Late Devonian extinction

The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when nearly all of the fossil agnathan fishes suddenly disappeared. A second strong pulse closed the Devonian period. Overall, 19% of all families and 50% of all genera went extinct.

Although it is clear that there was a massive loss of biodiversity towards the end of the Devonian, the extent of time during which these events took place is uncertain, with estimates ranging from 500,000 to 15 million years, the latter being the full length of the Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a series of smaller extinctions, though the latest research suggests multiple causes and a series of distinct extinction pulses through an interval of some three million years. Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, including particularly notable extinctions at the ends of the Givetian, Frasnian, and Famennian stages. Some references cite 250 million years as the span of time over which the extinctions occurred.

By the late Devonian, there were plants, insects, and amphibians on land, fish in the seas, and huge reefs built by corals and stromatoporoids. The continents of Euramerica and Gondwana were just beginning to move together to form Pangea. The extinction seems to have only affected marine life. Reef-building organisms were almost completely wiped out, so that coral reefs returned only with the development of modern corals in the Mesozoic. Brachiopods, trilobites, and other families were heavily affected. The causes of these extinctions are unclear. The leading theories suggest that changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism, were most likely responsible, although the impact of an extraterrestrial body such as a comet has also been considered. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.

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

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

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. Each of these two major events was preceded by a longer period of prolonged biodiversity loss.

The Kellwasser event
The Kellwasser event is the term given to the extinction pulse that occurs near the Frasnian/Famennian boundary. There may in fact have been two closely spaced events here.

The Hangenberg event
The Hangenberg event sits on or just below the Devonian/Carboniferous boundary and marks the final spike in the period of extinction.

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

Biological impact
The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms over cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the stromatoporoids, and the rugose and tabulate corals. Reefs of the later Devonian were dominated by sponges and calcifying bacteria, producing structures such as oncolites and stromatolites; the reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian or "stony" corals) did not recover until the Mesozoic era;

Further taxa to be severely affected include the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms. Freshwater species, including our tetrapod ancestors, and land plants were relatively unscathed.

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

The shape of conodonts' feeding apparatus varied with δ18O and thus seawater temperature; this may relate to them occupying different trophic levels as nutrient input changed.

As with all extinction events, specialist taxa occupying small niches were harder hit than generalists.

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

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

Major environmental changes
From the end of the Middle Devonian, into the Late Devonian, several environmental changes can be detected from the sedimentary record. There is evidence of widespread anoxia in oceanic bottom waters; the rate of carbon burial shot up, and benthic 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 boundary, with one sea level rise associated with the onset of anoxic deposits.

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 Alamo and Woodleigh, can generally not be dated with sufficient precision to link them to the event; those 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, Archaeopteris forests expanded rapidly during the closing stages of the Devonian.

Effect on weathering
These tall trees required deep rooting systems to acquire water and nutrients, and provide anchorage. These systems broke up the upper layers of bedrock and stabilised a deep layer of soil, which would have been on the order of metres thick. In contrast, early Devonian plants bore only rhizoids and rhizomes that could penetrate no more than a couple of centimetres. The mobilisation of a large portion of soil had a huge effect; soil promotes weathering, the chemical breakdown of rocks, releasing ions which act as nutrients to plants and algae. 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.

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

The 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 CO2 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.