Extinction event

Extinction event

Overview
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

 increases with respect to the rate of speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

. Because the majority of diversity and biomass
Biomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...

 on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...

 rather than the total diversity and abundance of life.

Over 99% of documented species are now extinct, but extinction occurs at an uneven rate.
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Encyclopedia
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

 increases with respect to the rate of speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

. Because the majority of diversity and biomass
Biomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...

 on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...

 rather than the total diversity and abundance of life.

Over 99% of documented species are now extinct, but extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

s on Earth is about two to five taxonomic
Alpha taxonomy
Alpha taxonomy is the discipline concerned with finding, describing and naming species of living or fossil organisms. This field is supported by institutions holding collections of these organisms, with relevant data, carefully curated: such institutes include natural history museums, herbaria and...

 families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 of marine invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s and vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s every million years.
Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms.

Since life began on Earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, Cretaceous–Tertiary extinction event, which occurred approximately 65.5 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. In the past 540 million years there have been five major events when over 50% of animal species died. There probably were mass extinctions in the Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...

 and Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...

 Eons, but before the Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...

 there were no animals with hard body parts to leave a significant fossil record.

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 the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.

Major extinction events


In a landmark paper published in 1982, Jack Sepkoski
Jack Sepkoski
J. John Sepkoski Jr., , was a University of Chicago paleontologist. Sepkoski studied the fossil record and the diversity of life on Earth. Sepkoski and David Raup contributed to the knowledge of extinction events...

 and David M. Raup
David M. Raup
David M. Raup is a University of Chicago paleontologist. Raup studied the fossil record and the diversity of life on Earth. Raup contributed to the knowledge of extinction events along with his colleague Jack Sepkoski...

 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, 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. Cretaceous–Tertiary extinction event (End Cretaceous or K-T extinction): 65.5 Ma at the Cretaceous
    Cretaceous
    The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

    .Maastrichtian
    Maastrichtian
    The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

    -Paleogene
    Paleogene
    The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...

    .Danian
    Danian
    The Danian is the oldest age or lowermost stage of the Paleocene epoch or series, the Paleogene period or system and the Cenozoic era or erathem. The beginning of the Danian age is at the Cretaceous-Tertiary extinction event...

     transition interval. The K–T event is now called the Cretaceous–Paleogene (or K–Pg) extinction event by many researchers. About 17% of all families, 50% of all genera
    Genera
    Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...

     and 75% of species became extinct. In the seas it reduced the percentage of sessile
    Sessility (zoology)
    In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...

     animals to about 33%. The majority of 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 clade
    Clade
    A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

    s. Mammal
    Mammal
    Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

    s and bird
    Bird
    Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

    s emerged as dominant land vertebrates in the age of new life.
  2. Triassic–Jurassic extinction event (End Triassic): 205 Ma at the Triassic
    Triassic
    The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

    -Jurassic
    Jurassic
    The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

     transition. About 23% of all families and 48% of all genera (20% of marine families and 55% of marine genera) went extinct. Most non-dinosaurian archosaur
    Archosaur
    Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...

    s, most therapsids
    Therapsida
    Therapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, including hair, lactation, and an erect posture. The earliest fossil attributed to Therapsida is believed to be...

    , and most of the large amphibians were eliminated, leaving dinosaur
    Dinosaur
    Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

    s 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
    Koolasuchus
    Koolasuchus is an extinct genus of brachyopoid temnospondyl in the family Chigutisauridae. Fossils have been found from Victoria, Australia and date back 120 Ma to the Aptian stage of the Early Cretaceous. Koolasuchus is the latest known temnospondyl. Koolasuchus is known from several fragments of...

    ).
  3. Permian–Triassic extinction event (End Permian): 251 Ma at the Permian
    Permian
    The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

    -Triassic
    Triassic
    The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

     transition. Earth's largest extinction killed 57% of all families and 83% of all genera (53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species) including insect
    Insect
    Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

    s. The evidence of plant
    Plant
    Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

    s 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
    Synapsid
    Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

    . The recovery of vertebrates took 30 million years, but the vacant niches
    Ecological niche
    In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

     created the opportunity for archosaur
    Archosaur
    Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...

    s to become ascendant. In the seas, the percentage of animals that were sessile
    Sessility (zoology)
    In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...

     dropped from 67% to 50%. The whole late Permian was a difficult time for at least marine life, even before the "Great Dying".
  4. Late Devonian extinction
    Late Devonian extinction
    The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...

