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{{annotated image/Extinction|caption=Comparison of the three episodes of extinction in the Late Devonian ("Late D") to other mass extinction events in [[History of Earth|Earth's history]]. Plotted is the extinction intensity, calculated from marine [[genus|genera]].}}
{{annotated image/Extinction|caption=Comparison of the three episodes of extinction in the Late Devonian ("Late D") to other mass extinction events in [[History of Earth|Earth's history]]. Plotted is the extinction intensity, calculated from marine [[genus|genera]].}}


The '''Late Devonian extinction''' was one of five major [[extinction event]]s in the history of the Earth's [[biota (ecology)|biota]]. A major extinction occurred at the boundary that marks the beginning of the last phase of the [[Devonian]] period, the [[Famennian]] [[faunal stage]], (the Frasnian-Famennian boundary), about 364 million years ago, when nearly all of the fossil [[agnatha]]n fishes suddenly disappeared. A second strong pulse closed the Devonian period. Overall, 19% of all families and 50% of all genera went extinct.<ref name="ucr">{{cite web|url= http://math.ucr.edu/home/baez/extinction|title= extinction}}</ref>
The '''Late Devonian extinction''' was one of five major [[extinction event]]s in the history of the Earth's [[biota (ecology)|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]], (the Frasnian-Famennian boundary), about 374 million years ago <ref>Racki, 2005</ref><ref>McGhee, 1996</ref>. Overall, 19% of all families and 50% of all genera went extinct.<ref name="ucr">{{cite web|url= http://math.ucr.edu/home/baez/extinction|title= extinction}}</ref>A second, distinct mass extinction - the [[Hangenberg Event]] - closed the Devonian period <ref>Caplan and Bustin, 1999</ref>


Although it is clear that there was a massive loss of biodiversity towards the end of the Devonian, the extent of time during which these events took place is uncertain, with estimates ranging from 500,000 to 15 million years, the latter being the full length of the Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a series of smaller extinctions, though the latest research suggests multiple causes and a series of distinct extinction pulses through an interval of some three million years.<ref>[http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_61972.htm Racki, Grzegorz, "Toward understanding of Late Devonian global evants: few answers, many question" GSA Annual meeting, Seattle 2003 (abstract)]; McGhee 1996.</ref> Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, with notable extinctions at the ends of the [[Givetian]], [[Frasnian]], and [[Famennian]] stages.<ref name="sole">Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record - Volume Two, '''The earth system: biological and ecological dimensions of global environment change'''" pp. 297-391, ''Encyclopedia of Global Environmental Change'' John Wiley & Sons.</ref>
Although it is clear that there was a massive loss of biodiversity in the Later Devonian, the extent of time during which these events took place is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian <ref>Stigall, 2011</ref>. Nor is it clear whether it concerned two sharp mass extinctions or a series of smaller extinctions, though the latest research suggests multiple causes and a series of distinct extinction pulses through an interval of some three million years.<ref>[http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_61972.htm Racki, Grzegorz, "Toward understanding of Late Devonian global evants: few answers, many question" GSA Annual meeting, Seattle 2003 (abstract)]; McGhee 1996.</ref> Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, with notable extinctions at the ends of the [[Givetian]], [[Frasnian]], and [[Famennian]] stages.<ref name="sole">Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record - Volume Two, '''The earth system: biological and ecological dimensions of global environment change'''" pp. 297-391, ''Encyclopedia of Global Environmental Change'' John Wiley & Sons.</ref>


