The Late Devonian extinction consisted of several extinction events in the
Late Devonian Epoch
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by ...
, which collectively represent one of the five largest
mass extinction events in the history of life on Earth. The term primarily refers to a major extinction, the Kellwasser event (also known as the Frasnian-Famennian extinction), which occurred around 372 million years ago, at the boundary between the
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage.
Major reef-building was under way during t ...
stage and the
Famennian
The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used ...
stage, the last
stage in the Devonian Period.
[Racki, 2005][McGhee, George R. Jr, 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis (Columbia University Press) ] Overall, 19% of all families and 50% of all genera became extinct.
A second mass extinction, the
Hangenberg event
The Hangenberg event, also known as the Hangenberg crisis or end-Devonian extinction, is a mass extinction that occurred at the end of the Famennian stage, the last stage in the Devonian Period (roughly 358.9 ± 0.4 million years ago). It is usu ...
(also known as the end-Devonian extinction), occurred 359 million years ago, bringing an end to the Famennian and Devonian, as the world transitioned into the
Carboniferous Period
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago (Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferous ...
.
Although it is well established that there was a massive
loss of biodiversity
Biodiversity loss includes the worldwide extinction of different species, as well as the local reduction or loss of species in a certain habitat, resulting in a loss of biological diversity. The latter phenomenon can be temporary or permanent, de ...
in the Late Devonian, the timespan of this event is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian. 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
The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian Stage. It is named after the town of Givet in ...
,
Frasnian
The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage.
Major reef-building was under way during t ...
, and
Famennian
The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used ...
stages.
[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.]
By the Late Devonian, the land had been colonized by
plants
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
and
insects
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs ...
. In the oceans, massive
reef
A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic component, abiotic processes—deposition (geology), deposition of ...
s were built by corals and
stromatoporoids.
Euramerica
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around (Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pa ...
and
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final st ...
were beginning to converge into what would become
Pangaea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 millio ...
. The extinction seems to have only affected
marine life
Marine life, sea life, or ocean life is the plants, animals and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the plane ...
. Hard-hit groups include
brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, ...
s,
trilobite
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...
s, and reef-building
organism
In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...
s; the latter almost completely disappeared. The causes of these extinctions are unclear. Leading hypotheses include changes in sea level and ocean
anoxia
The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:
* Anoxic waters, sea water, fresh water or groundwater that are depleted of di ...
, possibly triggered by
global cooling
Global cooling was a conjecture, especially during the 1970s, of imminent cooling of the Earth culminating in a period of extensive glaciation, due to the cooling effects of aerosols or orbital forcing.
Some press reports in the 1970s specul ...
or oceanic volcanism. The impact of a
comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or Coma (cometary), coma, and sometimes also a Comet ta ...
or another extraterrestrial body has also been suggested, such as the
Siljan Ring
The Siljan Ring ( sv, Siljansringen) is a prehistoric impact crater in Dalarna, central Sweden. It is one of the 15 largest known impact craters on Earth and the largest in Europe, with a diameter of about . The impact that created the Siljan Ring ...
event in Sweden. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in
speciation
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...
than by an increase in extinctions.
[Stigall, 2011] This might have been caused by invasions of cosmopolitan species, rather than by any single event.
Placoderms were hit hard by the Kellwasser event and completely died out in the Hangenberg event, but most other jawed vertebrates were less strongly impacted.
Agnatha
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present ( cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclos ...
ns (jawless fish) were in decline long before the end of the Frasnian and were nearly wiped out by the extinctions.
The extinction events were accompanied by widespread oceanic
anoxia
The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:
* Anoxic waters, sea water, fresh water or groundwater that are depleted of di ...
; 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 United States.
Late Devonian world
During the Late Devonian, the continents were arranged differently from today, with a supercontinent,
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final st ...
, covering much of the Southern Hemisphere. The
continent
A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...
of
Siberia
Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part o ...
occupied the Northern Hemisphere, while an equatorial continent,
Laurussia (formed by the collision of
Baltica
Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains.
The thick core of Baltica, the East European Craton, is ...
and
Laurentia
Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...
