The blue graph shows the apparent percentage (not the absolute number) of marine animalgenera
becoming extinct during any given time interval. It does not represent
all marine species, just those that are readily fossilized. The labels
of the traditional "Big Five" extinction events and the more recently
recognised Capitanian mass extinction event
are clickable links. The two extinction events occurring in the
Cambrian (far left) are very large in percentage magnitude, but small in
absolute numbers of known taxa due to the relative scarcity of
fossil-producing life at that time.
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the background extinction rate and the rate of speciation.
Estimates of the number of major mass extinctions in the last 540
million years range from as few as five to more than twenty. These
differences stem from disagreement as to what constitutes a "major"
extinction event, and the data chosen to measure past diversity.
The "Big Five" mass extinctions
In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five particular geological intervals with excessive diversity loss. They were originally identified as outliers on 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 at least 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. An earlier (first) event at the end of the Ediacaran is speculated.
Ordovician–Silurian extinction events (End Ordovician or O–S): 445–444 Ma, just prior to and at the Ordovician–Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 85% 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 that the causes of the mass extinction were global warming, related to volcanism, and anoxia, and not, as considered earlier, cooling and glaciation. However, this is at odds with numerous previous studies, which have indicated global cooling as the primary driver.
Most recently, the deposition of volcanic ash has been suggested to be
the trigger for reductions in atmospheric carbon dioxide leading to the
glaciation and anoxia observed in the geological record.
Late Devonian extinctions: 372–359 Ma, occupying much of the Late Devonian up to the Devonian–Carboniferous
transition. The Late Devonian was an interval of high diversity loss,
concentrated into two extinction events. The largest extinction was the Kellwasser Event (Frasnian-Famennian,
or F-F, 372 Ma), an extinction event at the end of the Frasnian, about
midway through the Late Devonian. This extinction annihilated coral reefs and numerous tropical benthic (seabed-living) animals such as jawless fish, brachiopods, and trilobites. Another major extinction was the Hangenberg Event
(Devonian-Carboniferous, or D-C, 359 Ma), which brought an end to the
Devonian as a whole. This extinction wiped out the armored placoderm fish and nearly led to the extinction of the newly evolved ammonoids. These two closely-spaced extinction events collectively eliminated about 19% of all families, 50% of all genera and at least 70% of all species. Sepkoski and Raup (1982) did not initially consider the Late Devonian extinction interval (Givetian, Frasnian, and Famennian stages) to be statistically significant. Regardless, later studies have affirmed the strong ecological impacts of the Kellwasser and Hangenberg Events.
Trilobites were highly successful marine animals until the Permian–Triassic extinction event wiped them all out.Permian–Triassic extinction event (End Permian): 252 Ma, at the Permian–Triassic transition. Earth's largest extinction killed 53% of marine families, 84% of marine genera, about 81% of all marine species and an estimated 70% of terrestrial vertebrate species. This is also the largest known extinction event for 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 early synapsids. 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
(unable to move about) dropped from 67% to 50%. The whole late Permian
was a difficult time, at least for marine life, even before the P–T
boundary extinction. More recent research has indicated that the End-Capitanian extinction event
that preceded the "Great Dying" likely constitutes a separate event
from the P–T extinction; if so, it would be larger than some of the "Big
Five" extinction events, and perhaps merit a separate place in this
list immediately before this one.
Triassic–Jurassic extinction event (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).
Badlands near Drumheller, Alberta, where erosion has exposed the Cretaceous–Paleogene boundary.Cretaceous–Paleogene extinction event (End Cretaceous, K–Pg extinction, or formerly K–T extinction): 66 Ma, at the Cretaceous (Maastrichtian) – Paleogene (Danian) transition.
The event was formerly called the Cretaceous-Tertiary or K–T extinction
or K–T boundary; it 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 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 former descended from the synapsids and the latter from theropod dinosaurs, emerged as dominant terrestrial 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, and other evidence such as fungal spikes (geologically rapid increase in fungal
abundance) 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.
