Late Devonian extinction
Encyclopedia
The Late Devonian extinction was one of five major extinction event
s 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. Overall, 19% of all families and 50% of all genera went extinct. A second, distinct mass extinction, the Hangenberg Event, closed the Devonian period
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. 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. 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.
By the late Devonian, the land had been colonized by plants and insects. In the oceans, there were massive reef
s 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 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
. 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. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation
than by an increase in extinctions. This might have been caused by invasions of cosmopolitan species, rather than any single event. 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.
, 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.
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 built huge forests on the highlands. Several different clades had developed a shrubby or tree-like habit by the Late Givetian, including the cladoxylalean
fern
s, lepidosigillarioid lycopsids, and aneurophyte and archaeopterid
progymnosperm
s.
Fish were also undergoing a huge radiation, and the first tetrapods were beginning to evolve leg-like structures.
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
.
; 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.
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 stark that bigger 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 starkly 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 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 level
s as nutrient input changed. As with most extinction events, specialist taxa occupying small niches were harder hit than generalists.
The Hangenberg event impacted both sea and freshwater communities. This mass extinction impacted ammonite
s and trilobite
s, as well as jawed vertebrates including our tetrapod
ancestors. The Hangenberg is linked to the extinction of 44% of high-level vertebrate clade
s, including all placoderms and most sarcopterygians, and the complete turnover of the vertebrate biota. This led 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. 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.
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 invertebrate
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 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.
in oceanic bottom waters; the rate of carbon burial shot up, and organisms were decimated, especially in the tropics, and especially reef communities. 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. The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall;.
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 Although some minor features of meteoric impact have been observed in places (iridium anomalies and microspherules), these were probably caused by other factors.
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 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. 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. Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting that anoxia
may have played the dominant role in extinction.
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 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 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
ice age
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
. 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 Earth state into the Icehouse that continued throughout the Carboniferous and Permian.
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...
s in the history of the Earth's biota
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...
. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of 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...
period, the Famennian
Famennian
The Famennian is one of two faunal stages in the Late Devonian epoch. It lasted from 374.5 ± 2.6 million years ago to 359.2 ± 2.5 million years ago. It was preceded by the Frasnian stage and followed by the Tournaisian stage and is named after Famenne, a natural region in southern Belgium.It was...
faunal stage
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...
, (the Frasnian-Famennian boundary), about 374 million years ago. Overall, 19% of all families and 50% of all genera went extinct. A second, distinct mass extinction, the Hangenberg Event, closed the Devonian period
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. 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. 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
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...
, and Famennian
Famennian
The Famennian is one of two faunal stages in the Late Devonian epoch. It lasted from 374.5 ± 2.6 million years ago to 359.2 ± 2.5 million years ago. It was preceded by the Frasnian stage and followed by the Tournaisian stage and is named after Famenne, a natural region in southern Belgium.It was...
stages.
By the late Devonian, the land had been colonized by plants and insects. In the oceans, there were massive reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
s built by corals and stromatoporoids
Stromatoporoidea
Stromatoporoidea is a class of aquatic invertebrates common in the fossil record from the Ordovician through the Cretaceous. They were especially abundant in the Silurian and Devonian. These invertebrates were important reef-formers throughout the Paleozoic and the Late Mesozoic. The group was...
on land. Euramerica
Euramerica
Euramerica was a minor supercontinent created in the Devonian as the result of a collision between the Laurentian, Baltica, and Avalonia cratons .300 million years ago in the Late Carboniferous tropical rainforests lay over the equator of Euramerica...
and Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
were beginning to converge into what would become Pangea. The extinction seems to have only affected marine life. Hard-hit groups include brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s, trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s, and reef-building organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s; the latter almost completely disappeared, with coral reefs only returning upon the evolution of modern corals during the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
. The causes of these extinctions are unclear. Leading theories include changes in sea level and 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...
, possibly triggered by global cooling or oceanic volcanism. The impact of a comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
or another extraterrestrial body has also been suggested. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in 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...
than by an increase in extinctions. This might have been caused by invasions of cosmopolitan species, rather than any single event. 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.
The Late Devonian world
During the Late Devonian, the continents were arranged differently, with a supercontinent, GondwanaGondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
, covering much of the southern hemisphere. The continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
of Siberia
Siberia (continent)
Siberia is the craton located in the heart of the region of Siberia. Siberia or "Angaraland" is today the Central Siberian Plateau...
occupied the northern hemisphere, while an equatorial continent, Laurussia (formed by the collision of Baltica
Baltica
Baltica is a name applied by geologists to a late-Proterozoic, early-Palaeozoic continent that now includes the East European craton of northwestern Eurasia. Baltica was created as an entity not earlier than 1.8 billion years ago. Before this time, the three segments/continents that now comprise...
and Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
) was drifting
Continental drift
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
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.
The biota was also very different. Plants, which had been on land in forms similar to mosses, liverworts, and lichens since 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...
, had just developed roots, seeds, and water transport
Vascular
Vascular in zoology and medicine means "related to blood vessels", which are part of the circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....
systems that allowed them to survive away from places that were constantly wet—and consequently built huge forests on the highlands. Several different clades had developed a shrubby or tree-like habit by the Late Givetian, including the cladoxylalean
Cladoxylopsid
The cladoxylopsids are a group of plants known only as fossils that are thought to be ancestors of ferns and horsetails.They had a central trunk, from the top of which several lateral branches were attached. Fossils of these plants originate in the Middle Devonian to Early Carboniferous periods ,...
fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s, lepidosigillarioid lycopsids, and aneurophyte and archaeopterid
Archaeopteris
Archaeopteris is an extinct genus of tree-like plants with fern-like leaves. A useful index fossil, this tree is found in strata dating from the Upper Devonian to Lower Carboniferous , and has a global distribution....
progymnosperm
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. They have been treated formally at the rank of division Progymnospermophyta or class Progymnospermopsida...
s.