), was
drifting
Drifting may refer to:
*Drifting (motorsport)
*Pipe drift or drifting, measuring a pipe's inner roundness
Film
* ''Drifting'' (1923 film), a film directed by Tod Browning
* ''Drifting'' (1982 film), the first Israeli gay-themed film
* ''Drifting'' ...
towards Gondwana, closing the
Rheic Ocean. The
Caledonian mountains
The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that o ...
were also growing across what is now the
Scottish Highlands
The Highlands ( sco, the Hielands; gd, a’ Ghàidhealtachd , 'the place of the Gaels') is a historical region of Scotland. Culturally, the Highlands and the Lowlands diverged from the Late Middle Ages into the modern period, when Lowland ...
and
Scandinavia
Scandinavia; Sámi languages: /. ( ) is a subregion in Northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. In English usage, ''Scandinavia'' most commonly refers to Denmark, Norway, and Swe ...
, while the
Appalachians
The Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. The ...
rose over America.
The biota was also very different. Plants, which had been on land in forms similar to mosses and liverworts since the
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...
, had just developed roots, seeds, and
water transport
Maritime transport (or ocean transport) and hydraulic effluvial transport, or more generally waterborne transport, is the transport of people ( passengers) or goods ( cargo) via waterways. Freight transport by sea has been widely used th ...
systems that allowed them to survive away from places that were constantly wet—and so grew huge forests on the highlands. Several clades had developed a shrubby or tree-like habitat by the Late Givetian, including the
cladoxylalean fern
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except ...
s,
lepidosigillarioid lycopsid
Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...
s, and
aneurophyte and
archaeopterid progymnosperm
The progymnosperms are an extinct group of woody, spore-bearing plants that is presumed to have evolved from the trimerophytes, and eventually gave rise to the gymnosperms, ancestral to acrogymnosperms and angiosperms (flowering plants). They ...
s.
Fish were also undergoing a huge radiation, and tetrapodomorphs, such as the Frasnian-age ''
Tiktaalik'', were beginning to evolve leg-like structures.
Extinction patterns
The Kellwasser event and most other Later Devonian pulses primarily affected the marine community, and had a greater effect on shallow warm-water organisms than on cool-water organisms. The Kellwasser event's effects were also stronger at low latitudes than high ones.
Reef destruction
The most hard-hit biological category affected by the Kellwasser event were the calcite-based reef-builders of the great Devonian reef-systems, including the
stromatoporoid
Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Ordovician through the Devonian. They were especially abundant and important reef-formers in the Silurian and most of the Devonian.Stock, C.W. 2001, Stro ...
sponges and the
rugose and
tabulate coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secre ...
s. After the Kellwasser event, reefs of the Famennian were primarily dominated by siliceous sponges and calcifying bacteria, producing structures such as
oncolites and
stromatolites
Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). ...
. The collapse of the reef system was so stark that it would take until the Mesozoic for reefs to recover their Middle Devonian extent. Mesozoic and modern reefs are based on
scleractinian ("stony") corals, which would not evolve until the Triassic period. Devonian reef-builders are entirely extinct in the modern day: Stromatoporoids died out in the end-Devonian Hangenberg event, while rugose and tabulate corals went extinct at the
Permian-Triassic extinction.
Marine invertebrates
Further taxa to be starkly affected include the
brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, ...
s,
trilobite
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...
s,
ammonite
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...
s,
conodont
Conodonts ( Greek ''kōnos'', " cone", + ''odont'', " tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, w ...
s,
acritarch
Acritarchs are organic microfossils, known from approximately 1800 million years ago to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity refl ...
and
graptolites.
Cystoids disappeared during this event. The surviving taxa show morphological trends through the event. Trilobites evolved smaller eyes in the run-up to the Kellwasser event, with eye size increasing again afterwards. This suggests 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. The brims are thought to have served a respiratory purpose, and the increasing anoxia of waters led to an increase in their brim area in response. The shape of conodonts' feeding apparatus varied with
the oxygen isotope ratio, and thus with the sea water temperature; this may relate to their occupying different
trophic level
The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it i ...
s as nutrient input changed.