    : 360–375 Ma near the Devonian
    Devonian
    The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

    -Carboniferous
    Carboniferous
    The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

     transition. At the end of the Frasnian Age
    Frasnian
    The Frasnian is one of two faunal stages in the Late Devonian epoch. It lasted from 385.3 ± 2.6 million years ago to 374.5 ± 2.6 million years ago. It was preceded by the Givetian stage and followed by the Famennian stage...

     in the later part(s) of the Devonian Period, a prolonged series of extinctions eliminated about 19% of all families, 50% of all genera
    Genera
    Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...

     and 70% of all species. This extinction event lasted perhaps as long as 20 Ma, and there is evidence for a series of extinction pulses within this period.
  5. Ordovician–Silurian extinction event (End Ordovician or O-S): 440–450 Ma at the Ordovician
    Ordovician
    The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

    -Silurian
    Silurian
    The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

     transition. Two events occurred that killed off 27% of all families and 57% of all genera. 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
    Genus
    In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

     that went extinct.


The older the fossil record gets, the more difficult it is to read. This is because:
  • Older fossils are harder to find because they are usually buried at a considerable depth in the rock.
  • Dating older fossils is more difficult.
  • Productive fossil beds are researched more than unproductive ones, therefore leaving certain periods unresearched.
  • Prehistoric environmental disturbances 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) provides reassurance that most widely accepted extinction events are indeed real. A quantification of the rock exposure of Western Europe does indicate that many of the minor events for which a biological explanation has been sought are most readily explained by sampling bias.

Lesser extinctions


Lesser extinction events include:
Period Start Date Extinction Date Cause
Quaternary extinction event
Quaternary extinction event
The Quaternary period saw the extinctions of numerous predominantly larger, especially megafaunal, species, many of which occurred during the transition from the Pleistocene to the Holocene epoch. However, the extinction wave did not stop at the end of the Pleistocene, but continued especially on...

 
50 ka to now Humans and/or climate change (ice age, etc.)
Neogene
Neogene
The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...

 
Middle Miocene disruption
Middle Miocene disruption
The term Middle Miocene disruption, alternatively the Middle Miocene extinction or Middle Miocene extinction peak, refers to a wave of extinctions of terrestrial and aquatic life forms that occurred around the middle of the Miocene Epoch, c...

 
14.5 Ma Nördlinger Ries
Nördlinger Ries
The Nördlinger Ries is a large circular depression in western Bavaria, Germany, located north of the Danube in the district of Donau-Ries. The city of Nördlingen is located about southwest of the centre of the depression....

 bolide impact? Volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es in African Rift Valley
Eocene–Oligocene extinction event  33.9 Ma Volcanoes? Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

 and Popigai crater
Popigai crater
The Popigai crater in Siberia, Russia is tied with Manicouagan Crater as the fourth largest verified impact crater on Earth. A large bolide impact created the diameter crater 35.7 ± 0.2 million years ago during the late Eocene . The crater is 1½ hours from the outpost of Khatanga...

 bolide impacts?
Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 
Aptian extinction
Aptian extinction
The Aptian extinction was an extinction event of the early Cretaceous Period. It is dated to c. 116 or 117 million years ago, in the middle of the Aptian stage of the geological time scale, and has sometimes been termed the mid-Aptian extinction event as a result.It is classified as a minor...

117 Ma Rahjamal Traps
Rahjamal Traps
The Rahjamal Traps are a volcanism episode in Bengal in India, associated with the Kerguelen "hot spot" of volcanic activity. At the time in question, c. 116–117 Ma, India in the southern Indian ocean over the Kerguelen hot spot; plate tectonics had not yet moved the Indian landmass into its...

 volcanism episode in Bengal
Bengal
Bengal is a historical and geographical region in the northeast region of the Indian Subcontinent at the apex of the Bay of Bengal. Today, it is mainly divided between the sovereign land of People's Republic of Bangladesh and the Indian state of West Bengal, although some regions of the previous...

?
End-Jurassic extinction 145.5 Ma
Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 
Toarcian turnover 183 Ma
Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 
Olson's Extinction
Olson's Extinction
Olson's Extinction was a mass extinction that occurred in the Early Guadalupian of the Permian period and which predated the Permian–Triassic extinction event.- Extinction Patterns :...

