The P-T extinction (a.k.a. the Great Dying) tends to be considered the worst - for example, Wikipedia states:
It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all families and 83% of all genera became extinct. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event, possibly up to 10 million years.
Presumably the "96% of all marine species" (which references two books, neither of which I have) is in terms of multicellular marine species or some similar subcategory that's easier to observe in the fossil record (I could be wrong).
My question is this: Would the Great Oxygenation Event not have killed a higher proportion of organisms (taking into account all organisms)?
I know that we don't have any records that can tell us how many died in the GOE and hence this question is necessarily speculative, but surely nearly all organisms pre-GOE would have been obligate anaerobes (and therefore found oxygen to be toxic)?
What I'm unsure about is how single-celled organisms fared in more recent (Phanerozoic) extinctions, as most discussion seems to be in terms of opisthokonts (i.e. animals + fungi) and archaeplastidans (i.e. plants + algae).
This is a great question. I decided to research it and here is what I found. For some reason the GOE is not on lists of the "big 5" mass extinctions. The question is why?
I think it's due to the pace of the event. The event is described as happening 2.45-2.32 billion years ago.
This is a timescale in billions of years. So it isn't really a proper mass extinction. Little is known about the time scale of the evolution of cyanobacterial lineages.
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K–T extinction, abbreviation of Cretaceous–Tertiary extinction, also called K–Pg extinction or Cretaceous–Paleogene extinction, a global extinction event responsible for eliminating approximately 80 percent of all species of animals at or very close to the boundary between the Cretaceous and Paleogene periods, about 66 million years ago. The K–T extinction was characterized by the elimination of many lines of animals that were important elements of the Mesozoic Era (251.9 million to 66 million years ago), including nearly all of the dinosaurs and many marine invertebrates. The event receives its name from the German word Kreide, meaning “chalk” (which references the chalky sediment of the Cretaceous Period), and the word Tertiary, which was traditionally used to describe the period of time spanning the Paleogene and Neogene periods. The K–T extinction ranks third in severity of the five major extinction episodes that punctuate the span of geologic time.
The only lines of archosaurs—the group of reptiles that contains the dinosaurs, birds, and crocodilians—that survived the extinction were the lineages that led to modern birds and crocodilians. Of the planktonic marine flora and fauna, only about 13 percent of the coccolithophore and planktonic foraminiferal genera remained alive. Among free-swimming mollusks, the ammonoids and belemnoids became extinct. Among other marine invertebrates, the larger foraminifers ( orbitoids) died out, and the hermatypic corals were reduced to about one-fifth of their genera. Rudist bivalves also disappeared, as did bivalves with a reclining (or partially buried) life habit, such as Exogyra and Gryphaea. The stratigraphically important inoceramids also died out.
The mass extinction was quite different between, and even among, other marine and terrestrial organisms. Land plants appear to have fared better than land animals however, there is evidence of widespread species extinctions of angiosperms and other dramatic shifts among North American plant communities. It is important to note that some groups of reptiles died out well before the K–T boundary, including flying reptiles (pterosaurs) and sea reptiles (plesiosaurs, mosasaurs, and ichthyosaurs). Among surviving reptile groups, turtles, crocodilians, lizards, and snakes were either not affected or affected only slightly. Effects on amphibians and mammals were also relatively mild. These patterns seem odd, considering how environmentally sensitive and habitat-restricted many of those groups are today.
Many hypotheses have been offered over the years to explain dinosaur extinction, but only a few have received serious consideration. The extermination of the dinosaurs has been a puzzle to paleontologists, geologists, and biologists for two centuries. Proposed causes include disease, heat waves and resulting sterility, freezing cold spells, the rise of egg-eating mammals, and X-rays from a nearby exploding supernova. Since the early 1980s, however, much attention has been focused on the so-called “asteroid theory” formulated by American scientists Walter Alvarez and Luis Alvarez. This theory states that a bolide (meteorite or comet) impact may have triggered the extinction event by ejecting a huge quantity of rock debris into the atmosphere, enshrouding Earth in darkness for several months or longer. With no sunlight able to penetrate this global dust cloud, photosynthesis ceased, resulting in the death of green plants and the disruption of the food chain.
There is much evidence in the rock record that supports this hypothesis. A huge crater 180 km (112 miles) in diameter dating to the end of the Cretaceous was discovered buried beneath sediments of the Yucatán Peninsula near Chicxulub, Mexico. A second, smaller crater, which predates the one at Chicxulub by about 2,000 to 5,000 years, was discovered at Boltysh in Ukraine in 2002. Its existence raises the possibility that the K–T extinction was the result of multiple bolide impacts. In addition, tektites (fractured sand grains characteristic of meteorite impacts) and the rare-earth element iridium, which is common only deep within Earth’s mantle and in extraterrestrial rocks, have been found in deposits associated with the extinction. There is also evidence for some spectacular side effects of bolide impact, including an enormous tsunami that washed up on the shores of the Gulf of Mexico and widespread wildfires triggered by a fireball from the impact.
Despite this strong evidence, the asteroid theory has met with skepticism among some paleontologists, with some agitating for terrestrial factors as the cause of the extinction and others claiming that the amount of iridium dispersed by an impact was caused by a smaller object, such as a comet. A huge outpouring of lava, known as the Deccan Traps, occurred in India at the end of the Cretaceous. Some paleontologists believe that the carbon dioxide that accompanied these flows created a global greenhouse effect that greatly warmed the planet. Others note that tectonic plate movements caused a major rearrangement of the world’s landmasses, particularly during the latter part of the Cretaceous. The climatic changes resulting from such continental drift could have caused a gradual deterioration of habitats favourable to the dinosaurs and other animal groups that suffered extinction. It is, of course, possible that sudden catastrophic phenomena such as an asteroid or comet impact contributed to an environmental deterioration already brought about by terrestrial causes.