Sixth mass extinction
Research
completed after the seminal 1982 paper (Sepkoski and Raup) has
concluded that a sixth mass extinction event is ongoing due to human
activities:
Holocene extinction: currently ongoing. Extinctions have occurred at over 1000 times the background extinction rate since 1900, and the rate is increasing. The mass extinction is a result of human activity (an ecocide) 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. In late 2021, WWF
Germany suggested that over a million species could go extinct within a
decade in the "largest mass extinction event since the end of the
dinosaur age." A 2023 study published in PNAS concluded that at least 73 genera
of animals have gone extinct since 1500. If humans had never existed,
it would have taken 18,000 years for the same genera to have disappeared
naturally, the report states.
Extinction events can be tracked by several methods, including
geological change, ecological impact, extinction vs. origination (speciation) rates, and most commonly diversity loss among taxonomic units. Most early papers used families as the unit of taxonomy, based on compendiums of marine animal families by Sepkoski (1982, 1992). Later papers by Sepkoski and other authors switched to genera, which are more precise than families and less prone to taxonomic bias or incomplete sampling relative to species.
These are several major papers estimating loss or ecological impact
from fifteen commonly-discussed extinction events. Different methods
used by these papers are described in the following section. The "Big
Five" mass extinctions are bolded.
Extinction proportions (diversity loss) of marine genera or ecological impact in estimates of mass extinction severity
Luis (left) and Walter Alvarez (right) at the K-Pg boundary in Gubbio, Italy
in 1981. This team discovered geological evidence for an asteroid
impact causing the K-Pg extinction, spurring a wave of public and
scientific interest in mass extinctions and their causes
For much of the 20th century, the study of mass extinctions was
hampered by insufficient data. Mass extinctions, though acknowledged,
were considered mysterious exceptions to the prevailing gradualistic
view of prehistory, where slow evolutionary trends define faunal
changes. The first breakthrough was published in 1980 by a team led by Luis Alvarez, who discovered trace metal evidence for an asteroid impact at the end of the Cretaceous period. The Alvarez hypothesis for the end-Cretaceous extinction gave mass extinctions, and catastrophic explanations, newfound popular and scientific attention.
Changes
in diversity among genera and families, according to Sepkoski (1997).
The "Big Five" mass extinctions are labelled with arrows, and taxa are
segregated into Cambrian- (Cm), Paleozoic- (Pz), and Modern- (Md) type
faunas.
Another landmark study came in 1982, when a paper written by David M. Raup and Jack Sepkoski was published in the journal Science. This paper, originating from a compendium of extinct marine animal families developed by Sepkoski,
identified five peaks of marine family extinctions which stand out
among a backdrop of decreasing extinction rates through time. Four of
these peaks were statistically significant: the Ashgillian (end-Ordovician), Late Permian, Norian (end-Triassic), and Maastrichtian (end-Cretaceous). The remaining peak was a broad interval of high extinction smeared over the later half of the Devonian, with its apex in the Frasnian stage.
Through the 1980s, Raup and Sepkoski continued to elaborate and
build upon their extinction and origination data, defining a
high-resolution biodiversity curve (the "Sepkoski curve") and successive evolutionary faunas with their own patterns of diversification and extinction.Though these interpretations formed a strong basis for subsequent
studies of mass extinctions, Raup and Sepkoski also proposed a more
controversial idea in 1984: a 26-million-year periodic pattern to mass
extinctions. Two teams of astronomers linked this to a hypothetical brown dwarf in the distant reaches of the solar system, inventing the "Nemesis hypothesis" which has been strongly disputed by other astronomers.
Around the same time, Sepkoski began to devise a compendium of marine animal genera,
which would allow researchers to explore extinction at a finer
taxonomic resolution. He began to publish preliminary results of this
in-progress study as early as 1986, in a paper which identified 29
extinction intervals of note.
By 1992, he also updated his 1982 family compendium, finding minimal
changes to the diversity curve despite a decade of new data.