As with most extinction events, specialist taxa occupying small niches were harder hit than generalists.
Vertebrates
Vertebrates were not strongly-affected by the Kellwasser event, but still experienced some diversity loss. Around half of placoderm families died out, primarily species-poor bottom-feeding groups. More diverse placoderm families survived the event only to succumb in the Hangenberg event at the end of the Devonian. Most lingering agnathan (jawless fish) groups, such as
osteostracans,
galeaspids
Galeaspida (from Latin, 'Helmet shields') is an extinct taxon of jawless marine and freshwater fish. The name is derived from ''galea'', the Latin word for ''helmet'', and refers to their massive bone shield on the head. Galeaspida lived in shal ...
, and
heterostracans
Heterostraci (Ancient Greek, ἕτερος+ὄστρακον "those itha different shell" i is pl. of -us is an extinct subclass of pteraspidomorph jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci exis ...
, also went extinct by the end of the Frasnian. The jawless
thelodonts only barely survived, succumbing early in the Famennian.
Among freshwater and shallow marine
tetrapodomorph fish, the tetrapod-like
elpistostegalians (such as ''
Tiktaalik'') disappeared at the Frasnian-Famennian boundary. True
tetrapods
Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...
(defined as four-limbed vertebrates with digits) apparently survived, though their
fossils
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
are rare until the mid-to-late Famennian.
The Hangenberg event was much more impactful on vertebrate evolution, affecting both marine and freshwater communities.
It is linked to the extinction of 44% of high-level vertebrate
clades, including all
placoderms and most
sarcopterygian
Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includ ...
s, and the complete turnover of the vertebrate biota.
Over 96% of vertebrate species disappeared, with only smaller forms surviving. After the event only sharks less than a meter and most fishes and tetrapods less than 10 centimeters remained, and it would take 40 million years before they started to increase in size again.
This led to the establishment of the modern vertebrate fauna in the
Carboniferous, consisting mostly of
actinopterygians,
chondrichthyans, and
tetrapod
Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...
s.
Romer's gap
Romer's gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist ...
, a 15 million-year hiatus in the early Carboniferous tetrapod record, has been linked to this event.
Also, the poor Famennian record for marine vertebrates suggests that some of the losses attributed to the Kellwasser event likely actually occurred during the Hangenberg extinction.
Magnitude of diversity loss
The late Devonian crash in
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic ('' genetic variability''), species ('' species diversity''), and ecosystem ('' ecosystem diversity' ...
was more drastic than the familiar
extinction event that closed the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
. A recent survey (McGhee 1996) estimates that 22% of all the '
families' of marine animals (largely
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s) were eliminated. The family is a great unit, and to lose so many signifies a deep loss of ecosystem diversity. On a smaller scale, 57% of genera and at least 75% of species did not survive into the Carboniferous. These latter estimates
[The species estimate is the toughest to assess and most likely to be adjusted.] 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
In statistics, sampling bias is a bias (statistics), bias in which a sample is collected in such a way that some members of the intended statistical population, population have a lower or higher sampling probability than others. It results in a bia ...
es during the Devonian.
Duration and timing
Extinction rates appear to have been higher than the background rate, for an extended interval covering the last 20–25 million years of the Devonian. During this time, about eight to ten distinct events can be seen, of which two stand out as particularly severe.
The Kellwasser event was preceded by a longer period of prolonged
biodiversity loss
Biodiversity loss includes the worldwide extinction of different species, as well as the local reduction or loss of species in a certain habitat, resulting in a loss of biological diversity. The latter phenomenon can be temporary or permanent, de ...
.
The fossil record of the first 15 million years of the
Carboniferous period which followed is largely void of terrestrial animal fossils, likely related to losses during the end-Devonian Hangenberg event. This period is known as
Romer's gap
Romer's gap is an example of an apparent gap in the tetrapod fossil record used in the study of evolutionary biology. Such gaps represent periods from which excavators have not yet found relevant fossils. Romer's gap is named after paleontologist ...
.