By the late Devonian, the land had been colonized by [[plants]], [[insects]], and [[amphibians]]. In the oceans, there were massive [[reef]]s built by corals and [[Stromatoporoidea|stromatoporoids]] on land. [[Euramerica]] and [[Gondwana]] were beginning to converge into what would become [[Pangea]]. The extinction seems to have only affected marine life. Hard-hit groups include [[brachiopod]]s, [[trilobite]]s, and reef-building [[organism]]s; the latter almost completely disappeared, with coral reefs only returning upon the evolution of modern corals during the [[Mesozoic]].<ref name="ucr"/> The causes of these extinctions are unclear. Leading theories include changes in sea level and ocean [[anoxic event|anoxia]], possibly triggered by global cooling or oceanic volcanism. The impact of a [[comet]] or another extraterrestrial body has also been suggested.<ref>Sole, R. V., and Newman, M. [http://www.santafe.edu/media/workingpapers/99-12-079.pdf Patterns of extinction and biodiversity in the fossil record]</ref> Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in [[speciation]] than by an increase in extinctions.<ref>{{Cite journal | last=Bambach | first=R.K. | last2=Knoll | first2=A.H. | last3=Wang | first3=S.C. | title=Origination, extinction, and mass depletions of marine diversity | journal=Paleobiology | volume=30 | issue=4 | pages=522–542 | date=December 2004 | url=http://www.bioone.org/perlserv/?request=get-document&issn=0094-8373&volume=30&page=522 | year=2004 | doi=10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2 }}</ref>
By the late Devonian, the land had been colonized by [[plants]], [[insects]], and [[amphibians]]. In the oceans, there were massive [[reef]]s built by corals and [[Stromatoporoidea|stromatoporoids]] on land. [[Euramerica]] and [[Gondwana]] were beginning to converge into what would become [[Pangea]]. The extinction seems to have only affected marine life. Hard-hit groups include [[brachiopod]]s, [[trilobite]]s, and reef-building [[organism]]s; the latter almost completely disappeared, with coral reefs only returning upon the evolution of modern corals during the [[Mesozoic]].<ref name="ucr"/> The causes of these extinctions are unclear. Leading theories include changes in sea level and ocean [[anoxic event|anoxia]], possibly triggered by global cooling or oceanic volcanism. The impact of a [[comet]] or another extraterrestrial body has also been suggested.<ref>Sole, R. V., and Newman, M. [http://www.santafe.edu/media/workingpapers/99-12-079.pdf Patterns of extinction and biodiversity in the fossil record]</ref> Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in [[speciation]] than by an increase in extinctions.<ref>{{Cite journal | last=Bambach | first=R.K. | last2=Knoll | first2=A.H. | last3=Wang | first3=S.C. | title=Origination, extinction, and mass depletions of marine diversity | journal=Paleobiology | volume=30 | issue=4 | pages=522–542 | date=December 2004 | url=http://www.bioone.org/perlserv/?request=get-document&issn=0094-8373&volume=30&page=522 | year=2004 | doi=10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2 }}</ref><ref>Stigall, 2011</ref>. This might have been caused by invasions of cosmopolitan species, rather than any single event<ref>Stigall, 2011</ref>. Surprisingly, jawed vertebrates seem to have been unaffected by the loss of reefs or other aspects of the Kellwasser event, while agnathans were in decline long before the end of the Frasnian<ref>Sallan and Coates, 2010</ref>.


==The Late Devonian world==
==The Late Devonian world==
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==Duration and timing of the extinction events==
==Duration and timing of the extinction events==
{{Expand section|date=February 2008}}
{{Expand section|date=February 2008}}
Extinction rates appear to be higher than the background rate for an extended period lasting the last 20-25 million years of the Devonian. During this period, about eight to ten distinct events can be seen, of which two stand out as particularly severe.<ref name=Algeo2001>{{cite book|author=Algeo, T.J., S.E. Scheckler and J. B. Maynard|chapter=Effects of the Middle to Late Devonian spread of vascular land plants on weathering regimes, marine biota, and global climate|pages=13–236|editor=P.G. Gensel and D. Edwards|year= 2001|title=Plants Invade the Land: Evolutionary and Environmental Approaches|publisher=Columbia Univ. Press: New York.}}</ref> Each of these two major events was preceded by a longer period of prolonged biodiversity loss.<ref name=Streel2000>{{cite journal
Extinction rates appear to be higher than the background rate for an extended period lasting the last 20-25 million years of the Devonian. During this period, about eight to ten distinct events can be seen, of which two stand out as particularly severe.<ref name=Algeo2001>{{cite book|author=Algeo, T.J., S.E. Scheckler and J. B. Maynard|chapter=Effects of the Middle to Late Devonian spread of vascular land plants on weathering regimes, marine biota, and global climate|pages=13–236|editor=P.G. Gensel and D. Edwards|year= 2001|title=Plants Invade the Land: Evolutionary and Environmental Approaches|publisher=Columbia Univ. Press: New York.}}</ref> The Kellwasser event was preceded by a longer period of prolonged biodiversity loss.<ref name=Streel2000>{{cite journal
| author = Streel, M.
| author = Streel, M.
| coauthors = Caputo, M.V.; Loboziak, S.; Melo, J.H.G.
| coauthors = Caputo, M.V.; Loboziak, S.; Melo, J.H.G.
Line 26: Line 26:
| doi=10.1016/S0012-8252(00)00026-X
| doi=10.1016/S0012-8252(00)00026-X
| pages = 121–173
| pages = 121–173
}}</ref> The fossil record of following first 15 million years of the [[Carboniferous]] is largely void of terrestrial animal fossils. This period is known as the [[Romer's gap]].<ref name=Ward>Ward, P. et al. (2006): Confirmation of Romer's Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization. ''[[Proceedings of the National Academy of Science]]'' no 103 (45): pp 16818-16822.</ref>
}}</ref> The fossil record of following first 15 million years of the [[Carboniferous]] is largely void of terrestrial animal fossils, likely related to losses during the end-Devonian Hangenberg event. This period is known as the [[Romer's gap]].<ref name=Ward>Ward, P. et al. (2006): Confirmation of Romer's Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization. ''[[Proceedings of the National Academy of Science]]'' no 103 (45): pp 16818-16822.</ref><ref>Sallan and Coates, 2010</ref>


===The Kellwasser event===
===The Kellwasser event===
The Kellwasser event is the term given to the extinction pulse that occurs near the Frasnian/Famennian boundary. There may in fact have been two closely spaced events here.
The Kellwasser event is the term given to the extinction pulse that occurs near the Frasnian/Famennian boundary. Most references to the "Late Devonian extinction" are in fact referring to the Kellwasser, which was the first event to be detected based on marine invertebrate record. There may in fact have been two closely spaced events here as shown by the presence of two distinct anoxic shale layers.