The Editors of Encyclopaedia Britannica This article was most recently revised and updated by John P. Rafferty, Editor.
Ordovician-silurian Extinction: 440 million years ago
Small marine organisms died out.
Devonian Extinction: 365 million years ago
Many tropical marine species went extinct.
Permian-triassic Extinction: 250 million years ago
The largest mass extinction event in Earth's history affected a range of species, including many vertebrates.
Triassic-jurassic Extinction: 210 million years ago
The extinction of other vertebrate species on land allowed dinosaurs to flourish.
Cretaceous-tertiary Extinction: 65 Million Years Ago
What to Call It?
Scientists refer to the major extinction that wiped out nonavian dinosaurs as the K-T extinction, because it happened at the end of the Cretaceous period and the beginning of the Tertiary period. Why not C-T? Geologists use "K" as a shorthand for Cretaceous. "C" is shorthand for an earlier period, the Cambrian.
Dawn of a New Age
The extinction that occurred 65 million years ago wiped out some 50 percent of plants and animals. The event is so striking that it signals a major turning point in Earth's history, marking the end of the geologic period known as the Cretaceous and the beginning of the Tertiary period.
6th Mass Extinction
While we generally consider mass extinction events as historical events, many scientists argue that we are currently in the beginnings of another mass extinction event. In fact, from the species that we can measure and observe going extinct, we can estimate the overall rate of extinction. This rate is much higher than in most periods of history. Furthermore, this 6th mass extinction may be entirely caused by human actions.
Starting around 10,000 years ago, humans developed agriculture. While they gained the ability to grow and store food, our ancestors also started modifying the environment to give us more room for agriculture. Starting around 300 years ago, the Industrial Revolution gave us an increased ability to change the environment. We developed tractors and chainsaws, to clear out forests on which to raise crops or animals. Further, these machines emit carbon dioxide and our animals emit methane, both of which are greenhouse gases. The release of these gases is changing the composition of the atmosphere, which in turn is disrupting the climate.
Unfortunately, these changes are happening so fast that scientists are not entirely sure we can now reverse them. The rate of change is important because animals can only adapt over long periods of time. If a change happens too quickly, many animals will go extinct because they are not able to adapt in time. We are already seeing major groups of animals suffering, such as amphibians and coral reefs, both of which rely on specific amounts of water and temperatures to survive.
As horrific as they must have been, mass extinction events were not all bad news for those that survived. The extinction of the large, dominant land dinosaurs allowed smaller animals to survive and thrive. New species emerged and took on new niches, driving the evolution of life on Earth and shaping the future of natural selection on various populations. The end of the dinosaurs particularly benefited mammals, whose ascendance led to the rise of humans and other species on Earth today.
Some scientists believe that in the early 21st century, we are in the middle of the sixth major mass extinction event. Because these events often span millions of years, it's possible that the climate changes and Earth changes—physical changes to the planet—that we are experiencing will trigger the extinction of several species and in the future will be seen as a mass extinction event.
End-Permian extinction, which wiped out most of Earth's species, was instantaneous in geological time
Sam Bowring (front) and former graduate student Seth Burgess inspecting the End-Permian extinction horizon at Penglaitan. Credit: Shuzhong Shen
The most severe mass extinction in Earth's history occurred with almost no early warning signs, according to a new study by scientists at MIT, China, and elsewhere.
The end-Permian mass extinction, which took place 251.9 million years ago, killed off more than 96 percent of the planet's marine species and 70 percent of its terrestrial life—a global annihilation that marked the end of the Permian Period.
The new study, published today in the GSA Bulletin, reports that in the approximately 30,000 years leading up to the end-Permian extinction, there is no geologic evidence of species starting to die out. The researchers also found no signs of any big swings in ocean temperature or dramatic fluxes of carbon dioxide in the atmosphere. When ocean and land species did die out, they did so en masse, over a period that was geologically instantaneous.
"We can say for sure that there were no initial pulses of extinction coming in," says study co-author Jahandar Ramezani, a research scientist in MIT's Department of Earth, Atmospheric, and Planetary Sciences. "A vibrant marine ecosystem was continuing until the very end of Permian, and then bang—life disappears. And the big outcome of this paper is that we don't see early warning signals of the extinction. Everything happened geologically very fast."
Ramezani's co-authors include Samuel Bowring, professor of geology at MIT, along with scientists from the Chinese Academy of Sciences, the National Museum of Natural History, and the University of Calgary.
Finding missing pieces
For over two decades, scientists have tried to pin down the timing and duration of the end-Permian mass extinction to gain insights into its possible causes. Most attention has been devoted to well-preserved layers of fossil-rich rocks in eastern China, in a place known to geologists as the Meishan section. Scientists have determined that this section of sedimentary rocks was deposited in an ancient ocean basin, just before and slightly after the end-Permian extinction. As such, the Meishan section is thought to preserve signs of how Earth's life and climate fared leading up to the calamitous event.
"However, the Meishan section was deposited in a deep water setting and is highly condensed," says Shuzhong Shen of the Nanjing Institute of Geology and Palaeontology in China, who led the study. "The rock record may be incomplete." The whole extinction interval at Meishan comprises just 30 centimeters of ancient sedimentary layers, and he says it's likely that there were periods in this particular ocean setting when sediments did not settle, creating "depositional gaps" during which any evidence of life or environmental conditions may not have been recorded.
In 1994, Shen took Bowring, along with paleobiologist Doug Erwin, now curator of paleozoic invertebrates at the National Museum of Natural History and a co-author of the paper, looking for a more complete extinction record in Penglaitan, a much less-studied section of rock in southern China's Guangxi province. The Penglaitan section is what geologists consider "highly expanded." Compared with Meishan's 30 centimeters of sediments, Penglaitan's sedimentary layers make up a much more expanded 27 meters that were deposited over the same period of time, just before the main extinction event occurred.