 
270 Ma
Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 
Carboniferous Rainforest Collapse
Carboniferous Rainforest Collapse
The Carboniferous Rainforest Collapse was an extinction event that occurred around 305 million years ago in the Carboniferous period). Vast coal forests covered the equatorial region of Euramerica...

318 Ma Climate change
End Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 
416 Ma
Lau event
Lau event
The Lau event was the last of three relatively minor mass extinctions during the Silurian period, having a major effect on the conodont fauna...

 
420 Ma
Mulde event
Mulde event
The Mulde event was a secundo-secundo event, and marked the second of three relatively minor mass extinctions during the Silurian period. It coincided with a global drop in sea level, and is closely followed by an excursion in geochemical isotopes...

 
424 Ma Global drop in sea level?
Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 
Ireviken event
Ireviken event
The Ireviken event was a minor extinction event at the Llandovery/Wenlock boundary .The event is best recorded at Ireviken, Gotland, where over 50% of trilobite species go extinct; 80% of the global conodont species also become extinct in this interval.-Anatomy of the event:The event lasted around...

 
428 Ma Deep-ocean anoxia
Anoxic event
Oceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused...

?
Cambrian–Ordovician extinction event  488 Ma Glaciation? Depletion of oxygen in marine waters?
Dresbachian
Dresbachian
The Dresbachian is the lower stage of the Late or Upper Cambrian Period in North America, equivalent to the Chinese Guzhangian which spans about 4 million years, from...

 
502 Ma
Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 
End Botomian  517 Ma
Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 
End-Ediacaran extinction
End-Ediacaran extinction
Evidence suggesting that a mass extinction occurred at the end of the Ediacaran period, , includes:*A mass extinction of acritarchs*The sudden disappearance of the Ediacara biota and calcifying organisms;...

 
542 Ma Ocean anoxia
Anoxic event
Oceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused...

?

Evolutionary importance


Mass extinctions have sometimes accelerated the evolution of life on Earth
Life on Earth
Life on Earth: A Natural History by David Attenborough is a television natural history series made by the BBC in association with Warner Bros. and Reiner Moritz Productions...

. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.

For example mammaliformes ("almost mammals") and then mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s existed throughout the reign of the dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s, but could not compete for 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
Crurotarsi
The Crurotarsi are a group of archosauriformes, represented today by the crocodiles,...

.

Another point of view put forward in the Escalation hypothesis
Escalation hypothesis
The Escalation Hypothesis is a theory put forward by Geerat J. Vermeij. It states that organisms are in constant conflict with one another and therefore devote lots of resources to thwarting the adaptations evolution brings to all competing organisms as time advances...

 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
Dead Clade Walking
The phrase Dead Clade Walking refers to the fact that some clades of organisms which survived mass extinctions, either become extinct a few million years after the mass extinction or fail to recover in numbers and diversity.-Origin of phrase:...

".
So analysing extinctions in terms of "what died and what survived" often fails to tell the full story.

Patterns in frequency


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
Nemesis (star)
Nemesis is a hypothetical hard-to-detect red dwarf star, white dwarf star or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU , somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to...

 to the sun, oscillations in the galactic plane, or passage through the Milky Way's spiral arms. However, other authors have concluded 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.
Mass extinctions are thought to result when a long-term stress is compounded by a short term shock. Over the course of the Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...

, individual taxa appear to be less likely to become extinct at any time, which may reflect more robust food webs as well as less 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,Dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication
Eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...

 and anoxic event
Anoxic event
Oceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused...

s; and marine ecosystems became more diversified so that food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

s were less likely to be disrupted.
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 (meteorite impacts, etc.) to produce the global effects observed.

Looking for the 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.

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
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...

 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 which are most often cited as causes of mass extinctions, using data from Courtillot et al. (1996),
Hallam (1992)
and Grieve et al. (1996):
  • Flood basalt
    Flood basalt
    A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...

     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.
  • Asteroid impacts; One large impact associated with a mass extinction; 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


The formation of large igneous province
Large igneous province
A Large Igneous Province is an extremely large accumulation of igneous rocks—intrusive, extrusive, or both—in the earth's crust...

s by flood basalt events could have:
  • produced dust and particulate aerosols which inhibited photosynthesis and thus caused food chain
    Food chain
    A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

    s to collapse both on land and at sea
  • emitted sulfur oxides which were precipitated as acid rain
    Acid rain
    Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...

     and poisoned many organisms, contributing further to the collapse of food chains
  • emitted carbon dioxide
    Carbon dioxide
    Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

     and thus possibly causing sustained global warming
    Greenhouse effect
    The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

     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-Cretaceous, End-Permian, and End Triassic
Triassic-Jurassic extinction event
The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, , and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas a whole class and twenty percent of all marine families...

 extinctions.