In 1996, Sepkoski published another paper which tracked marine genera
extinction (in terms of net diversity loss) by stage, similar to his
previous work on family extinctions. The paper filtered its sample in
three ways: all genera (the entire unfiltered sample size),
multiple-interval genera (only those found in more than one stage), and
"well-preserved" genera (excluding those from groups with poor or
understudied fossil records). Diversity trends in marine animal families
were also revised based on his 1992 update.
Revived interest in mass extinctions led many other authors to
re-evaluate geological events in the context of their effects on life. A 1995 paper by Michael Benton
tracked extinction and origination rates among both marine and
continental (freshwater & terrestrial) families, identifying 22
extinction intervals and no periodic pattern. Overview books by O.H. Wallister (1996) and A. Hallam and P.B. Wignall (1997) summarized the new extinction research of the previous two decades.
One chapter in the former source lists over 60 geological events which
could conceivably be considered global extinctions of varying sizes.
These texts, and other widely circulated publications in the 1990s,
helped to establish the popular image of mass extinctions as a "big
five" alongside many smaller extinctions through prehistory.
New data on genera: Sepkoski's compendium
Major Phanerozoic extinctions tracked via proportional genera extinctions by Bambach (2006)
Though Sepkoski passed away in 1999, his marine genera compendium was
formally published in 2002. This prompted a new wave of studies into
the dynamics of mass extinctions. These papers utilized the compendium to track origination rates (the rate that new species appear or speciate) parallel to extinction rates in the context of geological stages or substages.
A review and re-analysis of Sepkoski's data by Bambach (2006)
identified 18 distinct mass extinction intervals, including 4 large
extinctions in the Cambrian.
These fit Sepkoski's definition of extinction, as short substages with
large diversity loss and overall high extinction rates relative to their
surroundings.
Bambach et al. (2004) considered each of the "Big Five"
extinction intervals to have a different pattern in the relationship
between origination and extinction trends. Moreover, background
extinction rates were broadly variable and could be separated into more
severe and less severe time intervals. Background extinctions were least
severe relative to the origination rate in the middle Ordovician-early
Silurian, late Carboniferous-Permian, and Jurassic-recent. This argues
that the Late Ordovician, end-Permian, and end-Cretaceous extinctions
were statistically significant outliers in biodiversity trends, while
the Late Devonian and end-Triassic extinctions occurred in time periods
which were already stressed by relatively high extinction and low
origination.
Computer models run by Foote (2005) determined that abrupt pulses
of extinction fit the pattern of prehistoric biodiversity much better
than a gradual and continuous background extinction rate with smooth
peaks and troughs. This strongly supports the utility of rapid, frequent
mass extinctions as a major driver of diversity changes. Pulsed
origination events are also supported, though to a lesser degree which
is largely dependent on pulsed extinctions.
Similarly, Stanley (2007) used extinction and origination data to
investigate turnover rates and extinction responses among different
evolutionary faunas and taxonomic groups. In contrast to previous
authors, his diversity simulations show support for an overall
exponential rate of biodiversity growth through the entire Phanerozoic.
Tackling biases in the fossil record
An illustration of the Signor-Lipps effect,
a geological bias which posits that increased fossil sampling would
help to better constrain the exact time when an organism truly goes
extinct.
As data continued to accumulate, some authors began to re-evaluate Sepkoski's sample using methods meant to account for sampling biases. As early as 1982, a paper by Phillip W. Signor and Jere H. Lipps noted that the true sharpness of extinctions was diluted by the incompleteness of the fossil record. This phenomenon, later called the Signor-Lipps effect,
notes that a species' true extinction must occur after its last fossil,
and that origination must occur before its first fossil. Thus, species
which appear to die out just prior to an abrupt extinction event may
instead be a victim of the event, despite an apparent gradual decline
looking at the fossil record alone. A model by Foote (2007) found that
many geological stages had artificially inflated extinction rates due to
Signor-Lipps "backsmearing" from later stages with extinction events.
