLucas"/> In the mid to late Triassic the [[dinosaur]]s evolved from one group of archosaurs, and went on to dominate terrestrial ecosystems for the rest of the [[Mesozoic]].<ref name="BentonVertebratePaleontology">{{cite book|author=Benton, M.J.|date=2004|title=Vertebrate Paleontology|publisher=Blackwell Publishers|pages=xii-452|isbn=0-632-05614-2}}</ref> This "Triassic Takeover" may have contributed to the [[evolution of mammals]] by forcing the surviving therapsids and their [[mammaliformes| mammaliform]] successors to live as small, mainly nocturnal [[insectivore]]s; nocturnal life probably forced at least the mammaliforms to develop fur and higher [[metabolic rate]]s.<ref name="RubenJones2000FurAndFeathers">{{ cite journal | author=Ruben, J.A., and Jones, T.D. | title=Selective Factors Associated with the Origin of Fur and Feathers | journal=American Zoologist | date=2000 | volume=40 | issue=4 | pages=585–596 |doi=10.1093/icb/40.4.585 | url=http://icb.oxfordjournals.org/cgi/content/full/40/4/585 }}</ref> Some [[temnospondyl]] [[amphibian]]s made a relatively quick recovery, in spite of nearly becoming extinct. ''[[Mastodonsaurus]]'' and [[trematosauria]]ns were the main aquatic and semi-aquatic predators during most of the [[Triassic]], some preying on [[tetrapod]]s and others on fish.<ref> {{ cite journal|author=Yates AM & Warren AA|date=2000 |url=http://www.ingentaconnect.com/content/ap/zj/2000/00000128/00000001/art00184;jsessionid=f6bl337idrkcp.alice?format=print|title=The phylogeny of the 'higher' temnospondyls (Vertebrata: Choanata) and its implications for the monophyly and origins of the Stereospondyli|journal=Zoological Journal of the Linnean Society |volume=128|issue=1|pages=77-121|accessdate=2008-01-18 }} </ref> 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.<ref name="Benton"/> ==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 event]]s, increased [[volcanoes|volcanism]], or sudden release of [[methane hydrates]] from the sea floor. The latter include sea-level change, [[anoxia]], and increasing [[arid]]ity.<ref name="TannerLucas"> {{ cite journal|author=Tanner LH, Lucas SG & Chapman MG|title=Assessing the record and causes of Late Triassic extinctions|journal=Earth-Science Reviews|volume=65|issue=1-2|pages=103-139|date=2004|doi=10.1016/S0012-8252(03)00082-5|url=http://nmnaturalhistory.org/pdf_files/TJB.pdf|accessdate=2007-10-22 }} </ref> ===Impact event=== [[Image:Impact event.jpg|thumb|300px|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;<ref name="Retallack_etal_1998"> {{ cite journal | author=Retallack GJ, Seyedolali A, Krull ES, Holser WT, Ambers CP, Kyte FT | title=Search for evidence of impact at the Permian–Triassic boundary in Antarctica and Australia | journal=Geology | volume=26 | issue=11 | year=1998 | pages=979–982 | url=http://geology.geoscienceworld.org/cgi/content/abstract/26/11/979 }} <br/></ref><ref name="becker_etal_2004"> {{ cite journal | author=Becker L, Poreda RJ, Basu AR, Pope KO, Harrison TM, Nicholson C, Iasky R | title=Bedout: a possible end-Permian impact crater offshore of northwestern Australia | journal=Science | volume=304 | issue=5676 | year=2004 | pages=1469–1476 |doi=10.1126/science.1093925 }} </ref> [[fullerenes]] trapping extraterrestrial noble gases;<ref name="becker_etal_2001"> {{ cite journal | author=Becker L, Poreda RJ, Hunt AG, Bunch TE, Rampino M | title=Impact event at the Permian–Triassic boundary: Evidence from extraterrestrial noble gases in fullerenes | journal=Science | volume=291 | issue=5508 | year=2001 | pages=1530–1533 | doi=10.1126/science.1057243 }} </ref> meteorite fragments in Antarctica;<ref name="basu_etal_2003"> {{ cite journal | author=Basu AR, Petaev MI, Poreda RJ, Jacobsen SB, Becker L | title=Chondritic meteorite fragments associated with the Permian–Triassic boundary in Antarctica | journal=Science | volume=302 | issue=5649 | year=2003 | pages=1388–1392 |doi=10.1126/science.1090852 }} </ref> and grains rich in iron, nickel and silicon, which may have been created by an impact.