"It's from a different part of the ancient ocean basin, that was closer to the continent, where you might find coral reefs and a lot more sedimentation and biological activity," Ramezani says. "So we can see a lot more, as in what's happening in the environment and with life, in this same period of time."
The researchers painstakingly collected and analyzed samples from multiple layers of the Penglaitan section, including samples from ash beds that were deposited by volcanic activity that occurred as nearby seafloor was crushed slowly under continental crust. These ash beds contain zircons—tiny mineral grains that contain uranium and lead, the ratios of which researchers can measure to determine the age of the zircon, and the ash bed from which it came.
Ramezani and his colleagues used this geochronology technique, developed to a large extent by Bowring, to determine with high precision the age of multiple ash bed layers throughout the Penglaitan section. From their analysis, they were able to determine that the end-Permian extinction occurred suddenly, around 252 million years ago, give or take 31,000 years.
The team also analyzed sedimentary layers for fossils, as well as oxygen and carbon isotopes, which can tell something about the ocean temperature and the state of its carbon cycle at the time the sediments were deposited. From the fossil record, they expected to see waves of species going extinct in the lead-up to the final extinction horizon. Similarly, they anticipated big changes in ocean temperature and chemistry, that would signal the oncoming disaster.
"We thought we would see a gradual decline in the diversity of life forms or, for example, certain species that are known to be less resilient than others, we would expect them to die out early on, but we don't see that," Ramezani says. "Disappearances are very random and don't conform to any kind of physiologic process or environmental effect. That makes us believe that the changes we are seeing before the event horizon are not really reflecting extinction."
For example, the researchers found signs that the ocean temperature rose from 30 to 35 degrees Celsius from the base to the top of the 27-meter interval—a period that encompasses about 30,000 years before the main extinction event. This temperature swing, however, is not very significant compared with a much larger heat-up that took place after most species already had died out.
"Big changes in temperature come right after the extinction, when the ocean gets really hot and uncomfortable," Ramezani says. "So we can rule out that ocean temperature was a driver of the extinction."
So what could have caused the sudden, global wipeout? The leading hypothesis is that the end-Permian extinction was caused by massive volcanic eruptions that spewed more than 4 million cubic kilometers of lava over what is now known as the Siberian Traps, in Siberia, Russia. Such immense and sustained eruptions likely released huge amounts of sulfur dioxide and carbon dioxide into the air, heating the atmosphere and acidifying the oceans.
Prior work by Bowring and his former graduate student Seth Burgess determined that the timing of the Siberian Traps eruptions matches the timing of the end-Permian extinction. But according to the team's new data from the Penglaitan section, even though increased global volcanic activity dominated the last 400,000 years of the Permian, it doesn't appear that there were any dramatic die-outs of marine species or any significant changes in ocean temperature and atmospheric carbon in the 30,000 years leading up to the main extinction.
"We can say there was extensive volcanic activity before and after the extinction, which could have caused some environmental stress and ecologic instability. But the global ecologic collapse came with a sudden blow, and we cannot see its smoking gun in the sediments that record extinction," Ramezani says. "The key in this paper is the abruptness of the extinction. Any hypothesis that says the extinction was caused by gradual environmental change during the late Permian—all those slow processes, we can rule out. It looks like a sudden punch comes in, and we're still trying to figure out what it meant and what exactly caused it."
"This study adds very much to the growing evidence that Earth's major extinction events occur on very short timescales, geologically speaking," says Jonathan Payne, professor of geological sciences and biology at Stanford University, who was not involved in the research. "It is even possible that the main pulse of Permian extinction occurred in just a few centuries. If it turns out to reflect an environmental tipping point within a longer interval of ongoing environmental change, that should make us particularly concerned about potential parallels to global change happening in the world around us right now."
In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five mass extinctions. They were originally identified as outliers to a general trend of decreasing extinction rates during the Phanerozoic,  but as more stringent statistical tests have been applied to the accumulating data, it has been established that multicellular animal life has experienced five major and many minor mass extinctions.  The "Big Five" cannot be so clearly defined, but rather appear to represent the largest (or some of the largest) of a relatively smooth continuum of extinction events. 
- (End Ordovician or O–S): 450–440 Ma at the Ordovician–Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 60% to 70% of all species.  Together they are ranked by many scientists as the second-largest of the five major extinctions in Earth's history in terms of percentage of genera that became extinct. In May 2020, studies suggested the cause of the mass extinction was due to global warming, related to volcanism, and anoxia, and not due, as considered earlier, to cooling and glaciation.  : 375–360 Ma near the Devonian–Carboniferous transition. At the end of the Frasnian Age in the later part(s) of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera and at least 70% of all species.  This extinction event lasted perhaps as long as 20 million years, and there is evidence for a series of extinction pulses within this period. (End Permian): 252 Ma at the Permian–Triassic transition.  Earth's largest extinction killed 57% of all families, 83% of all genera and 90% to 96% of all species  (53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species,  including insects).  The highly successful marine arthropod, the trilobite, became extinct. The evidence regarding plants is less clear, but new taxa became dominant after the extinction.  The "Great Dying" had enormous evolutionary significance: on land, it ended the primacy of mammal-like reptiles. The recovery of vertebrates took 30 million years,  but the vacant niches created the opportunity for archosaurs to become ascendant. In the seas, the percentage of animals that were sessile dropped from 67% to 50%. The whole late Permian was a difficult time for at least marine life, even before the "Great Dying". (End Triassic): 201.3 Ma at the Triassic–Jurassic transition. About 23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species became extinct.  Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g., Koolasuchus). (End Cretaceous, K–Pg extinction, or formerly K–T extinction): 66 Ma at the Cretaceous (Maastrichtian) – Paleogene (Danian) transition interval.  The event formerly called the Cretaceous-Tertiary or K–T extinction or K–T boundary is now officially named the Cretaceous–Paleogene (or K–Pg) extinction event. About 17% of all families, 50% of all genera and 75% of all species became extinct.  In the seas all the ammonites, plesiosaurs and mosasaurs disappeared and the percentage of sessile animals (those unable to move about) was reduced to about 33%. All non-avian dinosaurs became extinct during that time.  The boundary event was severe with a significant amount of variability in the rate of extinction between and among different clades. Mammals and birds, the latter descended from theropod dinosaurs, emerged as dominant large land animals.