Fish were also undergoing a huge radiation, and the first tetrapods were beginning to evolve leg-like structures.
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. The Kellwasser event was preceded by a longer period of prolonged biodiversity loss. The fossil record of following first 15 million years of the CarboniferousCarboniferous
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"...
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
Romer's gap
Romer's Gap is an example of a gap in the fossil record used in the study of evolution. Such gaps represent a period from which excavators have found no or very few fossils. Romer's gap is named after paleontologist Dr...
.
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 last spike in the period of extinction. It is marked by an anoxic black shale layer and an overlying sandstone deposit. Unlike the Kellwasser event, the Hangenberg event affected marine and terrestrial habitats.Effects of the events
The extinction events are accompanied by widespread oceanic anoxiaAnoxic 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...
; 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 rugoseRugosa
Disambiguation:The Rugosa Rose is also sometimes just called "Rugosa". For the moon in Star Wars: The Clone Wars, see .The Rugosa, also called the Tetracoralla, are an extinct order of coral that were abundant in Middle Ordovician to Late Permian seas.Solitary rugosans are often referred to...
and tabulate coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s. Reefs of the later Devonian were dominated by sponges and calcifying bacteria, producing structures such as oncolite
Oncolite
Oncolites are sedimentary structures formed out of oncoids, which are layered spherical growth structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but instead of forming columns they form approximately spherical structures...
s and stromatolites; the reef system collapse was so stark that bigger reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian or "stony" corals
Scleractinia
Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...
) did not recover until the Mesozoic era.
Further taxa to be starkly affected include the brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s, trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s, ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
s, conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
s, and acritarch
Acritarch
Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
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 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...
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 level
Trophic level
The trophic level of an organism is the position it occupies in a food chain. The word trophic derives from the Greek τροφή referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism...
s as nutrient input changed. As with most extinction events, specialist taxa occupying small niches were harder hit than generalists.
The Hangenberg event impacted both sea and freshwater communities. This mass extinction impacted ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
s and trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s, as well as jawed vertebrates including our tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
ancestors. The Hangenberg is linked to the extinction of 44% of high-level vertebrate 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, including all placoderms and most sarcopterygians, and the complete turnover of the vertebrate biota. This led to the establishment of the modern vertebrate fauna, consisting mostly of actinopterygians, chondrichthyans, and tetrapods, in the 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"...
. Romers Gap, a 15 million year hiatus in the early Carboniferous tetrapod record, has been linked to this event. 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.
Magnitude
The late Devonian crash in biodiversityBiodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
was more drastic than the familiar extinction event that closed 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...
: a recent survey (McGhee 1996) estimates that 22 percent of all the 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 animals (largely 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) 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
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"...
. 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, 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 anoxiaAnoxic 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...
in oceanic bottom waters; the rate of carbon burial shot up, and organisms were decimated, especially in the tropics, and especially reef communities. 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. The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall;.
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, but no secure evidence of a specific extraterrestrial impact has been identified in this case. Impact craters, such as the Kellwasser-aged AlamoAlamo bolide impact
The Alamo bolide impact occurred 367 million years ago, when one or more hypervelocity objects from space slammed into shallow marine waters at a site that is now the Devonian Guilmette Formation of the Worthington Mountains and Schell Creek Range of southeastern Nevada; the event is named for...
and the Hangenberg-aged Woodleigh
Woodleigh crater
Woodleigh is a large meteorite impact crater in Western Australia, centred on Woodleigh Station east of Shark Bay. A team of four scientists at the Geological Survey of Western Australia and the Australian National University, led by Arthur J...
, can generally not 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.
Plant evolution
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 30 m 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 development 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, ArchaeopterisArchaeopteris
Archaeopteris is an extinct genus of tree-like plants with fern-like leaves. A useful index fossil, this tree is found in strata dating from the Upper Devonian to Lower Carboniferous , and has a global distribution....
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
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
, the chemical breakdown of rocks, releasing ions which act as nutrients to plants and algae. The relatively sudden input of nutrients into river water may have caused eutrophication and subsequent 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...
. 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. Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting that 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...
may have played the dominant role in extinction.
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 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 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
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
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...
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
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
. 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 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.Further reading
- McGhee, George R., Jr, 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis (Columbia University Press) ISBN 0231075049
External links
- Late Devonian mass extinctions at The Devonian Times. An excellent overview.
- Devonian Mass Extinction
- BBC "The Extinction files" "The Late Devonian Extinction"
- "Understanding Late Devonian and Permian-Triassic Biotic and Climatic Events: Towards an Integrated Approach": a Geological Society of AmericaGeological Society of AmericaThe Geological Society of America is a nonprofit organization dedicated to the advancement of the geosciences. The society was founded in New York in 1888 by Alexander Winchell, John J. Stevenson, Charles H. Hitchcock, John R. Proctor and Edward Orton and has been headquartered at 3300 Penrose...
conference in 2003 reflects current approaches - PBS: Deep Time