===The Hangenberg event===
===The Hangenberg event===
The [[Hangenberg event]] sits on or just below the Devonian/Carboniferous boundary and marks the final spike in the period of extinction.
The [[Hangenberg event]] sits on or just below the Devonian/Carboniferous boundary and marks the final spike in the period of extinction. It is marked by an anoxic black shale layer and an overlaying sandstone deposit<ref>Brezinski et al. 2009</ref>. Unlike the Kellwasser event, the Hangenberg event affected marine and terrestrial habitats <ref>Sallan and Coates, 2010</ref>


==Effects of the events==
==Effects of the events==
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===Biological impact===
===Biological impact===
The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms over cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the [[stromatoporoid]]s, and the [[rugosa|rugose]] and [[tabulate]] [[coral]]s. Reefs of the later Devonian were dominated by sponges and calcifying bacteria, producing structures such as [[oncolite]]s and [[stromatolites]]; the reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern [[scleractinia|scleractinian or "stony" corals]]) did not recover until the Mesozoic era;
The Kellwasser event and most other Later Devonian pulses primarily affected the marine community, and selectively affected shallow warm-water organisms over cool-water organisms. The most important group to be affected by the Kellwasser event were the reef-builders of the great Devonian reef-systems, including the [[stromatoporoid]]s, and the [[rugosa|rugose]] and [[tabulate]] [[coral]]s. Reefs of the later Devonian were dominated by sponges and calcifying bacteria, producing structures such as [[oncolite]]s and [[stromatolites]]; the reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern [[scleractinia|scleractinian or "stony" corals]]) did not recover until the Mesozoic era;
⚫
Further taxa to be severely affected include the [[brachiopod]]s, [[trilobite]]s, [[ammonite]]s, [[conodont]]s, and [[acritarch]]s. The surviving taxa show morphological trends through the event.

⚫
Trilobites evolve smaller eyes in the run up to the Kellwasser event, with eye size increasing again afterwards. This suggests that vision was less important around the event, perhaps due to increasing water depth or turbidity. The brims of trilobites (i.e. the rims of their heads) also expanded across this period. It is thought that the brims serve a respiratory purpose, and that the increasing [[anoxic event|anoxia]] of waters led to an increase in their brim area in response. The shape of conodonts' feeding apparatus varied with [[Δ18O|δ<sup>18</sup>O]] and thus seawater temperature; this may relate to them occupying different [[trophic level]]s as nutrient input changed.<ref>{{cite journal
⚫
Further taxa to be severely affected include the [[brachiopod]]s, [[trilobite]]s, [[ammonite]]s, [[conodont]]s, and [[acritarch]]s, as well as [[Agnatha|jawless fish]], and all [[placoderm]]s. Freshwater species, including our [[tetrapod]] ancestors, and land plants were relatively unscathed.

The surviving taxa show morphological trends through the event.
⚫
Trilobites evolve smaller eyes in the run up to the Kellwasser event, with eye size increasing again afterwards. This suggests that vision was less important around the event, perhaps due to increasing water depth or turbidity. The brims of trilobites (i.e. the rims of their heads) also expanded across this period. It is thought that the brims serve a respiratory purpose, and that the increasing [[anoxic event|anoxia]] of waters led to an increase in their brim area in response.

The shape of conodonts' feeding apparatus varied with [[Δ18O|δ<sup>18</sup>O]] and thus seawater temperature; this may relate to them occupying different [[trophic level]]s as nutrient input changed.<ref>{{cite journal
| title= Record of climate-driven morphological changes in 376 Ma Devonian fossils
| title= Record of climate-driven morphological changes in 376 Ma Devonian fossils
| author = Balter, Vincent| year = 2008 |journal = Geology |month=November | doi = 10.1130/G24989A.1
| author = Balter, Vincent| year = 2008 |journal = Geology |month=November | doi = 10.1130/G24989A.1
Line 55: Line 50:
| first3= Catherine
| first3= Catherine
| last4= Joachimski
| last4= Joachimski
⚫
| first4= Michael M.}}</ref> As with most extinction events, specialist taxa occupying small niches were harder hit than generalists.<ref>McGhee, George R., Jr, 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis (Columbia University Press) ISBN 0231075049</ref>
| first4= Michael M.}}</ref>

The Hangenberg event impacted both marine and freshwater communities. This mass extinction impacted [[ammonite]]s and [[trilobite]]s, as well as jawed vertebrates including our [[tetrapod]] ancestors <ref>Korn, 2004</ref><ref>Sallan and Coates, 2010</ref>. The Hangenberg is linked to the extinction of 44% of high-level vertebrate [[clade]]s, including all placoderms and most [[sarcopterygian]]s, and the complete turnover of the vertebrate biota<ref>Sallan and Coates, 2010</ref>. This lead to the establishment of the modern vertebrate fauna, consisting mostly of actinopterygians, chondrichthyans, and tetrapods, in the [[Carboniferous]]. [[Romers Gap]], a 15 million year hiatus in the early Carboniferous tetrapod record, has been linked to this event <ref>Sallan and Coates, 2010</ref>. It is also likely that some of the losses attributed to the Kellwasser event actually occurred during the Hangenberg extinction, due to the poor Famennian record for marine invertebrates<ref>Foote, 2005</ref><ref>Sallan and Coates, 2010</ref>.