<ref name="Kaiho_etal_2001"> {{ cite journal | author=Kaiho K, Kajiwara Y, Nakano T, Miura Y, Kawahata H, Tazaki K, Ueshima M, Chen Z, Shi GR | title=End-Permian catastrophe by a bolide impact: Evidence of a gigantic release of sulfur from the mantle | journal=Geology | volume=29 | issue=9 | year=2001 | pages=815–818 |url=http://geology.geoscienceworld.org/cgi/content/abstract/26/11/979|accessdate=2007-10-22 }} </ref> However, the veracity of most these claims has been challenged.<ref name="Farley_etal_2001"> {{ cite journal | author=Farley KA, Mukhopadhyay S, Isozaki Y, Becker L, Poreda RJ | title=An extraterrestrial impact at the Permian–Triassic boundary? | journal=Science | volume=293 | issue=5539 | year=2001 | pages=2343 | doi=10.1126/science.293.5539.2343a }} </ref><ref name="Koeberl_etal_2002"> {{ cite journal | author=Koeberl C, Gilmour I, Reimold WU, Philippe Claeys P, Ivanov B | title=End-Permian catastrophe by bolide impact: Evidence of a gigantic release of sulfur from the mantle: Comment and Reply | journal=Geology | volume=30 | issue=9 | year=2002 | pages=855–856|doi=10.1130/0091-7613(2002)030%3C0855:EPCBBI%3E2.0.CO;2 }} </ref><ref name="Isbell_etal_1999"> {{ cite journal | author=Isbell JL, Askin RA, Retallack GR | title=Search for evidence of impact at the Permian–Triassic boundary in Antarctica and Australia; discussion and reply | journal=Geology | volume=27 | issue=9 | year=1999 | pages=859–860|doi=10.1130/0091-7613(1999)027%3C0859:SFEOIA%3E2.3.CO;2 }} </ref><ref name="Koeberl_etal_2004"> {{ cite journal | author=Koeberl K, Farley KA, Peucker-Ehrenbrink B, Sephton MA | title=Geochemistry of the end-Permian extinction event in Austria and Italy: No evidence for an extraterrestrial component | journal=Geology | volume=32 | issue=12 | year=2004 | pages=1053–1056 |doi=10.1130/G20907.1 }} <br/></ref> 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 [[Deformation|plastic deformation]], consistent with formation in a [[tectonics|tectonic]] environment such as volcanism.<ref name="Langanhorst_etal_2005"> {{ cite conference | author=Langenhorst F, Kyte FT & Retallack GJ | title=Reexamination of quartz grains from the Permian–Triassic boundary section at Graphite Peak, Antarctica | booktitle=Lunar and Planetary Science Conference XXXVI |date=2005 |url=http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2358.pdf|accessdate=2007-07-13 }} </ref> Several possible impact craters have been proposed as possible causes of the P–Tr extinction, including the [[Bedout|Bedout structure]] off the northwest coast of Australia,<ref name="becker_etal_2004"> {{ cite journal | author=Becker L, Poreda RJ, Basu AR, Pope KO, Harrison TM, Nicholson C, Iasky R | title=Bedout: a possible end-Permian impact crater offshore of northwestern Australia | journal=Science | volume=304 | issue=5676 | year=2004 | pages=1469–1476 | url=http://www.sciencemag.org/cgi/content/abstract/304/5676/1469 }} </ref> and the so-called [[Wilkes Land crater]] of East Antarctica.<ref name="vfp06"> {{ cite journal | author=von Frese RR, Potts L, Gaya-Pique L, Golynsky AV, Hernandez O, Kim J, Kim H & Hwang J | title= Permian–Triassic mascon in Antarctica | journal=Eos Trans. AGU, Jt. Assem. Suppl. | volume=87 | issue=36 | year=2006 | pages=Abstract T41A-08 |url=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|accessdate=2007-10-22 }} </ref> 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 possible or even likely that the crater no longer exists. 70% of the Earth's surface is sea, so an asteroid or comet fragment is over twice as likely to hit sea as to hit land. But Earth has no ocean-floor crust over 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.