Major extinction events

Major extinction events
BY
PRAMODA G
Mysuru

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 rate of speciation.
Because most diversity and biomass on Earth is microbial,
and thus difficult to measure, recorded extinction events
affect the easily observed, biologically complex
component of the biosphere rather than the total diversity
and abundance of life.

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.

1. Ordovician–Silurian 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.

2. Late Devonian extinction: 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
algal 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.

3. Permian–Triassic extinction event (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".

4. 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 the rapsids, and most
of the large amphibians were Major extinction events Badlands
near Drumheller, Alberta, where erosion has exposed the K– Pg
boundary.
Trilobites were highly successful marine animals until the
Permian– Triassic extinction event wiped them all out
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 K.,oolasuchus).

5. Cretaceous–Paleogene extinction event(End Cretaceous, K–Pg
extinction, or formerly K–T extinction):
66Ma 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 oficially named the
Cretaceous–Paleogene (or K–Pg) 5. Cretaceous–Paleogene extinction
even t(End Cretaceous, K–Pg extinction, or formerly K–T extinction)
event. About 17% of all families, 50% of algl enera and 75% of all
species became extinct. In the seas all the ammonites, plesiosaurs
and mosasaurs disappeared and the percentage ofs essile animals
(those unable to move about) was reduced to about 33%. All non-
aviand inosaurs 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.

6. Holocene extinction: Currently ongoing.
Extinctions have occurred at over 1000 times the
background extinction rate since 1900. The mass extinction
is considered a result of human activity.
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 lagrer than many of the "Big Five"
extinction events.

List of extinction events
Period Extinction Date Possible causes
Quaternary Holocene
extinction
c. 10,000 BCE -
Ongoing
Humans
Cretaceous Cretaceous-
Paleogene
extinction event
66 Ma Chicxulub impactor
Triassic Triassic–Jurassic
extinction event
201 Ma Central Atlantic magmatic
province; impactor
Permian Permian–Triassic
extinction event
252 Ma Siberian Traps;Wilkes Land
Crater; Anoxic event
Devonian Late Devonian
extinction
375–360 Ma Viluy Traps
Ordovician–
Silurian extinction
events
450–440 Ma Global cooling and sea level
drop; Gamma-ray Burst

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 particular mass extinctions
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.

Most widely supported explanations
Macleod (2001) summarized the relationship between mass
extinctions and events which are most often cited as causes of mass
extinctions, using data from Courtillote t al. (1996), Hallam (1992) and
Grieve et al. (1996):
The most commonly suggested causes of mass extinctions are listed

Flood basalt events:
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 Myr. Recently such
possible correlation was extended for the whole Phanerozoic Eon.

1. Sea-level falls
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 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

2. Impact events
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 mega-
tsunamis and/or global forest fires.
Most paleontologists now agree that an asteroid did hit the Earth
about 66 Ma ago, but there is an ongoing dispute whether the impact
was the sole cause of the Cretaceous–Paleogene extinction even.

3.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.

4.Global warming
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.

5. Clathrate gun hypothesis
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.

6.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.
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, i.e. 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.

7.Hydrogen sulfide emissions from the seas
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.

8.Oceanic overturn
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 Devonian and Permian–Triassic extinctions.

9.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 supernova or gamma ray burst
caused the End-Ordovician extinction.

10.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

11.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 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

Major extinction events

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Major extinction events