Estimated extinction rates among genera through time. From Foote (2007), top, and Kocsis et al. (2019), bottom
Other biases include the difficulty in assessing taxa with high
turnover rates or restricted occurrences, which cannot be directly
assessed due to a lack of fine-scale temporal resolution. Many
paleontologists opt to assess diversity trends by randomized sampling
and rarefaction
of fossil abundances rather than raw temporal range data, in order to
account for all of these biases. But that solution is influenced by
biases related to sample size. One major bias in particular is the "Pull of the recent",
the fact that the fossil record (and thus known diversity) generally
improves closer to the modern day. This means that biodiversity and
abundance for older geological periods may be underestimated from raw
data alone.
Alroy (2010) attempted to circumvene sample size-related biases in diversity estimates using a method he called "shareholder
quorum subsampling" (SQS). In this method, fossils are sampled from a
"collection" (such as a time interval) to assess the relative diversity
of that collection. Every time a new species (or other taxon) enters the sample, it brings over all other fossils belonging to that species in the collection (its "share"
of the collection). For example, a skewed collection with half its
fossils from one species will immediately reach a sample share of 50% if
that species is the first to be sampled. This continues, adding up the
sample shares until a "coverage" or "quorum"
is reached, referring to a pre-set desired sum of share percentages. At
that point, the number of species in the sample are counted. A
collection with more species is expected to reach a sample quorum with
more species, thus accurately comparing the relative diversity change
between two collections without relying on the biases inherent to sample
size.
Alroy also elaborated on three-timer algorithms, which are meant
to counteract biases in estimates of extinction and origination rates. A
given taxon is a "three-timer" if it can be found before, after, and
within a given time interval, and a "two-timer" if it overlaps with a
time interval on one side. Counting "three-timers" and "two-timers" on
either end of a time interval, and sampling time intervals in sequence,
can together be combined into equations to predict extinction and
origination with less bias. In subsequent papers, Alroy continued to refine his equations to improve lingering issues with precision and unusual samples.
McGhee et al. (2013), a paper which primarily focused on
ecological effects of mass extinctions, also published new estimates of
extinction severity based on Alroy's methods. Many extinctions were
significantly more impactful under these new estimates, though some were
less prominent.
Stanley (2016) was another paper which attempted to remove two
common errors in previous estimates of extinction severity. The first
error was the unjustified removal of "singletons", genera unique to only
a single time slice. Their removal would mask the influence of groups
with high turnover rates or lineages cut short early in their
diversification. The second error was the difficulty in distinguishing
background extinctions from brief mass extinction events within the same
short time interval. To circumvent this issue, background rates of
diversity change (extinction/origination) were estimated for stages or
substages without mass extinctions, and then assumed to apply to
subsequent stages with mass extinctions. For example, the Santonian and Campanian stages were each used to estimate diversity changes in the Maastrichtian
prior to the K-Pg mass extinction. Subtracting background extinctions
from extinction tallies had the effect of reducing the estimated
severity of the six sampled mass extinction events. This effect was
stronger for mass extinctions which occurred in periods with high rates
of background extinction, like the Devonian.
Uncertainty in the Proterozoic and earlier eons
Because most diversity and biomass on Earth is microbial,
and thus difficult to measure via fossils, extinction events placed
on-record are those that affect the easily observed, biologically
complex component of the biosphere rather than the total diversity and abundance of life. For this reason, well-documented extinction events are confined to the Phanerozoic eon, before which all living organisms were either microbial or at most soft-bodied; the sole exception is the Great Oxidation Event in the Proterozoic. Perhaps due to the absence of a robust microbial fossil record, mass extinctions seem mainly to be a Phanerozoic phenomenon, with apparent extinction rates being low before large complex organisms arose.
The Great Oxidation Event, which occurred around 2.45 billion years ago in the Paleoproterozoic, was probably the first major extinction event. Since the Cambrian explosion,
five further major mass extinctions have significantly exceeded the
background extinction rate. The most recent and best-known, the Cretaceous–Paleogene extinction event,
which occurred approximately 66 Ma (million years ago), was a
large-scale mass extinction of animal and plant species in a
geologically short period of time. In addition to the five major Phanerozoic
mass extinctions, there are numerous minor ones as well, and the
ongoing mass extinction caused by human activity is sometimes called the
sixth extinction.