⚫
As with all extinction events, specialist taxa occupying small niches were harder hit than generalists.<ref>McGhee, George R., Jr, 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis (Columbia University Press) ISBN 0231075049</ref>


===Magnitude===
===Magnitude===
The late Devonian crash in [[biodiversity]] was more drastic than the familiar [[K-T extinction event|extinction event]] that closed the [[Cretaceous]]: a recent survey (McGhee 1996) estimates that 22 percent of all the ''[[Family (biology)|families]]'' of marine animals (largely [[invertebrate]]s) were eliminated. The family is a large unit, and to lose such a large number signifies a profound loss of ecosystem diversity. On a smaller scale, 57% of genera and at least 75% of species<ref>The species estimate is the hardest to assess and most likely to be adjusted.</ref> did not survive into the [[Carboniferous]]. These latter estimates need to be treated with a degree of caution, as the estimates of species loss depend on surveys of Devonian marine taxa that are perhaps not well enough known to assess their true rate of losses, so it is difficult to estimate the effects of differential preservation and [[sampling bias]]es during the Devonian.
The late Devonian crash in [[biodiversity]] was more drastic than the familiar [[K-T extinction event|extinction event]] that closed the [[Cretaceous]]: a recent survey (McGhee 1996) estimates that 22 percent of all the ''[[Family (biology)|families]]'' of marine animals (largely [[invertebrate]]s) were eliminated. The family is a large unit, and to lose such a large number signifies a profound loss of ecosystem diversity. On a smaller scale, 57% of genera and at least 75% of species<ref>The species estimate is the hardest to assess and most likely to be adjusted.</ref> did not survive into the [[Carboniferous]]. These latter estimates need to be treated with a degree of caution, as the estimates of species loss depend on surveys of Devonian marine taxa that are perhaps not well enough known to assess their true rate of losses, so it is difficult to estimate the effects of differential preservation and [[sampling bias]]es during the Devonian.


==Causes of the extinction==
==Causes of the extinctions==
Since the "extinction" occurred over such a long time period, it is difficult to assign a single cause, and indeed to separate cause from effect. The sedimentological record shows that the late Devonian was a time of environmental change,<!--this needs to be more specific to transmit information--> which directly affected organisms and caused extinction. What caused these changes is somewhat more open to debate.
Since the Kellwasser-related "extinctions" occurred over such a long time period, it is difficult to assign a single cause, and indeed to separate cause from effect. The sedimentological record shows that the late Devonian was a time of environmental change,<!--this needs to be more specific to transmit information--> which directly affected organisms and caused extinction. What caused these changes is somewhat more open to debate.


===Major environmental changes===
===Major environmental changes===
From the end of the Middle Devonian, into the Late Devonian, several environmental changes can be detected from the sedimentary record. There is evidence of widespread [[anoxic event|anoxia]] in oceanic bottom waters;<ref name=Algeo1998/> the rate of carbon burial shot up,<ref name=Algeo1998/> and {{wict|benthic}} organisms were decimated, especially in the tropics, and especially reef communities.<ref name=Algeo1998/> There is good evidence for high-frequency sea level changes around the Frasnian/Famennian boundary, with one [[sea level rise]] associated with the onset of anoxic deposits.<ref name=Bond2008>{{cite journal|doi=10.1016/j.palaeo.2008.02.015 |title=The role of sea-level change and marine anoxia in the Frasnian-Famennian (Late Devonian) mass extinction|year=2008|author=David P.G. Bond, Paul B. Wignalla |volume=In press |journal=Palaeogeography Palaeoclimatology Palaeoecology |pages=107}}</ref>
From the end of the Middle Devonian, into the Late Devonian, several environmental changes can be detected from the sedimentary record. There is evidence of widespread [[anoxic event|anoxia]] in oceanic bottom waters;<ref name=Algeo1998/> the rate of carbon burial shot up,<ref name=Algeo1998/> and {{wict|benthic}} organisms were decimated, especially in the tropics, and especially reef communities.<ref name=Algeo1998/> There is good evidence for high-frequency sea level changes around the Frasnian/Famennian Kellwasser event, with one [[sea level rise]] associated with the onset of anoxic deposits.<ref name=Bond2008>{{cite journal|doi=10.1016/j.palaeo.2008.02.015 |title=The role of sea-level change and marine anoxia in the Frasnian-Famennian (Late Devonian) mass extinction|year=2008|author=David P.G. Bond, Paul B. Wignalla |volume=In press |journal=Palaeogeography Palaeoclimatology Palaeoecology |pages=107}}</ref>. The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall<ref>Brezinski et al. 2009</ref>; <ref>Algeo et al., 2008</ref>.


