Everything about Permian Triassic Extinction Event totally explained
The
Permian–Triassic (P–Tr) extinction event, informally known as the
Great Dying, was an
extinction event that occurred, 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 and 70 percent of
terrestrial vertebrate species becoming
extinct; it's the only known mass extinction of
insects.
The extinction event occurred in two pulses, five million years apart. 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, and increasing
aridity. However,
radiometric study of rock sequences in China, and is sometimes used to identify the Permian-Triassic boundary in rocks that are unsuitable for radiometric dating.
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, 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. the spike didn't appear world-wide; ; and in many places it didn't fall on the Permian-Triassic boundary.
The duration of the overall extinction, and the timing and duration of various groups' extinctions within the greater event, is still uncertain. Some evidence suggests that it was spread out over a few million years, with a very sharp peak in the last 1 million years of the Permian. Statistical analyses of some highly fossiliferous strata in Meishan, South China suggest that the main extinction was clustered around one peak. 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. An older theory, still supported in some recent papers, 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.
For example, all but one of the surviving
dinocephalian genera died out at the end of the Guadalupian, as did the Verbeekinidae, a family of large-size
fusuline foraminifera.
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.
Extinction patterns
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.
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). The extinction rate of marine organisms was catastrophic.
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. In the case of the brachiopods at least, surviving taxa were generally small, rare members of a diverse community.
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 enviromental 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.
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.
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, 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. Dominant
gymnosperm genera were replaced post-boundary by
lycophytes - extant lycophytes are recolonizers of disturbed areas.
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 don't coincide with the shift in values, but occurs many years after.
The recovery of gymnosperm forests would take 4-5 million years. 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. these may have migrated to areas where we've no sedimentary record.
Possible explanations of these patterns
The most vulnerable marine organisms were those which produced calcareous hard parts (for example 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 didn't 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 (33% of genera died out).
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's 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. and some writers estimate that the recovery wasn't complete until 30M years after the P-Tr extinction, for example in the late Triassic.
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. The lack of coal deposits during this era is known as a coal gap.
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.
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, 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.
Crinoids suffered a selective extinction, resulting in a decrease in the amount of morphospace they occupied. 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.
Land vertebrates
Lystrosaurus, a pig-sized herbivorous
dicynodont therapsid, constituted as much as 90% of some earliest Triassic land vertebrate faunas. 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.
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.
Land vertebrates took an unusually long time to recover from the P-Tr extinction; one writer estimates that the recovery wasn't 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.
Impact event
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;
fullerenes trapping extraterrestrial noble gases; meteorite fragments in Antarctica; and grains rich in iron, nickel and silicon, which may have been created by an impact. However, the veracity of most these claims has been challenged. 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.
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, and the so-called
Wilkes Land crater of East Antarctica. In each of these cases the idea that an impact was responsible hasn't 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's 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.
One attraction of large impact theories is that they theoretically could trigger other cause-considered extinction-paralleling phenomena or the
antipode of an impact site. The flood basalt eruptions which produced the
Siberian Traps constituted one of the largest known volcanic events on Earth and covered over 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.
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. 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.
Methane hydrate gasification
This section focusses on the
ratio of two carbon isotopes,
Carbon-13 and
Carbon-12, in
carbonate rocks from the end-Permian. It uses a few simple scientific notations:
- 13C and 12C are common abbreviations for the isotopes Carbon-13 to Carbon-12.
- 13C/12C means the ratio of 13C to 12C.
- means the difference between the ratio 13C/12C in a sample and an internationally agreed standard value. means the change as measured in bulk carbonate rocks.
- ‰ means per mil (0.1%) or parts per thousand. So " of -10 ‰" means "the ratio 13C/12C was 10 parts per thousand (1%) below that of the standard".
Scientists have found worldwide evidence of a swift decrease of about 10 ‰ in the
13C/
12C ratio in carbonate rocks from the end-Permian ( of -10 ‰).
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:
A reduction in organic activity would extract 12C more slowly from the environment and leave more of it to be incorporated into sediments, thus reducing the 13C/12C 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 ‰ in 13C/12C (-3 to -4 ‰) 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 12C wouldn't have produced the smaller drop in the 13C/12C ratio of the rocks around the PETM. This, or another organic-based reason, may have been responsible for both this and a late Proterozoic/Cambrian pattern of fluctuating 13C/12C ratios.
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 (-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 deep. They can be found up to about below the sea floor, but usually only about below the sea floor.
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.
One would expect a vast release of methane to cause significant global warming, since methane is a very powerful 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 (18O/16O); 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. There is some correlation between incidents of pronounced sea level regression and mass extinctions, but other evidence indicates there's no relationship; with regression itself creating new habitats.
Anoxia
There is evidence that the oceans became anoxic 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.
The uranium/thorium ratios of several late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction.
This would have been devastating for marine life, producing massive dies 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. 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.
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
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 didn't join the supercontinent until very late in the Permian. 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 wasn't 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 relase of methane don't match the patterns seen throughout the early Triassic;
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