Late Devonian mass extinction
The Late Devonian mass extinction, also known as the Kellwasser event, was a mass extinction event which occurred around 372 million years ago, at the boundary between the Frasnian and Famennian ages of the Late Devonian period. It is placed as one of the "Big Five" most severe mass extinction events in Earth's history, with likely around 40% of marine species going extinct, though the degree of severity is contested. A second mass extinction called the Hangenberg event, also known as the end-Devonian extinction, occurred 13 million years later around 359 million years ago, bringing an end to the Famennian and Devonian, as the world transitioned into the Carboniferous Period. The effects of the two extinction events have historically been conflated, and both events collectively profoundly reshaped marine ecosystems.
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. 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 ages.
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 last 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, 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. 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. Agnathans were in decline long before the end of the Frasnian and were nearly wiped out by the extinctions.
The extinction event was 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 Canada and 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, 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.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 habit by the Late Givetian, including the cladoxylalean ferns, lepidosigillarioid lycopsids, and aneurophyte and archaeopterid progymnosperms. 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. Large differences are observed between the biotas before and after the Frasnian-Famennian boundary, demonstrating the extinction event's magnitude.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 sponges and the rugose and tabulate corals. It left communities of beloceratids and manticoceratids devastated. Following the Kellwasser event, reefs of the Famennian were primarily dominated by siliceous sponges and calcifying bacteria, producing structures such as oncolites and stromatolites, although there is evidence this shift in reef composition began prior to the Frasnian-Famennian boundary. 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 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 arthropods
were profoundly affected by the extinction event. Three trilobite orders went extinct: Corynexochida, Harpetida, and Odontopleurida. All three had been declining since the Taghanic event at the end of the Givetian, which also killed off the order Lichida. This left only two trilobites orders in the Famennian: Phacopida and Proetida. Trilobites which survived the Kellwasser event tended to prefer deep environments and tropical latitudes. A few small groups managed to thrive in the aftermath, namely phacopids and cyrtosymboline phillipsiids. These warm-water specialists would suffer during the cold snap of the Hangenberg event, cutting the post-Kellwasser recovery short.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 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.
Among ostracods, no families went extinct, but small-scale taxonomic units were severely impacted. Around 80% of ostracod species died out worldwide, though the extinction rate reached 91% in Eastern Europe. Both shallow and deep marine ostracods were impacted. Ostracods which could tolerate oxygen stress survived the extinction more easily, and endemic deep marine species diversified quickly in the aftermath.
Other marine invertebrates
Further taxa to be starkly affected include the brachiopods, ammonites, conodonts, acritarch and graptolites. Cystoids disappeared during this event. The surviving taxa show morphological trends through the event. Atrypid and strophomenid brachiopods became rarer, replaced in many niches by productids, whose spiny shells made them more resistant to predation and environmental disturbances.As with most extinction events, specialist taxa occupying small niches were harder hit than generalists. Marine invertebrates that lived in warmer ecoregions were devastated more compared to those living in colder biomes.
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 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. 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. Among freshwater and shallow marine tetrapodomorph fish, the tetrapod-like elpistostegalians disappeared at the Frasnian-Famennian boundary. True tetrapods survived and experienced an evolutionary radiation following the Kellwasser extinction, though their fossils are rare until the mid-to-late Famennian.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 estimates that 22% of all the 'families' of marine animals 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.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, the Kellwasser and the Hangenberg events, stand out as particularly severe. The Kellwasser event was preceded by a longer period of prolonged biodiversity loss.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. 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 and was the most severe of the extinction crises of the Late Devonian. There may in fact have been two closely spaced events here, as shown by the presence of two distinct anoxic shale layers.
There is evidence that the Kellwasser event was a two-pulsed event, with the two extinction pulses being separated by an interval of approximately 800,000 years. The second pulse was more severe than the first.