Despite the popularization of these five events, there is no definite line separating them from other extinction events using different methods of calculating an extinction's impact can lead to other events featuring in the top five. 
Older fossil records are more difficult to interpret. This is because:
- Older fossils are harder to find as they are usually buried at a considerable depth.
- Dating of older fossils is more difficult.
- Productive fossil beds are researched more than unproductive ones, therefore leaving certain periods unresearched.
- Prehistoric environmental events can disturb the deposition process.
- The preservation of fossils varies on land, but marine fossils tend to be better preserved than their sought after land-based counterparts. 
It has been suggested that the apparent variations in marine biodiversity may actually be an artifact, with abundance estimates directly related to quantity of rock available for sampling from different time periods.  However, statistical analysis shows that this can only account for 50% of the observed pattern, [ citation needed ] and other evidence (such as fungal spikes) [ clarification needed ] provides reassurance that most widely accepted extinction events are real. A quantification of the rock exposure of Western Europe indicates that many of the minor events for which a biological explanation has been sought are most readily explained by sampling bias. 
Research completed after the seminal 1982 paper (Sepkoski and Raup) has concluded that a sixth mass extinction event is ongoing:
6. Holocene extinction: Currently ongoing. Extinctions have occurred at over 1000 times the background extinction rate since 1900.   The mass extinction is a result of human activity,    driven by population growth and overconsumption of the earth's natural resources.  The 2019 global biodiversity assessment by IPBES asserts that out of an estimated 8 million species, 1 million plant and animal species are currently threatened with extinction.    
More recent research has indicated that the End-Capitanian extinction event likely constitutes a separate extinction event from the Permian–Triassic extinction event if so, it would be larger than many of the "Big Five" extinction events.
Mass extinctions have sometimes accelerated the evolution of life on Earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the newly dominant group is "superior" to the old but usually because an extinction event eliminates the old, dominant group and makes way for the new one, a process known as adaptive radiation.  
For example, mammaliformes ("almost mammals") and then mammals existed throughout the reign of the dinosaurs, but could not compete in the large terrestrial vertebrate niches which dinosaurs monopolized. The end-Cretaceous mass extinction removed the non-avian dinosaurs and made it possible for mammals to expand into the large terrestrial vertebrate niches. Ironically, the dinosaurs themselves had been beneficiaries of a previous mass extinction, the end-Triassic, which eliminated most of their chief rivals, the crurotarsans.
Another point of view put forward in the Escalation hypothesis predicts that species in ecological niches with more organism-to-organism conflict will be less likely to survive extinctions. This is because the very traits that keep a species numerous and viable under fairly static conditions become a burden once population levels fall among competing organisms during the dynamics of an extinction event.
Furthermore, many groups which survive mass extinctions do not recover in numbers or diversity, and many of these go into long-term decline, and these are often referred to as "Dead Clades Walking".  However, clades that survive for a considerable period of time after a mass extinction, and which were reduced to only a few species, are likely to have experienced a rebound effect called the "push of the past". 
Darwin was firmly of the opinion that biotic interactions, such as competition for food and space—the ‘struggle for existence’—were of considerably greater importance in promoting evolution and extinction than changes in the physical environment. He expressed this in The Origin of Species: "Species are produced and exterminated by slowly acting causes…and the most import of all causes of organic change is one which is almost independent of altered…physical conditions, namely the mutual relation of organism to organism-the improvement of one organism entailing the improvement or extermination of others". 
It has been suggested variously that extinction events occurred periodically, every 26 to 30 million years,   or that diversity fluctuates episodically every
62 million years.  Various ideas attempt to explain the supposed pattern, including the presence of a hypothetical companion star to the sun,   oscillations in the galactic plane, or passage through the Milky Way's spiral arms.  However, other authors have concluded that the data on marine mass extinctions do not fit with the idea that mass extinctions are periodic, or that ecosystems gradually build up to a point at which a mass extinction is inevitable.  Many of the proposed correlations have been argued to be spurious.   Others have argued that there is strong evidence supporting periodicity in a variety of records,  and additional evidence in the form of coincident periodic variation in nonbiological geochemical variables. 
Mass extinctions are thought to result when a long-term stress is compounded by a short-term shock.  Over the course of the Phanerozoic, individual taxa appear to have become less likely to suffer extinction,  which may reflect more robust food webs, as well as fewer extinction-prone species, and other factors such as continental distribution.  However, even after accounting for sampling bias, there does appear to be a gradual decrease in extinction and origination rates during the Phanerozoic.  This may represent the fact that groups with higher turnover rates are more likely to become extinct by chance or it may be an artefact of taxonomy: families tend to become more speciose, therefore less prone to extinction, over time  and larger taxonomic groups (by definition) appear earlier in geological time. 
It has also been suggested that the oceans have gradually become more hospitable to life over the last 500 million years, and thus less vulnerable to mass extinctions, [note 1]   but susceptibility to extinction at a taxonomic level does not appear to make mass extinctions more or less probable. 
There is still debate about the causes of all mass extinctions. In general, large extinctions may result when a biosphere under long-term stress undergoes a short-term shock.  An underlying mechanism appears to be present in the correlation of extinction and origination rates to diversity. High diversity leads to a persistent increase in extinction rate low diversity to a persistent increase in origination rate. These presumably ecologically controlled relationships likely amplify smaller perturbations (asteroid impacts, etc.) to produce the global effects observed. 