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, mammaliaformes ("almost mammals") and then mammals existed throughout the reign of the dinosaurs, but could not compete in the large terrestrial vertebrate niches that 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. 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 that 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".
Patterns in frequency
Various authors have suggested that extinction events occurred periodically, every 26 to 30 million years, or that diversity fluctuates episodically about every 62 million years. Various ideas, mostly regarding astronomical influences, 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 or lacking statistical significance. 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 such as Strontium isotopes,
flood basalts, anoxic events, orogenies, and evaporite deposition. One
explanation for this proposed cycle is carbon storage and release by
oceanic crust, which exchanges carbon between the atmosphere and mantle.
Phanerozoic biodiversity as shown by the fossil record
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,
but susceptibility to extinction at a taxonomic level does not appear to make mass extinctions more or less probable.
Causes
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
A good theory for a particular mass extinction should:
explain all of the losses, not just focus on a few groups (such as dinosaurs);
explain why particular groups of organisms died out and why others survived;
provide mechanisms that are strong enough to cause a mass extinction but not a total extinction;
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 that
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
MacLeod (2001)
summarized the relationship between mass extinctions and events that
are most often cited as causes of mass extinctions, using data from
Courtillot, Jaeger & Yang et al. (1996), Hallam (1992) and Grieve & Pesonen (1992):
Flood basalt events (giant volcanic eruptions): 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.
Asteroid impacts:
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, or cannot be dated precisely enough. The impact that created the Siljan Ring either was just before the Late Devonian Extinction or coincided with it.
The most commonly suggested causes of mass extinctions are listed below.
Flood basalt events
The scientific consensus is that the main cause of the End-Permian extinction event was the large amount of carbon dioxide emitted by the volcanic eruptions that created the Siberian Traps, which elevated global temperatures.
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.
These
are often clearly marked by worldwide sequences of contemporaneous
sediments that 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, along with the more recently recognised Capitanian mass extinction of comparable severity to the Big Five.
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.
Extraterrestrial threats
Impact events
An artist's rendering of an asteroid
a few kilometers across colliding with the Earth. Such an impact can
release the equivalent energy of several million nuclear weapons
detonating simultaneously.
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.
The Permian-Triassic extinction event has also been hypothesised to have been caused by an asteroid impact that formed the Araguainha crater due to the estimated date of the crater's formation overlapping with the end-Permian extinction event. However, this hypothesis has been widely challenged, with the impact hypothesis being rejected by most researchers.
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 maps of the spiral structure of the Milky Way
in CO molecular line emission has failed to find a correlation.
A nearby nova, supernova or gamma ray burst
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 gamma ray burst caused the End-Ordovician extinction, while a supernova has been proposed as the cause of the Hangenberg event.
Global cooling
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.
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.
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
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.
The bio-availability of essentialtrace 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 (Se6+) is converted into much less soluble selenide (Se2-),
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".
Oceanic overturn is a disruption of thermo-haline circulation
that 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
that 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.
Geomagnetic reversal
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
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 that 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 that 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 that 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
Many species of plants and animals are at high risk of extinction due to the destruction of the Amazon rainforest
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 that are contrary to the
available evidence; they are based on other theories that 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.
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 would be completely sterilized):
rising global temperatures caused by the expanding Sun would gradually
increase the rate of weathering, which would in turn remove more and
more CO2 from the atmosphere. When CO2 levels get
too low (perhaps at 50 ppm), most plant life will die out, although
simpler plants like grasses and mosses can survive much longer, until CO2 levels drop to 10 ppm.
With all photosynthetic organisms gone, atmospheric oxygen can no
longer be replenished, and it 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 around 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, it would represent the final mass
extinction in Earth's history (albeit a very long extinction event).
Effects and recovery
The effects 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 Phanerozoic 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 that
inhibited recovery, as well as prolonged environmental stress that
continued into the Early Triassic. Recent research indicates that
recovery did not begin until the start of the mid-Triassic, four to six
million years after the extinction;
and some writers estimate that the recovery was not complete until 30
million 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.
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