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
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...

. 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
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

.

Sea-level falls are associated with most of the mass extinctions, including all of the "Big Five"—End-Ordovician
Ordovician-Silurian extinction events
The Ordovician–Silurian extinction event, or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440...

, Late Devonian
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...

, End-Permian
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...

, End-Triassic
Triassic-Jurassic extinction event
The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, , and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas a whole class and twenty percent of all marine families...

, and End-Cretaceous.

A study, published in the journal Nature (online June 15, 2008) 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


The impact of a sufficiently large asteroid or comet could have caused food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

s to collapse both on land and at sea by producing dust and particulate aerosols and thus inhibiting photosynthesis. Impacts on sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

-rich rocks could have emitted sulfur oxides precipitating as poisonous acid rain
Acid rain
Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...

, contributing further to the collapse of food chains. Such impacts could also have caused megatsunami
Megatsunami
Megatsunami is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis...

s and / or global forest fires.

Most paleontologists now agree that an asteroid did hit the Earth about 65 Ma, but there is an ongoing dispute whether the impact was the sole cause of the Cretaceous–Tertiary extinction event. There is evidence that there was an interval of about 300 ka from the impact to the mass extinction. In 1997, paleontologist Sankar Chatterjee
Sankar Chatterjee
Sankar Chatterjee is a paleontologist, and is the Paul W. Horn Professor of Geosciences at Texas Tech University and Curator of Paleontology at the Museum of Texas Tech University. He earned his Ph. D. from the University of Calcutta in 1970 and was a Post-doctoral Fellow at the Smithsonian...

 drew attention to the proposed and much larger 600 km (370 mi) Shiva crater
Shiva crater
The Shiva crater is a sea floor structure located beneath the Indian Ocean, west of Mumbai, India. It was named by the paleontologist Sankar Chatterjee after Shiva, the Hindu god of destruction and renewal....

 and the possibility of a multiple-impact scenario.

In 2007, a hypothesis was put forth that argued the impactor that killed the dinosaurs 65 Ma years ago belonged to the Baptistina family
Baptistina family
The Baptistina family is an asteroid family that was likely produced by the breakup of an asteroid across 80 million years ago following an impact with a smaller body. The largest presumed remnant of this parent asteroid is 298 Baptistina....

 of asteroids. Concerns have been raised regarding the reputed link, in part because very few solid observational constraints exist of the asteroid or family. Indeed, it was recently discovered that 298 Baptistina does not share the same chemical signature as the source of the K-T impact. Although this finding may make the link between the Baptistina family and K-T impactor more difficult to substantiate, it does not preclude the possibility.

In 2010, another hypothesis was offered which implicated the newly discovered asteroid P/2010 A2
P/2010 A2
P/2010 A2 is a small Solar System body that displayed characteristics of both an asteroid and a comet, and thus, was initially given a cometary designation. Since it has the orbit of a main-belt asteroid and showed the tail of a comet, it was listed as a main-belt comet...

, a member of the Flora family
Flora family
The Flora family of asteroids is a large grouping of S-type asteroids in the inner main belt, whose origin and properties are relatively poorly understood at present...

 of asteroids, as a possible remnant cohort of the K/T impactor.

The Shiva hypothesis
Shiva Hypothesis
Named after the Hindu god of destruction, the Shiva Hypothesis is a hypothesis that purports to explain an apparent pattern in mass extinctions caused by impact events....

 proposes that periodic gravitational disturbances cause comets from the Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...

 to bombard earth every 26 to 30 million years.

Sustained and significant global cooling


Sustained global cooling could kill many polar
Polar circle
A polar circle is either the Arctic Circle or the Antarctic Circle. On Earth, the Arctic Circle is located at a latitude of  N, and the Antarctic Circle is located at a latitude of  S....

 and temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 species and force others to migrate towards the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

; reduce the area available for tropical
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

 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
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

 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
Ordovician-Silurian extinction events
The Ordovician–Silurian extinction event, or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440...

, Permian-Triassic
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...

, Late Devonian
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...

 extinctions, and possibly others. Sustained global cooling is distinguished from the temporary climatic effects of flood basalt events or impacts.