<ref name="Jones_etal_2002"> {{ cite journal | author=Jones AP, Price GD, Price NJ, DeCarli PS, Clegg RA | title=Impact induced melting and the development of large igneous provinces | journal=Earth and Planetary Science Letters | volume=202 | issue=3 | year=2002 | pages=551–561 |doi=10.1016/S0012-821X(02)00824-5 }} </ref> One attraction of large impact theories is that they theoretically could trigger other cause-considered extinction-paralleling phenomena<ref name="White"/>, such as the [[Siberian Traps]] eruptions (see below) as being either an impact site<ref name="Hager, Bradford H, 2001; Elkins Tanton, Linda T"> {{ cite conference | author=AHager, Bradford H | title=Giant Impact Craters Lead To Flood Basalts: A Viable Model | booktitle=CCNet 33/2001: Abstract 50470 |date=2001 |url=http://abob.libs.uga.edu/bobk/ccc/cc030101.html }} </ref> or the [[antipode]] of an impact site.<ref name="Hagstrum, Jonathan T, 2001"> {{ cite conference | author=Hagstrum, Jonathan T | title=Large Oceanic Impacts As The Cause Of Antipodal Hotspots And Global Mass Extinctions | booktitle=CCNet 33/2001: Abstract 50288 |date=2001 |url=http://abob.libs.uga.edu/bobk/ccc/cc030101.html }} </ref> <ref name="White"/> 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 [[Rapid modes of evolution| rapidly evolve]] in adaptation to more slowly-manifesting and/or less than global-in-scope phenomena. ===Volcanism=== [[Image:LatePermianGlobal.jpg| right | thumb | 300px | 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.<ref>{{cite journal|author=Zhou, M-F., Malpas, J, Song, X-Y, Robinson, PT, Sun, M, Kennedy, AK, Lesher, CM & Keays, RR|date=2002|title=A temporal link between the Emeishan large igneous province (SW China) and the end-Guadalupian mass extinction|journal=Earth and Planetary Science Letters |volume=196 |issue=3–4 |pages=113–122|doi=10.1016/S0012-821X(01)00608-2}}</ref> The flood basalt eruptions which produced the [[Siberian Traps]] constituted one of the largest known volcanic events on Earth and covered over {{sq km to sq mi|200000}} 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&nbsp;Ma — immediately before the end of the Permian.<ref name="Jin2000"/><ref>{{cite journal|author=Mundil, R., Ludwig, K.R., Metcalfe, I. & Renne, P.R|date=2004|title=Age and Timing of the Permian Mass Extinctions: U/Pb Dating of Closed-System Zircons |journal=Science |volume=305 |issue=5691 |pages=1760–1763 |doi=10.1126/science.1101012}}</ref> The Emeishan and Siberian Traps eruptions may have caused dust clouds and acid [[particulate|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 [[mollusk]]s and [[plankton]]ic 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.<ref name="White"/> 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.<ref>[http://hoopermuseum.earthsci.carleton.ca/pt_boundary/Causes/volcanics.html "Permian–Triassic Extinction - Volcanism"]</ref> 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.<ref name="White"/> 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 CO<sub>2</sub> 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.<ref name="White">{{cite journal|author=White RV|date=2002|journal=Phil. Trans. Royal Society of London |volume=360 |pages=2963-2985 |url=http://www.le.ac.uk/gl/ads/SiberianTraps/Documents/White2002-P-Tr-whodunit.pdf|title=Earth’s biggest 'whodunnit': unravelling the clues in the case of the end-Permian mass extinction|doi=10.1098/rsta.2002.1097|accessdate=2008-01-12}}</ref> <br "clear=all" /> 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&nbsp;[[per mil|‰]] (parts per thousand) in the [[Carbon 13|<sup>13</sup>C]]/[[Carbon 12|<sup>12</sup>C]] [[isotope analysis|isotope ratio]] in [[Carbonate minerals|carbonate]] rocks from the end-Permian ({{delta|13|C|sub=carbonate}} of -10&nbsp;‰).<ref> {{ cite journal|author=Palfy J, Demeny A, Haas J, Htenyi M, Orchard MJ, & Veto I|date=2001|title=Carbon isotope anomaly at the Triassic– Jurassic boundary from a marine section in Hungary|journal=Geology|volume=29|issue=11|pages=1047–1050|doi=10.1130/0091-7613(2001)029%3C1047:CIAAOG%3E2.0.CO;2 }} </ref><ref name="Twitchett"/> This is the first, largest and most rapid of a series of negative and positive excursions (decreases and increases in <sup>13</sup>C/<sup>12</sup>C 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).