===Possible triggers===
===Possible triggers===
====Bolide impact====
====Bolide impact====
[[Bolide]] impacts can be dramatic triggers of mass extinctions. It has been posited that an asteroid impact was the prime cause of this faunal turnover,<ref>Digby McLaren, 1969; McGhee (1996)</ref> but no secure evidence of a specific extraterrestrial impact has been identified in this case. Impact craters, such as the [[Alamo bolide impact|Alamo]] and [[Woodleigh crater|Woodleigh]], can generally not be dated with sufficient precision to link them to the event; those dated precisely are not contemporaneous with the extinction.{{Verify source|date=December 2009}} Although some minor features of meteoric impact have been observed in places (iridium anomalies and microspherules), these were probably caused by other factors.<ref name=Algeo1995/>
[[Bolide]] impacts can be dramatic triggers of mass extinctions. It has been posited that an asteroid impact was the prime cause of this faunal turnover,<ref>Digby McLaren, 1969; McGhee (1996)</ref> but no secure evidence of a specific extraterrestrial impact has been identified in this case. Impact craters, such as the Kellwasser=aged [[Alamo bolide impact|Alamo]] and the Hangenberg-aged [[Woodleigh crater|Woodleigh]], can generally not be dated with sufficient precision to link them to the event; others dated precisely are not contemporaneous with the extinction <ref>Racki, 2005</ref> Although some minor features of meteoric impact have been observed in places (iridium anomalies and microspherules), these were probably caused by other factors.<ref name=Algeo1995/>


====Plant evolution====
====Plant evolution====
Line 99: Line 96:


=====Effect on CO<sub>2</sub>=====
=====Effect on CO<sub>2</sub>=====
The "greening" of the continents occurred during Devonian time. The covering of the planet's continents with massive photosynthesizing land plants in the first forests may have reduced carbon dioxide levels in the atmosphere. Since {{co2}} is a greenhouse gas, reduced levels might have helped produce a chillier climate. Evidence such as glacial deposits in northern Brazil (located near the Devonian south pole) suggest widespread glaciation at the end of the Devonian, as a large continental mass covered the polar region. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period.
The "greening" of the continents occurred during Devonian time. The covering of the planet's continents with massive photosynthesizing land plants in the first forests may have reduced carbon dioxide levels in the atmosphere. Since {{co2}} is a greenhouse gas, reduced levels might have helped produce a chillier climate. Evidence such as glacial deposits in northern Brazil (located near the Devonian south pole) suggest widespread glaciation at the end-Devonian, as a large continental mass covered the polar region. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. The Hangenberg event has been also been linked to glaciation in the tropics equivalent to that of the [[Pleistocene]] [[ice age]] <ref> Brezinski et al., 2009</ref>


The weathering of silicate rocks also draws down carbon dioxide from the atmosphere. This acted in concert with the burial of organic matter to decrease atmospheric carbon dioxide concentrations from ~15 to ~3 times present levels.
The weathering of silicate rocks also draws down carbon dioxide from the atmosphere. This acted in concert with the burial of organic matter to decrease atmospheric carbon dioxide concentrations from ~15 to ~3 times present levels.

Revision as of 15:39, 6 January 2011

CambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
Marine extinction intensity during Phanerozoic
%
Millions of years ago
CambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
Comparison of the three episodes of extinction in the Late Devonian ("Late D") to other mass extinction events in Earth's history. Plotted is the extinction intensity, calculated from marine genera.

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, (the Frasnian-Famennian boundary), about 374 million years ago [1][2]. Overall, 19% of all families and 50% of all genera went extinct.[3]A second, distinct mass extinction - the Hangenberg Event - closed the Devonian period [4]

Although it is clear that there was a massive loss of biodiversity in the Later Devonian, the extent of time during which these events took place is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian [5]. Nor is it clear whether it concerned two sharp mass extinctions or a series of smaller extinctions, though the latest research suggests multiple causes and a series of distinct extinction pulses through an interval of some three million years.[6] Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, with notable extinctions at the ends of the Givetian, Frasnian, and Famennian stages.[7]

By the late Devonian, the land had been colonized by plants, insects, and amphibians. In the oceans, there were massive reefs built by corals and stromatoporoids on land. Euramerica and Gondwana were beginning to converge into what would become Pangea. The extinction seems to have only affected marine life. Hard-hit groups include brachiopods, trilobites, and reef-building organisms; the latter almost completely disappeared, with coral reefs only returning upon the evolution of modern corals during the Mesozoic.[3] The causes of these extinctions are unclear. Leading theories include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested.[8] Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.[9][10]. This might have been caused by invasions of cosmopolitan species, rather than any single event[11]. Surprisingly, jawed vertebrates seem to have been unaffected by the loss of reefs or other aspects of the Kellwasser event, while agnathans were in decline long before the end of the Frasnian[12].