Sustained and significant global warming


This would have the opposite effects: expand the area available for tropical
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

 species; kill temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 species or force them to migrate towards the poles
Polar circle
A polar circle is either the Arctic Circle or the Antarctic Circle. On Earth, the Arctic Circle is located at a latitude of  N, and the Antarctic Circle is located at a latitude of  S....

; 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
Water cycle
The water cycle, also known as the hydrologic cycle or H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. Water can change states among liquid, vapor, and solid at various places in 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
Paleocene-Eocene Thermal Maximum
The most extreme change in Earth surface conditions during the Cenozoic Era began at the temporal boundary between the Paleocene and Eocene epochs . This event, the Paleocene–Eocene Thermal Maximum , was associated with rapid global...

, which was associated with one of the smaller mass extinctions. It has also been suggested to have caused the Triassic-Jurassic extinction event
Triassic-Jurassic extinction event
The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, , and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas a whole class and twenty percent of all marine families...

, during which 20% of all marine families went extinct. Furthermore, the Permian–Triassic extinction event has been suggested to have been caused by warming.

Clathrate gun hypothesis


Clathrates are composites in which a lattice of one substance forms a cage around another. Methane clathrate
Methane clathrate
Methane clathrate, also called methane hydrate, hydromethane, methane ice, "fire ice", natural gas hydrate or just gas hydrate, is a solid clathrate compound in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice...

s (in which water molecules are the cage) form on continental shelves
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

. 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
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 or a sudden drop in sea level or even earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

s. Methane is a much more powerful greenhouse
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 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
Isotope analysis
Isotope analysis is the identification of isotopic signature, the distribution of certain stable isotopes and chemical elements within chemical compounds. This can be applied to a food web to make it possible to draw direct inferences regarding diet, trophic level, and subsistence...

 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 event
Anoxic event
Oceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused...

s 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
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...

, Permian–Triassic and Triassic–Jurassic extinctions, as well as a number of lesser extinctions (such as the Ireviken
Ireviken event
The Ireviken event was a minor extinction event at the Llandovery/Wenlock boundary .The event is best recorded at Ireviken, Gotland, where over 50% of trilobite species go extinct; 80% of the global conodont species also become extinct in this interval.-Anatomy of the event:The event lasted around...

, Mulde
Mulde event
The Mulde event was a secundo-secundo event, and marked the second of three relatively minor mass extinctions during the Silurian period. It coincided with a global drop in sea level, and is closely followed by an excursion in geochemical isotopes...

, Lau
Lau event
The Lau event was the last of three relatively minor mass extinctions during the Silurian period, having a major effect on the conodont fauna...

, Toarcian
Toarcian turnover
The term Toarcian turnover, alternatively the Toarcian extinction, the Pliensbachian-Toarcian extinction, or the Early Jurassic extinction, refers to the wave of extinctions that marked the end of the Pliensbachian stage and the start of the Toarcian stage of the Early Jurassic period, c...

 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.

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 photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

ing plankton and deep-water sulfate-reducing bacteria
Sulfate-reducing bacteria
Sulfate-reducing bacteria are those bacteria and archaea that can obtain energy by oxidizing organic compounds or molecular hydrogen while reducing sulfate to hydrogen sulfide...

, causing massive emissions of hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 which poisoned life on both land and sea and severely weakened the ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...

, exposing much of the life that still remained to fatal levels of UV radiation.

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 late Devonian
Late Devonian extinction
The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...

 and Permian–Triassic extinctions.

A nearby nova, supernova or gamma ray burst



A nearby gamma ray burst
Gamma ray burst
Gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...

 (less than 6000 light years away) would be powerful enough to destroy the Earth's ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...

, leaving organisms vulnerable to ultraviolet radiation
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 from the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. Gamma ray bursts are fairly rare, occurring only a few times in a given galaxy per million years. A proposal that a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 or gamma ray burst had caused a mass extinction would also have to be backed up by astronomical evidence of such an explosion at the right place and time.

It has been suggested that a supernova or gamma ray burst caused the End-Ordovician
Ordovician-Silurian extinction events
The Ordovician–Silurian extinction event, or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440...

 extinction.