<ref name=Payne2004>{{cite journal | author = Payne, J.L. | coauthors = Lehrmann, D.J.; Wei, J.; Orchard, M.J.; Schrag, D.P.; Knoll, A.H. | year = 2004 | title = Large Perturbations of the Carbon Cycle During Recovery from the End-Permian Extinction | journal = Science | volume = 305 | issue = 5683 | pages = 506 | doi = 10.1126/science.1097023 | url=http://www.sciencemag.org/cgi/content/abstract/305/5683/506 | pmid = 15273391 }}</ref> A variety of factors may have contributed to this drop in the [[Carbon-13| <sup>13</sup>C]]/[[Carbon-12| <sup>12</sup>C]] ratio, but most turn out to be insufficient to account fully for it:<ref name="Berner2002"/> *Gases from volcanic eruptions have a <sup>13</sup>C/<sup>12</sup>C ratio about 5 to 8&nbsp;‰ below standard ({{delta|13|C}} about -5 to -8&nbsp;‰). But the amount required to produce a reduction of about 10&nbsp;‰ worldwide would require eruptions greater by [[order of magnitude|orders of magnitude]] than any for which evidence has been found.<ref name="Dickens1995">{{cite journal |author=Dickens GR, O'Neil JR, Rea DK & Owen RM|date=1995|title=Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene |journal=Paleoceanography |volume=10 |issue=6 |pages=965–71 |doi=10.1029/95PA02087}}</ref> *A reduction in organic activity would extract <sup>12</sup>C more slowly from the environment and leave more of it to be incorporated into sediments, thus reducing the <sup>13</sup>C/<sup>12</sup>C ratio. Biochemical processes use the lighter isotopes, since chemical reactions are ultimately driven by electromagnetic forces between atoms and lighter isotopes respond more quickly to these forces. But a study of a smaller drop of 3 to 4 &nbsp;‰ in <sup>13</sup>C/<sup>12</sup>C ({{delta|13|C}} -3 to -4&nbsp;‰) at the [[Paleocene-Eocene Thermal Maximum]] (PETM) concluded that even transferring all the organic carbon (in organisms, soils, and dissolved in the ocean) into sediments would be insufficient: even such a large burial of material rich in <sup>12</sup>C would not have produced the ''smaller'' drop in the <sup>13</sup>C/<sup>12</sup>C ratio of the rocks around the PETM.<ref name="Dickens1995"/> *Buried sedimentary organic matter has a <sup>13</sup>C/<sup>12</sup>C ratio 20 to 25&nbsp;‰ below normal ({{delta|13|C}} -20 to -25&nbsp;‰). Theoretically if the sea level fell sharply shallow marine sediments would be exposed to oxidization. But 6,500-8,400 [[gigaton]]s (1 gigaton = 10<sup>9</sup> [[metric ton]]s) of organic carbon would have to be oxidized and returned to the ocean-atmosphere system within less than a few hundred thousand years to reduce the <sup>13</sup>C/<sup>12</sup>C ratio by 10&nbsp;‰. This is not thought to be a realistic possibility.<ref name="Erwin1993"/> *Rather than a sudden decline in sea level, intermittent periods of ocean-bottom oxia and [[Anoxic sea water| anoxia]] (high-oxygen and low- / zero-oxygen conditions) may have caused the <sup>13</sup>C/<sup>12</sup>C ratio fluctuations in the Early Triassic;<ref name=Payne2004/> and global anoxia may have been responsible for the end-Permian blip. The continents of the end-Permian and early Triassic were more clustered in the tropics than they are now (see map above), and large tropical rivers would have dumped sediment into smaller, partially enclosed ocean basins in low latitudes. Such conditions favor oxic and anoxic episodes; oxic / anoxic conditions would result in a rapid release / burial respectively of large amounts of organic carbon, which has a low <sup>13</sup>C/<sup>12</sup>C ratio because biochemical processes use the lighter isotopes.