The Kellwasser event
The Kellwasser event, named for its
type locality
Type locality may refer to:
* Type locality (biology)
* Type locality (geology)
See also
* Local (disambiguation)
* Locality (disambiguation)
{{disambiguation ...
, the Kellwassertal in
Lower Saxony
Lower Saxony (german: Niedersachsen ; nds, Neddersassen; stq, Läichsaksen) is a German state (') in northwestern Germany. It is the second-largest state by land area, with , and fourth-largest in population (8 million in 2021) among the 16 ...
, Germany, is the term given to the extinction pulse that occurred near the Frasnian–Famennian boundary (372.2 ± 1.6 Ma). 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, also known as the Hangenberg crisis or end-Devonian extinction, is a mass extinction that occurred at the end of the Famennian stage, the last stage in the Devonian Period (roughly 358.9 ± 0.4 million years ago). It is usu ...
occurred at or shortly before the Devonian–Carboniferous boundary (358.9 ± 0.4 Ma) and marks the last spike in the period of extinction. It is marked by an anoxic
black shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
layer and an overlying sandstone deposit.
Unlike the Kellwasser event, the Hangenberg event affected both marine and terrestrial habitats.
Potential causes
Since the Kellwasser-related extinctions occurred over such a long time, it is difficult to assign a single cause, and indeed to separate cause from effect. The sedimentary 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.
From the end of the Middle Devonian (382.7 ± 1.6 Ma), into the Late Devonian (382.7 ± 1.6-358.9 ± 0.4 Ma), several environmental changes can be detected from the sedimentary record. Evidence exists of widespread anoxia in oceanic bottom waters;
[ the rate of carbon burial shot up,][ and ]benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
organisms were devastated, especially in the tropics, and especially reef communities.[ Good evidence has been found for high-frequency sea-level changes around the Frasnian–Famennian Kellwasser event, with one sea-level rise associated with the onset of anoxic deposits.] The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall.
Possible triggers for the Kellwasser event are as follows:
Weathering and cooling
During the Devonian, land plants underwent a hugely significant phase of evolution. Their maximum height went from 30 cm at the start of the Devonian, to archaeopterids, 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. In conjunction with this, the evolution of seeds permitted reproduction and dispersal in areas which were not waterlogged, allowing plants to colonise previously inhospitable inland and upland areas.[ The two factors combined to greatly magnify the role of plants on the global scale. In particular, '']Archaeopteris
''Archaeopteris'' is an extinct genus of progymnosperm tree with fern-like leaves. A useful index fossil, this tree is found in strata dating from the Upper Devonian to Lower Carboniferous (), the oldest fossils being 385 million years old, a ...
'' forests expanded rapidly during the closing stages of the Devonian.
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 stabilized a deep layer of soil, which would have been of the order of metres thick. In contrast, early Devonian plants bore only rhizoids and rhizomes that could penetrate no more than a few centimeters. The mobilization of a large portion of soil had a huge effect: soil promotes weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movemen ...
, the chemical breakdown of rocks, releasing ions which are nutrients for plants and algae.[ The relatively sudden input of nutrients into river water as rooted plants expanded into upland regions may have caused ]eutrophication
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
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 stromatoporoids 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.[ Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting anoxia may have played the dominant role in extinction.]
The "greening" of the continents occurred during the Devonian. The covering of the planet's continents with massive photosynthesizing land plants in the first forests may have reduced CO2 levels in the atmosphere. Since is a greenhouse gas, reduced levels might have helped produce a chillier climate. Evidence such as glacial deposits in northern Brazil (near the Devonian South Pole) suggests widespread glaciation at the end of the Devonian, as a broad 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 also been linked to glaciation in the tropics equivalent to that of the Pleistocene
The Pleistocene ( , often referred to as the '' Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed ...
ice age
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...
.
The weathering of silicate rocks also draws down CO2 from the atmosphere, and CO2 sequestration by mountain building has been suggested as a cause of the decline in greenhouse gases during the Frasnian-Famennian transition. This mountain-building may have also enhanced biological sequestration through an increase in nutrient runoff. The combination of silicate weathering and the burial of organic matter to decreased atmospheric CO2 concentrations from about 15 to three 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
A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...