Plate tectonics


Movement of the continents into some configurations can cause or contribute to extinctions in several ways: by initiating or ending ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

s; 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
Meridiungulata
Meridiungulata is an extinct clade with the rank of cohort or super-order, containing the South-American ungulates: Pyrotheria , Astrapotheria, Notoungulata and Litopterna...

 and all of its large metatherians
Sparassodonta
Sparassodonta is an extinct order of carnivorous metatherian mammals native to South America. They were once considered to be true marsupials, but are now thought to be a sister taxon to them. A number of these mammalian predators closely resemble placental predators that evolved separately on...

 after the creation of a land bridge between North and South America
Great American Interchange
The Great American Interchange was an important paleozoogeographic event in which land and freshwater fauna migrated from North America via Central America to South America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor and bridged the formerly separated continents...

). 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
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

 (the most species-rich part of the ocean) and produce a vast, arid continental interior which may have extreme seasonal variations.

It is widely thought that the creation of the super-continent Pangaea
Pangaea
Pangaea, Pangæa, or Pangea is hypothesized as a supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration....

 contributed to the End-Permian
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...

 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 other hypotheses have been proposed, such as the spread of a new disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

, 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.

Effects


The impact of mass extinction events varied widely. The worst event, the Permian–Triassic extinction event, devastated life on earth and is estimated to have killed off over 90% of species. Life on Earth seemed to recover quickly after this extinction, but this was mostly in the form of disaster taxa
Pioneer organism
A pioneer organism is an organism that populates a region after a natural disaster or any other event that may have caused most life in that area to disappear. Common pioneer organisms include lichens and algae. Mosses usually follow lichens in colonization, but cannot serve as pioneer organisms...

, such as the hardy Lystrosaurus
Lystrosaurus
Lystrosaurus was a genus of Late Permian and Early Triassic Period dicynodont therapsids, which lived around 250 million years ago in what is now Antarctica, India, and South Africa...

. The most recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover. It is thought that this long recovery was due to the successive waves of extinction which inhibited recovery, as well as to prolonged environmental stress to organisms which continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction; and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.

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.

Recovery


After a major extinction event, new DNA appear (Hardy–Weinberg principle), species diversify and occupy empty niches.

See also


  • Background extinction rate
    Background extinction rate
    Background extinction rate, also known as ‘normal extinction rate’, refers to the standard rate of extinction in earth’s geological and biological history before humans became a primary contributor to extinctions...

  • Olson's Extinction
    Olson's Extinction
    Olson's Extinction was a mass extinction that occurred in the Early Guadalupian of the Permian period and which predated the Permian–Triassic extinction event.- Extinction Patterns :...

  • Doomsday event
    Doomsday event
    A doomsday event is a specific, plausibly verifiable or hypothetical occurrence which has an exceptionally destructive effect on the human race...

  • Elvis taxon
    Elvis taxon
    In paleontology, an Elvis taxon is a taxon which has been misidentified as having re-emerged in the fossil record after a period of presumed extinction, but is not actually a descendant of the original taxon, instead having developed a similar morphology through convergent evolution...

  • Endangered species
    Endangered species
    An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...

  • Impact event
    Impact event
    An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

  • Lazarus taxon
    Lazarus taxon
    In paleontology, a Lazarus taxon is a taxon that disappears from one or more periods of the fossil record, only to appear again later. The term refers to the account in the Gospel of John, in which Jesus raised Lazarus from the dead...

  • Great dying
  • Medea hypothesis
    Medea Hypothesis
    The Medea Hypothesis is a term coined by paleontologist Peter Ward for the anti-Gaian hypothesis that multicellular life, understood as a superorganism, is suicidal; in this view microbial-triggered mass extinctions are attempts to return the Earth to the microbial dominated state it has been for...

  • Middle Miocene disruption
    Middle Miocene disruption
    The term Middle Miocene disruption, alternatively the Middle Miocene extinction or Middle Miocene extinction peak, refers to a wave of extinctions of terrestrial and aquatic life forms that occurred around the middle of the Miocene Epoch, c...

  • Rare species
    Rare species
    A rare species is a group of organisms that are very uncommon or scarce. This designation may be applied to either a plant or animal taxon, and may be distinct from the term "endangered" or "threatened species" but not "extinct"....

  • Signor–Lipps effect
  • Snowball Earth
    Snowball Earth
    The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...

  • Timeline of extinctions
    Timeline of extinctions
    The timeline of extinctions is an historical account of species that have gone extinct during the time that modern humans have occupied the earth....

  • Geologic time scale
    Geologic time scale
    The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...



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