The Late Devonian world

The world was a very different place in the late Devonian. The continents were arranged differently, with a supercontinent, Gondwana, covering much of the southern hemisphere. The continent of Siberia occupied the northern hemisphere, while an equatorial continent, Laurussia (formed by the collision of Baltica and Laurentia) was drifting towards Gondwana. The Caledonian mountains were also growing across what is now the Scottish highlands and Scandinavia, while the Appalachians rose over America; these mountain belts were the equivalent of the Himalaya today.[verification needed]

The biota was also very different. Plants, which had been on land in forms similar to mosses, liverworts and lichens since the Ordovician, had just developed roots, seeds and water transport systems that allowed them to survive away from places that were constantly wet - and consequently formed huge forests on the highlands. Several different clades had developed a shrubby or tree-like habit by the Late Givetian, including the cladoxylalean ferns, lepidosigillarioid lycopsids, and aneurophyte and archaeopterid progymnosperms.[17] Fish were also undergoing a huge radiation, and the first tetrapods were beginning to evolve leg-like structures.[18]

Duration and timing of the extinction events

Extinction rates appear to be higher than the background rate for an extended period lasting the last 20-25 million years of the Devonian. During this period, about eight to ten distinct events can be seen, of which two stand out as particularly severe.[19] The Kellwasser event was preceded by a longer period of prolonged biodiversity loss.[20] The fossil record of following first 15 million years of the Carboniferous is largely void of terrestrial animal fossils, likely related to losses during the end-Devonian Hangenberg event. This period is known as the Romer's gap.[21][22]

The Kellwasser event

The Kellwasser event is the term given to the extinction pulse that occurs near the Frasnian/Famennian boundary. Most references to the "Late Devonian extinction" are in fact referring to the Kellwasser, which was the first event to be detected based on marine invertebrate record. There may in fact have been two closely spaced events here as shown by the presence of two distinct anoxic shale layers.

The Hangenberg event

The Hangenberg event sits on or just below the Devonian/Carboniferous boundary and marks the final spike in the period of extinction. It is marked by an anoxic black shale layer and an overlaying sandstone deposit[23]. Unlike the Kellwasser event, the Hangenberg event affected marine and terrestrial habitats [24]

Effects of the events

The extinction events are accompanied by widespread oceanic anoxia - that is, a lack of oxygen, prohibiting decay and allowing the preservation of organic matter. This, combined with the ability of porous reef rocks to hold oil, has led to Devonian rocks being an important source of oil, especially in the USA.

Biological impact

The Kellwasser event and most other Later Devonian pulses primarily affected the marine community, and selectively affected shallow warm-water organisms over cool-water organisms. The most important group to be affected by the Kellwasser event were the reef-builders of the great Devonian reef-systems, including the stromatoporoids, and the rugose and tabulate corals. Reefs of the later Devonian were dominated by sponges and calcifying bacteria, producing structures such as oncolites and stromatolites; the reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian or "stony" corals) did not recover until the Mesozoic era; Further taxa to be severely affected include the brachiopods, trilobites, ammonites, conodonts, and acritarchs. The surviving taxa show morphological trends through the event. Trilobites evolve smaller eyes in the run up to the Kellwasser event, with eye size increasing again afterwards. This suggests that vision was less important around the event, perhaps due to increasing water depth or turbidity. The brims of trilobites (i.e. the rims of their heads) also expanded across this period. It is thought that the brims serve a respiratory purpose, and that the increasing anoxia of waters led to an increase in their brim area in response. The shape of conodonts' feeding apparatus varied with δ18O and thus seawater temperature; this may relate to them occupying different trophic levels as nutrient input changed.[25] As with most extinction events, specialist taxa occupying small niches were harder hit than generalists.[26]

The Hangenberg event impacted both marine and freshwater communities. This mass extinction impacted ammonites and trilobites, as well as jawed vertebrates including our tetrapod ancestors [27][28]. The Hangenberg is linked to the extinction of 44% of high-level vertebrate clades, including all placoderms and most sarcopterygians, and the complete turnover of the vertebrate biota[29]. This lead to the establishment of the modern vertebrate fauna, consisting mostly of actinopterygians, chondrichthyans, and tetrapods, in the Carboniferous. Romers Gap, a 15 million year hiatus in the early Carboniferous tetrapod record, has been linked to this event [30]. It is also likely that some of the losses attributed to the Kellwasser event actually occurred during the Hangenberg extinction, due to the poor Famennian record for marine invertebrates[31][32].


Magnitude

The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous: a recent survey (McGhee 1996) estimates that 22 percent of all the families of marine animals (largely invertebrates) were eliminated. The family is a large unit, and to lose such a large number signifies a profound loss of ecosystem diversity. On a smaller scale, 57% of genera and at least 75% of species[33] did not survive into the Carboniferous. These latter estimates need to be treated with a degree of caution, as the estimates of species loss depend on surveys of Devonian marine taxa that are perhaps not well enough known to assess their true rate of losses, so it is difficult to estimate the effects of differential preservation and sampling biases during the Devonian.