<ref name="SchragBernerEtAl2002SnowballEarth">{{ cite journal | author=Schrag, D.P., Berner, R.A., Hoffman, P.F., and Halverson, G.P. | title=On the initiation of a snowball Earth | journal=Geochemistry Geophysics Geosystems | volume=3| issue=6 pages=1036 | doi=10.1029/2001GC000219 | date=2002 | url=http://www.agu.org/pubs/crossref/2002/2001GC000219.shtml | pages=1036 }} Preliminary abstract at {{ cite web | url=http://gsa.confex.com/gsa/2001ESP/finalprogram/abstract_8038.htm | title=On the initiation of a snowball Earth | author=Schrag, D.P. | publisher=Geological Society of America | date=June 2001}}</ref> This, or another organic-based reason, may have been responsible for both this and a late Proterozoic/Cambrian pattern of fluctuating <sup>13</sup>C/<sup>12</sup>C ratios.<ref name=Payne2004/> Other hypotheses include mass oceanic poisoning releasing vast amounts of {{co2}}<ref name="Benton2003"/> and a long-term reorganisation of the global carbon cycle.<ref name="Berner2002"/> However, only one sufficiently powerful cause has been proposed for the ''global'' 10&nbsp;‰ reduction in the <sup>13</sup>C/<sup>12</sup>C ratio: the release of [[methane]] from [[methane clathrate]]s;<ref name="Erwin1993"/> and carbon-cycle models confirm that it would have been sufficient to produce the observed reduction.<ref name="Berner2002">{{cite journal | author = Berner, R.A. | year = 2002 | title = Examination of hypotheses for the Permo-Triassic boundary extinction by carbon cycle modeling | journal = Proceedings of the National Academy of Sciences | pages = 32095199 | doi = 10.1073/pnas.032095199 | volume = 99 | pmid = 11917102 }}</ref> <ref name="Benton2003">{{cite journal | author = Benton, M.J. | coauthors = Twitchett, R.J. | year = 2003 | title = How to kill (almost) all life: the end-Permian extinction event | journal = Trends in Ecology & Evolution | volume = 18 | issue = 7 | pages = 358–365 | doi = 10.1016/S0169-5347(03)00093-4 }}</ref> 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 <sup>13</sup>C/<sup>12</sup>C ratio about 60&nbsp;‰ below normal ({{delta|13|C}} -60 &nbsp;‰). 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 shelf|continental shelves]] and the deeper seabed close to them). Oceanic methane hydrates are usually found buried in sediments where the seawater is at least {{m to ft|300}} deep. They can be found up to about {{m to ft|2000}} below the sea floor, but usually only about {{m to ft|1100}} below the sea floor.<ref name="Dickens2001"> {{ cite journal|author=Dickens GR|date=2001|title=The potential volume of oceanic methane hydrates with variable external conditions|journal=Organic Geochemistry|volume=32|issue=10|pages=1179–1193|doi=10.1016/S0146-6380(01)00086-9 }} </ref> 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.<ref> {{ cite journal|author=Reichow MK, Saunders AD, White RV, Pringle MS, Al'Muhkhamedov AI, Medvedev AI & Kirda NP|date=2002|title=<sup>40</sup>Ar/<sup>39</sup>Ar Dates from the West Siberian Basin: Siberian Flood Basalt Province Doubled|journal=Science|volume=296|issue=5574|pages=1846–1849|doi=10.1126/science.1071671 }} </ref> One would expect a vast release of methane to cause significant global warming, since methane is a very powerful [[Methane#Methane_as_a_greenhouse_gas | greenhouse gas]]. 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 (<sup>18</sup>O/<sup>16</sup>O);<ref> {{ cite journal|author=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|date=1989|title=A unique geochemical record at the Permian/Triassic boundary|journal=Nature|volume=337|issue=6202|pages=39–44|doi=10.1038/337039a0 }} </ref> 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.<ref> {{ cite journal|author=Dobruskina IA|date=1987|title=Phytogeography of Eurasia during the early Triassic|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=58|issue=1-2|pages=75–86|doi= 10.1016/0031-0182(87)90007-1 }} </ref><ref name="TannerLucas"/> 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,<ref name="Payne2004"/> but it would also have to be re-buried at an unrealistically high rate to account for the rapid increases in the <sup>13</sup>C/<sup>12</sup>C ratio (episodes of high positive {{delta|13|C}}) throughout the early Triassic, before being released again several times.