Identifying causes of specific mass extinctions Edit
A good theory for a particular mass extinction should: (i) explain all of the losses, not just focus on a few groups (such as dinosaurs) (ii) explain why particular groups of organisms died out and why others survived (iii) provide mechanisms which are strong enough to cause a mass extinction but not a total extinction (iv) be based on events or processes that can be shown to have happened, not just inferred from the extinction.
It may be necessary to consider combinations of causes. For example, the marine aspect of the end-Cretaceous extinction appears to have been caused by several processes which partially overlapped in time and may have had different levels of significance in different parts of the world. 
Arens and West (2006) proposed a "press / pulse" model in which mass extinctions generally require two types of cause: long-term pressure on the eco-system ("press") and a sudden catastrophe ("pulse") towards the end of the period of pressure.  Their statistical analysis of marine extinction rates throughout the Phanerozoic suggested that neither long-term pressure alone nor a catastrophe alone was sufficient to cause a significant increase in the extinction rate.
Most widely supported explanations Edit
Macleod (2001)  summarized the relationship between mass extinctions and events which are most often cited as causes of mass extinctions, using data from Courtillot et al. (1996),  Hallam (1992)  and Grieve et al. (1996): 
- events: 11 occurrences, all associated with significant extinctions  But Wignall (2001) concluded that only five of the major extinctions coincided with flood basalt eruptions and that the main phase of extinctions started before the eruptions. 
- Sea-level falls: 12, of which seven were associated with significant extinctions.  : one large impact is associated with a mass extinction, that is, the Cretaceous–Paleogene extinction event there have been many smaller impacts but they are not associated with significant extinctions. 
The most commonly suggested causes of mass extinctions are listed below.
Flood basalt events Edit
The formation of large igneous provinces by flood basalt events could have:
- produced dust and particulate aerosols which inhibited photosynthesis and thus caused food chains to collapse both on land and at sea 
- emitted sulfur oxides which were precipitated as acid rain and poisoned many organisms, contributing further to the collapse of food chains
- emitted carbon dioxide and thus possibly causing sustained global warming once the dust and particulate aerosols dissipated.
Flood basalt events occur as pulses of activity punctuated by dormant periods. As a result, they are likely to cause the climate to oscillate between cooling and warming, but with an overall trend towards warming as the carbon dioxide they emit can stay in the atmosphere for hundreds of years.
It is speculated that massive volcanism caused or contributed to the End-Permian, End-Triassic and End-Cretaceous extinctions.  The correlation between gigantic volcanic events expressed in the large igneous provinces and mass extinctions was shown for the last 260 million years.   Recently such possible correlation was extended across the whole Phanerozoic Eon. 
Sea-level falls Edit
These are often clearly marked by worldwide sequences of contemporaneous sediments which show all or part of a transition from sea-bed to tidal zone to beach to dry land – and where there is no evidence that the rocks in the relevant areas were raised by geological processes such as orogeny. Sea-level falls could reduce the continental shelf area (the most productive part of the oceans) sufficiently to cause a marine mass extinction, and could disrupt weather patterns enough to cause extinctions on land. But sea-level falls are very probably the result of other events, such as sustained global cooling or the sinking of the mid-ocean ridges.
Sea-level falls are associated with most of the mass extinctions, including all of the "Big Five"—End-Ordovician, Late Devonian, End-Permian, End-Triassic, and End-Cretaceous.
A 2008 study, published in the journal Nature, established a relationship between the speed of mass extinction events and changes in sea level and sediment.  The study suggests changes in ocean environments related to sea level exert a driving influence on rates of extinction, and generally determine the composition of life in the oceans. 
Impact events Edit
The impact of a sufficiently large asteroid or comet could have caused food chains to collapse both on land and at sea by producing dust and particulate aerosols and thus inhibiting photosynthesis.  Impacts on sulfur-rich rocks could have emitted sulfur oxides precipitating as poisonous acid rain, contributing further to the collapse of food chains. Such impacts could also have caused megatsunamis and/or global forest fires.
Most paleontologists now agree that an asteroid did hit the Earth about 66 Ma, but there is an ongoing dispute whether the impact was the sole cause of the Cretaceous–Paleogene extinction event.  
Nonetheless, in October 2019, researchers reported that the Cretaceous Chicxulub asteroid impact that resulted in the extinction of non-avian dinosaurs 66 Ma, also rapidly acidified the oceans producing ecological collapse and long-lasting effects on the climate, and was a key reason for end-Cretaceous mass extinction.  
According to the Shiva Hypothesis, the Earth is subject to increased asteroid impacts about once every 27 million years because of the Sun's passage through the plane of the Milky Way galaxy, thus causing extinction events at 27 million year intervals. Some evidence for this hypothesis has emerged in both marine and non-marine contexts.  Alternatively, the Sun's passage through the higher density spiral arms of the galaxy could coincide with mass extinction on Earth, perhaps due to increased impact events.  However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation. 
Global cooling Edit
Sustained and significant global cooling could kill many polar and temperate species and force others to migrate towards the equator reduce the area available for tropical species often make the Earth's climate more arid on average, mainly by locking up more of the planet's water in ice and snow. The glaciation cycles of the current ice age are believed to have had only a very mild impact on biodiversity, so the mere existence of a significant cooling is not sufficient on its own to explain a mass extinction.
It has been suggested that global cooling caused or contributed to the End-Ordovician, Permian–Triassic, Late Devonian extinctions, and possibly others. Sustained global cooling is distinguished from the temporary climatic effects of flood basalt events or impacts.
Global warming Edit
This would have the opposite effects: expand the area available for tropical species kill temperate species or force them to migrate towards the poles possibly cause severe extinctions of polar species often make the Earth's climate wetter on average, mainly by melting ice and snow and thus increasing the volume of the water cycle. It might also cause anoxic events in the oceans (see below).
Global warming as a cause of mass extinction is supported by several recent studies. 