Causes of the extinctions

Since the Kellwasser-related "extinctions" occurred over such a long time period, it is difficult to assign a single cause, and indeed to separate cause from effect. The sedimentological record shows that the late Devonian was a time of environmental change, which directly affected organisms and caused extinction. What caused these changes is somewhat more open to debate.

Major environmental changes

From the end of the Middle Devonian, into the Late Devonian, several environmental changes can be detected from the sedimentary record. There is evidence of widespread anoxia in oceanic bottom waters;[17] the rate of carbon burial shot up,[17] and Template:Wict organisms were decimated, especially in the tropics, and especially reef communities.[17] There is good evidence for high-frequency sea level changes around the Frasnian/Famennian Kellwasser event, with one sea level rise associated with the onset of anoxic deposits.[34]. The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall[35]; [36].

Possible triggers

Bolide impact

Bolide impacts can be dramatic triggers of mass extinctions. It has been posited that an asteroid impact was the prime cause of this faunal turnover,[37] but no secure evidence of a specific extraterrestrial impact has been identified in this case. Impact craters, such as the Kellwasser=aged Alamo and the Hangenberg-aged Woodleigh, can generally not be dated with sufficient precision to link them to the event; others dated precisely are not contemporaneous with the extinction [38] Although some minor features of meteoric impact have been observed in places (iridium anomalies and microspherules), these were probably caused by other factors.[39]

Plant evolution

During the Devonian, land plants underwent a hugely significant phase of evolution. Their maximum height went from 30 cm[citation needed] at the start of the Devonian, to 30 m[40] at the end of the period. This increase in height was made possible by the evolution of advanced vascular systems, which permitted the growth of complex branching and rooting systems.[17] In conjunction with this, the development of seeds permitted reproduction and dispersal in areas which were not waterlogged, allowing plants to colonise previously inhospitable inland and upland areas.[17] The two factors combined to greatly magnify the role of plants on the global scale. In particular, Archaeopteris forests expanded rapidly during the closing stages of the Devonian.

Effect on weathering

These tall trees required deep rooting systems to acquire water and nutrients, and provide anchorage. These systems broke up the upper layers of bedrock and stabilised a deep layer of soil, which would have been on the order of metres thick. In contrast, early Devonian plants bore only rhizoids and rhizomes that could penetrate no more than a couple of centimetres. The mobilisation of a large portion of soil had a huge effect; soil promotes weathering, the chemical breakdown of rocks, releasing ions which act as nutrients to plants and algae.[17] The relatively sudden input of nutrients into river water may have caused eutrophication and subsequent anoxia. For example, during an algal bloom, organic material formed at the surface can sink at such a rate that decomposing organisms use up all available oxygen by decaying them, creating anoxic conditions and suffocating bottom-dwelling fish. The fossil reefs of the Frasnian were dominated by stromatolites and (to a lesser degree) corals - organisms which only thrive in low nutrient conditions. Therefore the postulated influx of high levels of nutrients may have caused an extinction, just as phosphate run-off from Australian farmers is causing unmeasurable damage to the great barrier reef today.[17] Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting that anoxia may have played the dominant role in extinction.[39]

Effect on CO2

The "greening" of the continents occurred during Devonian time. The covering of the planet's continents with massive photosynthesizing land plants in the first forests may have reduced carbon dioxide levels in the atmosphere. Since CO2 is a greenhouse gas, reduced levels might have helped produce a chillier climate. Evidence such as glacial deposits in northern Brazil (located near the Devonian south pole) suggest widespread glaciation at the end-Devonian, as a large continental mass covered the polar region. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. The Hangenberg event has been also been linked to glaciation in the tropics equivalent to that of the Pleistocene ice age [41]

The weathering of silicate rocks also draws down carbon dioxide from the atmosphere. This acted in concert with the burial of organic matter to decrease atmospheric carbon dioxide concentrations from ~15 to ~3 times present levels. Carbon in the form of plant matter would be produced on prodigious scales, and given the right conditions could be stored and buried, eventually producing vast coal measures (e.g. in China) which locked the carbon out of the atmosphere and into the lithosphere.[42] This reduction in atmospheric CO2 would have caused global cooling and resulted in at least one period of late Devonian glaciation (and subsequent sea level fall),[43] probably fluctuating in intensity alongside the 40ka Milankovic cycle. The continued drawdown of organic carbon eventually pulled the Earth out of its Greenhouse Earth state into the Icehouse that continued throughout the Carboniferous and Permian.