. This reduction in atmospheric would have caused global cooling and resulted in at least one period of late Devonian glaciation (and subsequent sea level fall), probably fluctuating in intensity alongside the 40ka Milankovic cycle. The continued drawdown of organic carbon eventually pulled the Earth out of its greenhouse
A greenhouse (also called a glasshouse, or, if with sufficient heating, a hothouse) is a structure with walls and roof made chiefly of transparent material, such as glass, in which plants requiring regulated climatic conditions are grown.These ...
state during the Famennian into the icehouse
Icehouse or ice house may refer to:
* Ice house (building), a building where ice is stored
* Ice shanty, a shelter for ice fishing also known as an ''Icehouse''
* Ice skating rink, a facility for ice skating.
* Ice hockey arena, an area where ice ...
that continued throughout the Carboniferous and Permian.
Volcanic activity
Magmatism was suggested as a cause of the Late Devonian extinction in 2002. The end of the Devonian Period had extremely widespread trap magmatism and rifting in the Russian and Siberian platforms, which were situated above the hot mantle plumes and suggested as a cause of the Frasnian / Famennian and end-Devonian extinctions. The Viluy Large igneous province, located in the Vilyuysk
Vilyuysk ( rus, Вилюйск, p=vʲɪˈlʲʉjsk; sah, Бүлүү, ''Bülüü'') is a town and the administrative center of Vilyuysky District in the Sakha Republic, Russia, located on the Vilyuy River (left tributary of the Lena), about f ...
region on the Siberian Craton, covers most of the present day north-eastern margin of the Siberian Platform. The triple-junction rift system was formed during the Devonian Period; the Viluy rift is the western remaining branch of the system and two other branches form the modern margin of the Siberian Platform. Volcanic rocks are covered with post Late Devonian–Early Carboniferous sediments.
Volcanic rocks, dyke belts, and sills that cover more than 320,000 km2, and a gigantic amount of magmatic material (more than 1 million km3) formed in the Viluy branch.[ The Viluy and Pripyat-Dnieper-Donets large igneous provinces were suggested to correlate with the Frasnian / Famennian extinction,] with the Kola and Timan-Pechora magmatic provinces being suggested to be related to the Hangenberg event at the Devonian-Carboniferous boundary.[ Viluy magmatism may have injected enough and into the atmosphere to have generated a destabilised ]greenhouse
A greenhouse (also called a glasshouse, or, if with sufficient heating, a hothouse) is a structure with walls and roof made chiefly of transparent material, such as glass, in which plants requiring regulated climatic conditions are grown.These ...
and ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
, causing rapid global cooling, sea-level falls and marine anoxia occur during Kellwasser black shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
deposition.
Coronene
Coronene (also known as superbenzene and cyclobenzene) is a polycyclic aromatic hydrocarbon (PAH) comprising seven peri-fused benzene rings. Its chemical formula is . It is a yellow material that dissolves in common solvents including benzene, tol ...
and mercury enrichment has been found in deposits dating back to the Kellwasser event, with similar enrichments found in deposits coeval with the Frasnes event at the Givetian-Frasnian boundary and in ones coeval with the Hangenberg event. Because coronene enrichment is only known in association with large igneous province emissions and extraterrestrial impacts and the fact that there is no confirmed evidence of the latter occurring in association with the Kellwasser event, this enrichment strongly suggests a causal relationship between volcanism and the Kellwasser extinction event.
Recent studies have confirmed a correlation between Viluy traps in the Vilyuysk
Vilyuysk ( rus, Вилюйск, p=vʲɪˈlʲʉjsk; sah, Бүлүү, ''Bülüü'') is a town and the administrative center of Vilyuysky District in the Sakha Republic, Russia, located on the Vilyuy River (left tributary of the Lena), about f ...
region on the Siberian Craton and the Kellwasser extinction by 40Ar/39Ar dating. Ages show that the two volcanic phase hypotheses are well supported and the weighted mean ages of each volcanic phase are and Ma, or and Ma, which the first volcanic phase is in agreement with the age of Ma proposed for the Kellwasser event. However, the second volcanic phase is slightly older than Hangenberg event which place at Ma.