<ref name="Payne2004"/> ===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.<ref> {{ cite journal|author=Newell ND|date=1971|title=An Outline History of Tropical Organic Reefs|journal=American Museum novitates|volume=2465|pages=1-37|url=http://digitallibrary.amnh.org/dspace/bitstream/2246/2673/1/N2465.pdf|accessdate=2007-11-03 }} </ref> 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.<ref name="TannerLucas"/> 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.<ref> {{ cite journal|author=McRoberts, C.A., Furrer, H., Jones, D.S.|date=1997|title=Palaeoenvironmental interpretation of a Triassic– Jurassic boundary section from western Austria based on palaeoecological and geochemical data. |journal=Palaeogeography Palaeoclimatology Palaeoecology|volume=136|issue=1-4|pages=79– 95|doi=10.1016/S0031-0182(97)00074-6 }} </ref> ===Anoxia=== {{Cleanup-section|date=January 2008}} There is evidence that the oceans became [[anoxic event|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.<ref> {{ cite journal|author=Wignall PB & Twitchett RJ|date=2002|title=Permian–Triassic sedimentology of Jameson Land, East Greenland: Incised submarine channels in an anoxic basin|journal=Journal of the Geological Society|volume=159|issue=6|pages=691-703|doi=110.1144/0016-764900-120 }} </ref> The [[uranium]]/[[thorium]] ratios of several late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction.<ref name="MonasterskyScienceNews19960525"> {{ Citation | last=Monastersky | first=R. | publication-date=May 25, 1996 | title=Oxygen starvation decimated Permian oceans | periodical=Science News | url=http://findarticles.com/p/articles/mi_m1200/is_n21_v149/ai_18351222 }} </ref> This would have been devastating for marine life, producing massive dies offs except for [[Anaerobic respiration|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.<ref name="MonasterskyScienceNews19960525" /> The most likely causes of the hypothetical global warming include Siberian Traps eruptions, which certainly happened in a coal-rich area and emitted large amounts of carbon dioxide (see above). Also methane hydrate gasification might have resulted from the eruptions, indicated possibly by the change in the ration of carbon-13 to carbon-12 in the atmosphere. A warming period could have also been produced by meteorite impact(s), if the impact(s) released large amounts of geologically stored carbon. However, some research 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.<ref name="ZhangEtAl2001" /> ===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]].<ref> {{ cite journal|author=Kump LR, Pavlov A, & Arthur MA|date=2005|title=Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia|journal=Geology|volume=33|issue=5|pages=397–400|doi=10.1130/G21295.1 }} </ref> 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.<ref> {{ cite journal|author=Grice K, Cao C, Love GD, Bottcher ME, Twitchett RJ, Grosjean E, Summons RE, Turgeon SC, Dunning W & Yugan J|date=2005|title= Photic Zone Euxinia During the Permian–Triassic Superanoxic Event|journal=Science|volume=307|issue=5710|pages=706-709|doi=10.1126/science.110432 }} </ref> 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=== [[Image:Pangaea continents.png| thumb | right | 200px | Map of [[Pangaea]] showing where to-day's continents were at the P-Tr 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.<ref>[http://www.palaeos.com/Paleozoic/Permian/Permian.htm The Permian - Palaeos]</ref> 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 [[monsoon]]s 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. <br clear="all" /> ===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 <sup>13</sup>C/<sup>12</sup>C ratio expected to result from a massive relase of methane do not match the patterns seen throughout the early Triassic;<ref name="Payne2004"/> 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.<ref name="ZhangEtAl2001">{{ cite journal| author=Zhang R, Follows, MJ, Grotzinger, JP, & Marshall J| title =Could the Late Permian deep ocean have been anoxic?