Other suggestions

Other mechanisms that have been put forwards to explain the extinctions include tectonic driven climate change; sea level change; and oceanic overturning. These have all been discounted because they are unable to explain the duration, selectivity and periodicity of the extinctions.[39]

See also

References

  1. ^ Racki, 2005
  2. ^ McGhee, 1996
  3. ^ a b "extinction".
  4. ^ Caplan and Bustin, 1999
  5. ^ Stigall, 2011
  6. ^ Racki, Grzegorz, "Toward understanding of Late Devonian global evants: few answers, many question" GSA Annual meeting, Seattle 2003 (abstract); McGhee 1996.
  7. ^ Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record - Volume Two, The earth system: biological and ecological dimensions of global environment change" pp. 297-391, Encyclopedia of Global Environmental Change John Wiley & Sons.
  8. ^ Sole, R. V., and Newman, M. Patterns of extinction and biodiversity in the fossil record
  9. ^ Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004). "Origination, extinction, and mass depletions of marine diversity". Paleobiology. 30 (4): 522–542. doi:10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2.{{cite journal}}: CS1 maint: date and year (link)
  10. ^ Stigall, 2011
  11. ^ Stigall, 2011
  12. ^ Sallan and Coates, 2010
  13. ^ Parry, S. F.; Noble, S. R.; Crowley, Q. G.; Wellman, C. H. (2011). "A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications". Journal of the Geological Society. 168 (4). London: Geological Society: 863–872. doi:10.1144/0016-76492010-043.
  14. ^ Kaufmann, B.; Trapp, E.; Mezger, K. (2004). "The numerical age of the Upper Frasnian (Upper Devonian) Kellwasser horizons: A new U-Pb zircon date from Steinbruch Schmidt(Kellerwald, Germany)". The Journal of Geology. 112 (4): 495–501. Bibcode:2004JG....112..495K. doi:10.1086/421077.
  15. ^ Algeo, T. J. (1998). "Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events". Philosophical Transactions of the Royal Society B: Biological Sciences. 353 (1365): 113–130. doi:10.1098/rstb.1998.0195.
  16. ^ "Chart/Time Scale". www.stratigraphy.org. International Commission on Stratigraphy.
  17. ^ a b c d e f g h Algeo, T.J.; Scheckler, S. E. (1998). "Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events". Philosophical Transactions of the Royal Society B: Biological Sciences. 353 (1365): 113–130. doi:10.1098/rstb.1998.0195.
  18. ^ See Tiktaalik.
  19. ^ Algeo, T.J., S.E. Scheckler and J. B. Maynard (2001). "Effects of the Middle to Late Devonian spread of vascular land plants on weathering regimes, marine biota, and global climate". In P.G. Gensel and D. Edwards (ed.). Plants Invade the Land: Evolutionary and Environmental Approaches. Columbia Univ. Press: New York. pp. 13–236.{{cite book}}: CS1 maint: multiple names: authors list (link)
  20. ^ Streel, M. (2000). "Late Frasnian--Famennian climates based on palynomorph analyses and the question of the Late Devonian glaciations". Earth Science Reviews. 52 (1–3): 121–173. doi:10.1016/S0012-8252(00)00026-X. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  21. ^ Ward, P. et al. (2006): Confirmation of Romer's Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization. Proceedings of the National Academy of Science no 103 (45): pp 16818-16822.
  22. ^ Sallan and Coates, 2010
  23. ^ Brezinski et al. 2009
  24. ^ Sallan and Coates, 2010
  25. ^ Balter, Vincent; Renaud, Sabrina; Girard, Catherine; Joachimski, Michael M. (2008). "Record of climate-driven morphological changes in 376 Ma Devonian fossils". Geology. 36: 907. doi:10.1130/G24989A.1. {{cite journal}}: Unknown parameter |month= ignored (help)
  26. ^ McGhee, George R., Jr, 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis (Columbia University Press) ISBN 0231075049
  27. ^ Korn, 2004
  28. ^ Sallan and Coates, 2010
  29. ^ Sallan and Coates, 2010
  30. ^ Sallan and Coates, 2010
  31. ^ Foote, 2005
  32. ^ Sallan and Coates, 2010
  33. ^ The species estimate is the hardest to assess and most likely to be adjusted.
  34. ^ David P.G. Bond, Paul B. Wignalla (2008). "The role of sea-level change and marine anoxia in the Frasnian-Famennian (Late Devonian) mass extinction". Palaeogeography Palaeoclimatology Palaeoecology. In press: 107. doi:10.1016/j.palaeo.2008.02.015.
  35. ^ Brezinski et al. 2009
  36. ^ Algeo et al., 2008
  37. ^ Digby McLaren, 1969; McGhee (1996)
  38. ^ Racki, 2005
  39. ^ a b c Algeo, T.J. (1995). "Late Devonian Oceanic Anoxic Events and Biotic Crises: "Rooted" in the Evolution of Vascular Land Plants?". GSA Today. 5 (3). {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) [dead link]
  40. ^ Archaeopterids, see Beck (1981) in Algeo 1998
  41. ^ Brezinski et al., 2009
  42. ^ Carbon locked in Devonian coal, the earliest of Earth's coal deposits, is currently being returned to the atmosphere.
  43. ^ (Caputo 1985; Berner 1992, 1994) in Algeo 1998

Further reading

  • McGhee, George R., Jr, 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis (Columbia University Press) ISBN 0231075049
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