The most dramatic example of sustained warming is the Paleocene–Eocene Thermal Maximum, which was associated with one of the smaller mass extinctions. It has also been suggested to have caused the Triassic–Jurassic extinction event, during which 20% of all marine families became extinct. Furthermore, the Permian–Triassic extinction event has been suggested to have been caused by warming.   
Clathrate gun hypothesis Edit
Clathrates are composites in which a lattice of one substance forms a cage around another. Methane clathrates (in which water molecules are the cage) form on continental shelves. These clathrates are likely to break up rapidly and release the methane if the temperature rises quickly or the pressure on them drops quickly—for example in response to sudden global warming or a sudden drop in sea level or even earthquakes. Methane is a much more powerful greenhouse gas than carbon dioxide, so a methane eruption ("clathrate gun") could cause rapid global warming or make it much more severe if the eruption was itself caused by global warming.
The most likely signature of such a methane eruption would be a sudden decrease in the ratio of carbon-13 to carbon-12 in sediments, since methane clathrates are low in carbon-13 but the change would have to be very large, as other events can also reduce the percentage of carbon-13. 
It has been suggested that "clathrate gun" methane eruptions were involved in the end-Permian extinction ("the Great Dying") and in the Paleocene–Eocene Thermal Maximum, which was associated with one of the smaller mass extinctions.
Anoxic events Edit
Anoxic events are situations in which the middle and even the upper layers of the ocean become deficient or totally lacking in oxygen. Their causes are complex and controversial, but all known instances are associated with severe and sustained global warming, mostly caused by sustained massive volcanism. 
It has been suggested that anoxic events caused or contributed to the Ordovician–Silurian, late Devonian, Permian–Triassic and Triassic–Jurassic extinctions, as well as a number of lesser extinctions (such as the Ireviken, Mulde, Lau, Toarcian and Cenomanian–Turonian events). On the other hand, there are widespread black shale beds from the mid-Cretaceous which indicate anoxic events but are not associated with mass extinctions.
The bio-availability of essential trace elements (in particular selenium) to potentially lethal lows has been shown to coincide with, and likely have contributed to, at least three mass extinction events in the oceans, that is, at the end of the Ordovician, during the Middle and Late Devonian, and at the end of the Triassic. During periods of low oxygen concentrations very soluble selenate (Se 6+ ) is converted into much less soluble selenide (Se 2- ), elemental Se and organo-selenium complexes. Bio-availability of selenium during these extinction events dropped to about 1% of the current oceanic concentration, a level that has been proven lethal to many extant organisms. 
British oceanologist and atmospheric scientist, Andrew Watson, explained that, while the Holocene epoch exhibits many processes reminiscent of those that have contributed to past anoxic events, full-scale ocean anoxia would take "thousands of years to develop". 
Hydrogen sulfide emissions from the seas Edit
Kump, Pavlov and Arthur (2005) have proposed that during the Permian–Triassic extinction event the warming also upset the oceanic balance between photosynthesising plankton and deep-water sulfate-reducing bacteria, causing massive emissions of hydrogen sulfide which poisoned life on both land and sea and severely weakened the ozone layer, exposing much of the life that still remained to fatal levels of UV radiation.   
Oceanic overturn Edit
Oceanic overturn is a disruption of thermo-haline circulation which lets surface water (which is more saline than deep water because of evaporation) sink straight down, bringing anoxic deep water to the surface and therefore killing most of the oxygen-breathing organisms which inhabit the surface and middle depths. It may occur either at the beginning or the end of a glaciation, although an overturn at the start of a glaciation is more dangerous because the preceding warm period will have created a larger volume of anoxic water. 
Unlike other oceanic catastrophes such as regressions (sea-level falls) and anoxic events, overturns do not leave easily identified "signatures" in rocks and are theoretical consequences of researchers' conclusions about other climatic and marine events.
It has been suggested that oceanic overturn caused or contributed to the late Devonian and Permian–Triassic extinctions.
A nearby nova, supernova or gamma ray burst Edit
A nearby gamma-ray burst (less than 6000 light-years away) would be powerful enough to destroy the Earth's ozone layer, leaving organisms vulnerable to ultraviolet radiation from the Sun.  Gamma ray bursts are fairly rare, occurring only a few times in a given galaxy per million years.  It has been suggested that a supernova or gamma ray burst caused the End-Ordovician extinction. 
Geomagnetic reversal Edit
One theory is that periods of increased geomagnetic reversals will weaken Earth's magnetic field long enough to expose the atmosphere to the solar winds, causing oxygen ions to escape the atmosphere in a rate increased by 3–4 orders, resulting in a disastrous decrease in oxygen. 
Plate tectonics Edit
Movement of the continents into some configurations can cause or contribute to extinctions in several ways: by initiating or ending ice ages by changing ocean and wind currents and thus altering climate by opening seaways or land bridges which expose previously isolated species to competition for which they are poorly adapted (for example, the extinction of most of South America's native ungulates and all of its large metatherians after the creation of a land bridge between North and South America). Occasionally continental drift creates a super-continent which includes the vast majority of Earth's land area, which in addition to the effects listed above is likely to reduce the total area of continental shelf (the most species-rich part of the ocean) and produce a vast, arid continental interior which may have extreme seasonal variations.
Another theory is that the creation of the super-continent Pangaea contributed to the End-Permian mass extinction. Pangaea was almost fully formed at the transition from mid-Permian to late-Permian, and the "Marine genus diversity" diagram at the top of this article shows a level of extinction starting at that time which might have qualified for inclusion in the "Big Five" if it were not overshadowed by the "Great Dying" at the end of the Permian. 
Other hypotheses Edit
Many other hypotheses have been proposed, such as the spread of a new disease, or simple out-competition following an especially successful biological innovation. But all have been rejected, usually for one of the following reasons: they require events or processes for which there is no evidence they assume mechanisms which are contrary to the available evidence they are based on other theories which have been rejected or superseded.