| journal =Paleoceanography| volume =16| issue =3| pages =317–329|date=2001| doi = 10.1029/2000PA000522 | url=http://www.agu.org/pubs/crossref/2001/2000PA000522.shtml}} </ref> ==References== <!--See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for an explanation of how to generate footnotes using the <ref(erences/)> tags--> {{reflist|2}} ==Further reading== *{{aut|Over, Jess}} (editor), ''Understanding Late Devonian and Permian–Triassic Biotic and Climatic Events'', (Volume 20 in series Developments in Palaeontology and Stratigraphy (2006). The state of the inquiry into the extinction events. *{{aut|Sweet, Walter C.}} (editor), ''Permo–Triassic Events in the Eastern Tethys : Stratigraphy Classification and Relations with the Western Tethys'' (in series World and Regional Geology) ==External links== *[http://palaeo.gly.bris.ac.uk/Palaeofiles/Permian/intro.html "The Permo–Triassic extinction"] Introduction. *[http://www.geocities.com/earthhistory/permo.htm "The Permo–Triassic extinction"] A more detailed introduction. Bibliography. *[http://www.bbc.co.uk/science/horizon/2002/dayearthdied.shtml BBC2 'The Day the Earth Nearly Died' website.] *[http://www.pbs.org/wgbh/evolution/library/03/2/l_032_02.html PBS series ''Evolution'': "Extinction!"] video segment *[http://beckerantarctica.crustal.ucsb.edu/research_papers.html Luann Becker, "Exploring Antarctica: Understanding Life on Earth and Beyond"]: includes links to scientific papers *[http://www.spaceref.com/news/viewpr.rss.html?pid=19996/ SpaceRef: "Big Bang in Antarctica: Killer Crater Found Under Ice"] Radar images courtesy of Ohio State University. *[http://www.sciencedaily.com/releases/2005/02/050223130549.htm Science Daily: Global warming led to atmospheric hydrogen sulfide and Permian extinction] *[http://www.sciencedaily.com/releases/2006/06/060601174729.htm Science Daily: Big Bang In Antarctica: Killer Crater Found Under Ice] *[http://www.space.com/scienceastronomy/planetearth/extinction_permian_000907.html Lee Siegel, "Rocks Reveal Details of Mass Extinction"] Based on [http://www.sciencemag.org/cgi/content/abstract/289/5485/1740 Peter D. Ward, David R. Montgomery, Roger Smith, "Altered River Morphology in South Africa Related to the Permian–Triassic Extinction", in ''Science'' [[8 September]] [[2000]]] *[http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=286 David Morrison, "Did an Impact Trigger the Permian–Triassic Extinction?"] *[http://www.space.com/scienceastronomy/060601_big_crater.html Giant Crater Found: Tied to Worst Mass Extinction Ever] Robert Roy Britt (SPACE.com) [[1 June]] [[2006]] 06:07 p.m. ET *[http://www.space.com/scienceastronomy/planetearth/extinction_permian_000907.html Rocks Reveal Details of Mass Extinction] Lee Siegel (SPACE.com) 02:44 p.m. ET [[7 September]] [[2000]] *The History Files: [http://www.historyfiles.co.uk/FeaturesPrehistory/Permian_Extinction01.htm Permian Extinction Event] BBC News extract *Ward, P.D. (2006) [http://www.sciam.com/article.cfm?articleID=00037A5D-A938-150E-A93883414B7F0000&pageNumber=1&catID=2 "Impact from the Deep"]. ''Scientific American'' October 2006. {{ExtEvent nav}} [[Category:Extinction events]] [[Category:History of climate]] [[Category:Permian]] [[Category:Triassic]] [[Category:Evolutionary biology]] [[Category:Climate forcing agents]] [[Category:Impact events]] [[Category:Planetary science]] [[bg:Масово измиране Перм-Триас]] [[ca:Extinció del Permià-Triàsic]] [[es:Extinción masiva del Pérmico-Triásico]] [[eu:Desagerpen masibo Permo-Triasikoa]] [[fr:Extinction du Permien]] [[lt:Permo-triaso masinis rūšių išnykimas]] [[hu:Perm-triász kihalási esemény]] [[nl:Perm-Trias-massa-extinctie]] [[ja:P-T境界]] [[pl:Wymieranie permskie]] [[pt:Extinção do Permiano-Triássico]] [[ru:Массовое пермское вымирание]] [[sk:Veľké permské vymieranie]] [[sl:Permsko-triasno izumrtje]] [[fi:Permikauden joukkotuho]] [[sv:Perm-trias-utdöendet]] [[zh:二叠纪-三叠纪灭绝事件]]