Another overlooked contributor to the Kellwasser mass extinction could be the now extinct Cerberean Caldera
This is a sortable summary of the pages Timeline of volcanism on Earth, List of Quaternary volcanic eruptions, and Large volume volcanic eruptions in the Basin and Range Province. Uncertainties as to dates and tephra volumes are not restated, a ...
which was active in the Late Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...
period and thought to have undergone a supereruption approximately 374 million years ago. Remains of this caldera can be found in the modern day state of Victoria, Australia. Eovariscan volcanic activity in present-day Europe may have also played a role in conjunction with the Viluy Traps.
Other hypotheses
Other mechanisms put forward to explain the extinctions include tectonic
Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
-driven climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
, 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.
Bolide
A bolide is normally taken to mean an exceptionally bright meteor, but the term is subject to more than one definition, according to context. It may refer to any large crater-forming body, or to one that explodes in the atmosphere. It can be a ...
impacts can be dramatic triggers of mass extinctions. An asteroid impact was proposed as the prime cause of this faunal turnover. The impact that created the Siljan Ring
The Siljan Ring ( sv, Siljansringen) is a prehistoric impact crater in Dalarna, central Sweden. It is one of the 15 largest known impact craters on Earth and the largest in Europe, with a diameter of about . The impact that created the Siljan Ring ...
either was just before the Kellwasser event or coincided with it. Most impact craters, such as the Kellwasser-aged Alamo
The Battle of the Alamo (February 23 – March 6, 1836) was a pivotal event in the Texas Revolution. Following a 13-day siege, Mexican troops under President General Antonio López de Santa Anna reclaimed the Alamo Mission near San An ...
and the Hangenberg-aged Woodleigh, cannot generally be dated with sufficient precision to link them to the event; others dated precisely are not contemporaneous with the extinction. Although some minor features of meteoric impact have been observed in places (iridium anomalies and microspherules), these were probably caused by other factors.
A more recent explanation suggests that a nearby supernova explosion was the cause for the specific Hangenberg event
The Hangenberg event, also known as the Hangenberg crisis or end-Devonian extinction, is a mass extinction that occurred at the end of the Famennian stage, the last stage in the Devonian Period (roughly 358.9 ± 0.4 million years ago). It is usu ...
, which marks the boundary between the Devonian and Carboniferous periods, or even for a sequence of events covering several millions of years towards the end of the Devonian period. This could offer a possible explanation for the dramatic drop in atmospheric ozone that could have permitted massive ultraviolet damage to the genetic material of lifeforms, triggering a mass extinction. Recent research offers evidence of ultraviolet damage to pollen and spores over many thousands of years during this event as observed in the fossil record and that, in turn, points to a possible long-term destruction of the ozone layer. A supernova explosion is an alternative explanation to global temperature rise, that could account for the drop in atmospheric ozone. Detecting either of the long-lived, extra-terrestrial radioisotopes 146Sm or 244Pu in one or more end-Devonian extinction strata would confirm a supernova origin. However, there is currently no direct evidence for this hypothesis.
See also
* Evolutionary history of plants
Notes
References
Sources
*
* Racki, Grzegorz, "Toward understanding Late Devonian global events: few answers, many questions" in Jeff Over, Jared Morrow, P. Wignall (eds.), ''Understanding Late Devonian and Permian-Triassic Biotic and Climatic Events'', Elsevier, 2005.
External links
Late Devonian mass extinctions
at The Devonian Times. An excellent overview.
"The Late Devonian Extinction"
*
: a Geological Society of America
The Geological Society of America (GSA) is a nonprofit organization dedicated to the advancement of the geosciences.
History
The society was founded in Ithaca, New York, in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hitch ...
conference in 2003 reflects current approaches
{{DEFAULTSORT:Late Devonian Extinction
Extinction events
History of climate variability and change
Devonian events