Scientists have been concerned that human activities could cause more plants and animals to become extinct than any point in the past. Along with human-made changes in climate (see above), some of these extinctions could be caused by overhunting, overfishing, invasive species, or habitat loss. A study published in May 2017 in Proceedings of the National Academy of Sciences argued that a “biological annihilation” akin to a sixth mass extinction event is underway as a result of anthropogenic causes, such as over-population and over-consumption. The study suggested that as much as 50% of the number of animal individuals that once lived on Earth were already extinct, threatening the basis for human existence too.  
Future biosphere extinction/sterilization Edit
The eventual warming and expanding of the Sun, combined with the eventual decline of atmospheric carbon dioxide could actually cause an even greater mass extinction, having the potential to wipe out even microbes (in other words, the Earth is completely sterilized), where rising global temperatures caused by the expanding Sun will gradually increase the rate of weathering, which in turn removes more and more carbon dioxide from the atmosphere. When carbon dioxide levels get too low (perhaps at 50 ppm), all plant life will die out, although simpler plants like grasses and mosses can survive much longer, until CO
2 levels drop to 10 ppm.  
With all photosynthetic organisms gone, atmospheric oxygen can no longer be replenished, and is eventually removed by chemical reactions in the atmosphere, perhaps from volcanic eruptions. Eventually the loss of oxygen will cause all remaining aerobic life to die out via asphyxiation, leaving behind only simple anaerobic prokaryotes. When the Sun becomes 10% brighter in about a billion years,  Earth will suffer a moist greenhouse effect resulting in its oceans boiling away, while the Earth's liquid outer core cools due to the inner core's expansion and causes the Earth's magnetic field to shut down. In the absence of a magnetic field, charged particles from the Sun will deplete the atmosphere and further increase the Earth's temperature to an average of
420 K (147 °C, 296 °F) in 2.8 billion years, causing the last remaining life on Earth to die out. This is the most extreme instance of a climate-caused extinction event. Since this will only happen late in the Sun's life, such will cause the final mass extinction in Earth's history (albeit a very long extinction event).  
The impact of mass extinction events varied widely. After a major extinction event, usually only weedy species survive due to their ability to live in diverse habitats.  Later, species diversify and occupy empty niches. Generally, it takes millions of years for biodiversity to recover after extinction events.  In the most severe mass extinctions it may take 15 to 30 million years. 
The worst event, the Permian–Triassic extinction, devastated life on earth, killing over 90% of species. Life seemed to recover quickly after the P-T extinction, but this was mostly in the form of disaster taxa, such as the hardy Lystrosaurus. The most recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover. It is thought that this long recovery was due to successive waves of extinction which inhibited recovery, as well as prolonged environmental stress 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  and some writers estimate that the recovery was not complete until 30M years after the P-T extinction, that is, in the late Triassic.  Subsequent to the P-T extinction, there was an increase in provincialization, with species occupying smaller ranges – perhaps removing incumbents from niches and setting the stage for an eventual rediversification. 
The effects of mass extinctions on plants are somewhat harder to quantify, given the biases inherent in the plant fossil record. Some mass extinctions (such as the end-Permian) were equally catastrophic for plants, whereas others, such as the end-Devonian, did not affect the flora. 
Recent improvements in genetic engineering have raised questions about bringing extinct species back to life. Since Dolly the sheep was cloned in 1996, scientists know it is possible to create an organism from the DNA in a single cell. Stored in museum collections throughout the world are specimens of extinct animals containing DNA. The idea of using DNA to revive extinct species and repopulating them is controversial. How would we choose which ones? How would they impact species still on Earth?
Shallow warm-water marine invertebrates, which included the trilobites, rugose and tabulate corals, and two large groups of echinoderms ( blastoids and crinoids), show the most-protracted and greatest losses during the Permian extinction. Using the maximum number of different genera in the middle part of the Guadalupian Epoch (about 272.3 million to 259.8 million years ago) as a benchmark, extinction within marine invertebrate faunas significantly reduced the number of different genera by 12 to 70 percent by the beginning of the Capitanian Age some 266 million years ago. The diversity levels of many of these faunas plummeted to levels lower than at any prior time in the Permian Period. Extinctions at the boundary between the Guadalupian and Lopingian epochs (259.8 million to 252.2 million years ago) were even more severe—bordering on catastrophic—with a reduction of 70 to 80 percent from the Guadalupian generic maxima. A great many invertebrate families, which were highly successful prior to these extinctions, were affected.
By the early part of the Lopingian, specifically the Wuchiapingian Age (some 259.8 million to 254 million years ago), the now substantially reduced invertebrate fauna attempted to diversify again, but with limited success. Many were highly specialized groups, and more than half of these became extinct before the beginning of the Changhsingian Age (some 254 million years ago), the last age of the period. Marine invertebrate faunas during the Lopingian accounted for only about 10 percent or less of the Guadalupian faunal maxima that is, about 90 percent of the Permian extinctions were accomplished before the start of the Changhsingian Age.
The series of extinction episodes that occurred during both the last stage of the Guadalupian Epoch and throughout the Lopingian Epoch, each apparently more severe than the previous one, extended over about 15 million years. Disruptive ecological changes eventually reduced marine invertebrates to crisis levels (about 5 percent of their Guadalupian maxima)—their lowest diversity since the end of the Ordovician Period. The final extinction episode, sometimes referred to as the terminal Permian crisis, while very real, took 15 million years to materialize and likely eliminated many ecologically struggling faunas that had already been greatly reduced by previous extinction episodes leading up to the terminal Permian crisis.
The Permian extinction was not restricted to marine invertebrates. Several groups of aquatic vertebrates, such as the acanthodians, thought to be the earliest jawed fishes, and the placoderms, a group of jawed fishes with significant armour, were also eliminated. Notable terrestrial groups, such as the pelycosaurs (fin-backed reptiles), Moschops (a massive mammal-like reptile), and numerous families of insects also met their demise. In addition, a number of groups (such as sharks, bony fishes, brachiopods, bryozoans, ammonoids, therapsids, reptiles, and amphibians) experienced significant declines by the end of the Permian Period.
