Late Devonian extinction

The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, 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 stage and the Famennian stage, the last stage in the Devonian Period.[1][2] Overall, 19% of all families and 50% of all genera became extinct.[3] A second mass extinction, the Hangenberg event (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.[4]

Marine extinction intensity during the Phanerozoic
%
Millions of years ago
(H)
Cap
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.

Although it is well established that there was a massive loss of biodiversity 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.[5] 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.[6]

By the Late Devonian, the land had been colonized by plants and insects. In the oceans, massive reefs were built by corals and stromatoporoids. Euramerica and Gondwana were beginning to converge into what would become Pangaea. The extinction seems to have only affected marine life. Hard-hit groups include brachiopods, trilobites, and reef-building organisms; the latter almost completely disappeared. The causes of these extinctions are unclear. Leading hypotheses 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,[7] such as the Siljan Ring event in Sweden. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.[8][9] This might have been caused by invasions of cosmopolitan species, rather than by any single event.[9] 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. Agnathans (jawless fish) were in decline long before the end of the Frasnian and were nearly wiped out by the extinctions.[10]

The extinction events were 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 United States.

Late Devonian world

During the Late Devonian, the continents were arranged differently from today, 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, closing the Rheic Ocean. The Caledonian mountains were also growing across what is now the Scottish Highlands and Scandinavia, while the Appalachians rose over America.[15]

The biota was also very different. Plants, which had been on land in forms similar to mosses and liverworts 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 so grew huge forests on the highlands. Several clades had developed a shrubby or tree-like habitat by the Late Givetian, including the cladoxylalean ferns, lepidosigillarioid lycopsids, and aneurophyte and archaeopterid progymnosperms.[16] 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.

Reef destruction
Side view of a stromatoporoid showing laminae and pillars; Columbus Limestone (Devonian) of Ohio

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 sponges and the rugose and tabulate corals. After the Kellwasser event, reefs of the Famennian were primarily dominated by siliceous sponges and calcifying bacteria, producing structures such as oncolites and stromatolites.[17] 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 brachiopods, trilobites, ammonites, conodonts, acritarch 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 levels as nutrient input changed.[18] As with most extinction events, specialist taxa occupying small niches were harder hit than generalists.[2]

Vertebrates
Tiktaalik, an early air-breathing elpistostegalian. They were among the vertebrates which died out due to the Kellwasser event

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, and heterostracans, also went extinct by the end of the Frasnian. The jawless thelodonts only barely survived, succumbing early in the Famennian.[19] Among freshwater and shallow marine tetrapodomorph fish, the tetrapod-like elpistostegalians (such as Tiktaalik) disappeared at the Frasnian-Famennian boundary. True tetrapods (defined as four-limbed vertebrates with digits) apparently survived, though their fossils are rare until the mid-to-late Famennian.

The Hangenberg event was much more impactful on vertebrate evolution, affecting both marine and freshwater communities.[10][20] It 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.[10] 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.[21] This led to the establishment of the modern vertebrate fauna in the Carboniferous, consisting mostly of actinopterygians, chondrichthyans, and tetrapods. Romer's gap, a 15 million-year hiatus in the early Carboniferous tetrapod record, has been linked to this event.[10] 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.[10][22]

Magnitude of diversity loss

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% of all the 'families' of marine animals (largely invertebrates) 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[lower-alpha 1] 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.

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.[23] The Kellwasser event was preceded by a longer period of prolonged biodiversity loss.[24] 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.[25]

The Kellwasser event

The Kellwasser event, named for its type locality, the Kellwassertal in Lower Saxony, 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 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 layer and an overlying sandstone deposit.[26] Unlike the Kellwasser event, the Hangenberg event affected both marine and terrestrial habitats.[10]

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;[16] the rate of carbon burial shot up,[16] and benthic organisms were devastated, especially in the tropics, and especially reef communities.[16] 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.[27] The Hangenberg event has been associated with sea-level rise followed swiftly by glaciation-related sea-level fall.[26][28]

Possible triggers 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 30 m archaeopterids,[29] 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.[16] 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.[16] 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.

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, the chemical breakdown of rocks, releasing ions which are nutrients for plants and algae.[16] 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 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.[16] Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting anoxia may have played the dominant role in extinction.[30]

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 CO2 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 ice age.[26]

The weathering of silicate rocks also draws down CO2 from the atmosphere. This acted together with the burial of organic matter to decrease 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.[31] 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),[32] 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.

Volcanic activity

Magmatism was suggested as a cause of the Late Devonian extinction in 2002.[33] 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.[34] The Viluy and Pripyat-Dnieper-Donets large igneous provinces were suggested to correlate with the Frasnian / Famennian extinction and the Kola and Timan-Pechora magmatism was suggested to correspond to the end Devonian-Carboniferous extinction.[34]

Most recently, scientists have confirmed a correlation between Viluy traps (in the Vilyuysk region) on the Siberian Craton and the Kellwasser extinction by 40Ar/39Ar dating.[35][36]

The Viluy Large igneous province 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.[37] 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.[37]

Ages show that the two volcanic phase hypotheses are well supported and the weighted mean ages of each volcanic phase are 376.7±3.4 and 364.4±3.4 Ma, or 373.4±2.1 and 363.2±2.0 Ma, which the first volcanic phase is in agreement with the age of 372.2±3.2 Ma proposed for the Kellwasser event. However, the second volcanic phase is slightly older than Hangenberg event which place at 358.9±1.2 Ma.[36] Viluy magmatism may have injected enough CO2 and SO2 into the atmosphere to have generated a destabilised greenhouse and ecosystem, causing rapid global cooling, sea-level falls and marine anoxia occur during Kellwasser black shale deposition.[38][18][39][40]

Another overlooked contributor could be the now extinct Cerberean Caldera which was active in the Late Devonian period and thought to have undergone a supereruption approximately 374 million years ago.[lower-alpha 2][42] Remains of this caldera can be found in the modern day state of Victoria, Australia.

Other hypotheses

Other mechanisms put forward 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.[30]

Bolide impacts can be dramatic triggers of mass extinctions. An asteroid impact was proposed as the prime cause of this faunal turnover.[2][43] The impact that created the Siljan Ring either was just before the Kellwasser event or coincided with it.[44] Most impact craters, such as the Kellwasser-aged Alamo 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.[1] Although some minor features of meteoric impact have been observed in places (iridium anomalies and microspherules), these were probably caused by other factors.[30][45]

A more recent explanation suggests that a nearby supernova explosion was the cause for the specific Hangenberg event, 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.[46] 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

Notes
  1. The species estimate is the toughest to assess and most likely to be adjusted.
  2. Though a super eruption on its own would have devastating effects in both short term and long term, the Late Devonian extinction was caused by a series of events which contributed to the extinction.[41]

References
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Sources
  • McGhee, George R. (1 January 1996). The Late Devonian Mass Extinction: The Frasnian/Famennian Crisis. Columbia University Press. p. 9. ISBN 978-0-231-07505-3. Retrieved 23 July 2015.
  • 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.
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