What caused Earth's biggest mass extinction?
Scientists have debated until now what made Earth's oceans so inhospitable to life that some 96 percent of marine species died off at the end of the Permian period. New research shows the "Great Dying" was caused by global warming that left ocean animals unable to breathe.
The largest extinction in Earth's history marked the end of the Permian period, some 252 million years ago. Long before dinosaurs, our planet was populated with plants and animals that were mostly obliterated after a series of massive volcanic eruptions in Siberia.
Fossils in ancient seafloor rocks display a thriving and diverse marine ecosystem, then a swath of corpses. Some 96 percent of marine species were wiped out during the "Great Dying," followed by millions of years when life had to multiply and diversify once more.
What has been debated until now is exactly what made the oceans inhospitable to life – the high acidity of the water, metal and sulfide poisoning, a complete lack of oxygen, or simply higher temperatures.
New research from the University of Washington and Stanford University combines models of ocean conditions and animal metabolism with published lab data and paleoceanographic records to show that the Permian mass extinction in the oceans was caused by global warming that left animals unable to breathe. As temperatures rose and the metabolism of marine animals sped up, the warmer waters could not hold enough oxygen for them to survive. The study is published in the Dec. 7 issue of Science.
"This is the first time that we have made a mechanistic prediction about what caused the extinction that can be directly tested with the fossil record, which then allows us to make predictions about the causes of extinction in the future," said first author Justin Penn , a UW doctoral student in oceanography.
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We’ve never been able to gain such insight into exactly how and why different stressors affected different parts of the global ocean.
Researchers ran a climate model with Earth's configuration during the Permian, when the land masses were combined in the supercontinent of Pangaea. Before ongoing volcanic eruptions in Siberia created a greenhouse-gas planet, oceans had temperatures and oxygen levels similar to today's. The researchers then raised greenhouse gases in the model to the level required to make tropical ocean temperatures at the surface some 10 degrees Celsius (20 degrees Fahrenheit) higher, matching conditions at that time.
The model reproduces the resulting dramatic changes in the oceans. Oceans lost about 80 percent of their oxygen. About half the oceans' seafloor, mostly at deeper depths, became completely oxygen-free.
To analyze the effects on marine species, the researchers considered the varying oxygen and temperature sensitivities of 61 modern marine species – including crustaceans, fish, shellfish, corals and sharks – using published lab measurements. The tolerance of modern animals to high temperature and low oxygen is expected to be similar to Permian animals because they had evolved under similar environmental conditions. The researchers then combined the species' traits with the paleoclimate simulations to predict the geography of the extinction.
"Very few marine organisms stayed in the same habitats they were living in – it was either flee or perish," said second author Curtis Deutsch, a UW associate professor of oceanography.
According to study co-author Jonathan Payne, a professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth), “The conventional wisdom in the paleontological community has been that the Permian extinction was especially severe in tropical waters.” Yet the model shows the hardest hit were organisms most sensitive to oxygen found far from the tropics. Many species that lived in the tropics also went extinct in the model, but it predicts that high-latitude species, especially those with high oxygen demands, were nearly completely wiped out.
The study builds on previous work led by Deutsch showing that as oceans warm, marine animals' metabolism speeds up, meaning they require more oxygen, while warmer water holds less. That earlier study shows how warmer oceans push animals away from the tropics.
To test this prediction, Payne and co-author Erik Sperling, an assistant professor of geological sciences at Stanford Earth, analyzed late-Permian fossil distributions from the Paleobiology Database, a virtual archive of published fossil collections. The fossil record shows where species were before the extinction, and which were wiped out completely or restricted to a fraction of their former habitat.
The fossil record confirms that species far from the equator suffered most during the event. "The signature of that kill mechanism, climate warming and oxygen loss, is this geographic pattern that's predicted by the model and then discovered in the fossils," Penn said. "The agreement between the two indicates this mechanism of climate warming and oxygen loss was a primary cause of the extinction."
“We’ve never been able to gain such insight into exactly how and why different stressors affected different parts of the global ocean,” said Sperling, an assistant professor of geological sciences at Stanford Earth. “This was really exciting to see.”
The new study combines the changing ocean conditions with various animals' metabolic needs at different temperatures. Results show that the most severe effects of oxygen deprivation are for species living near the poles.
"Since tropical organisms' metabolisms were already adapted to fairly warm, lower-oxygen conditions, they could move away from the tropics and find the same conditions somewhere else," Deutsch said. "But if an organism was adapted for a cold, oxygen-rich environment, then those conditions ceased to exist in the shallow oceans."
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The conventional wisdom in the paleontological community has been that the Permian extinction was especially severe in tropical waters.
The so-called "dead zones" that are completely devoid of oxygen were mostly below depths where species were living, and played a smaller role in the survival rates.
"At the end of the day, it turned out that the size of the dead zones really doesn't seem to be the key thing for the extinction," Deutsch said. "We often think about anoxia, the complete lack of oxygen, as the condition you need to get widespread uninhabitability. But when you look at the tolerance for low oxygen, most organisms can be excluded from seawater at oxygen levels that aren't anywhere close to anoxic."
Warming leading to insufficient oxygen explains more than half of the marine diversity losses. The authors say that other changes, such as acidification or shifts in the productivity of photosynthetic organisms, likely acted as additional causes.
The situation in the late Permian — increasing greenhouse gases in the atmosphere that create warmer temperatures on Earth — is similar to today.
"Under a business-as-usual emissions scenarios, by 2100 warming in the upper ocean will have approached 20 percent of warming in the late Permian, and by the year 2300 it will reach between 35 and 50 percent," Penn said. "This study highlights the potential for a mass extinction arising from a similar mechanism under anthropogenic climate change."
The research was funded by the Gordon and Betty Moore Foundation and the National Science Foundation.