Late Devonian mass extinction
The Late Devonian mass extinction wiped out around 40 percent of marine species, yet most people have never heard its name. It struck the oceans around 372.15 million years ago, at the boundary between the Frasnian and Famennian ages. Scientists also call it the Kellwasser event, after a valley in Germany where its traces were first found. It ranks among the so-called Big Five, the most severe extinction crises in Earth's history. A second blow, the Hangenberg event, followed roughly 13 million years later, around 358.86 million years ago. That later catastrophe closed the Devonian period entirely and ushered in the Carboniferous. For a long time the two events were conflated, blurred together as a single disaster. How could a crisis that touched only the seas reshape the planet so deeply? Why did some creatures vanish while others barely noticed? And what role did the rise of forests on dry land play in killing life beneath the waves? The answers lie in black shale, in the eyes of trilobites, and in the deep roots of the first great trees.
Massive reefs once dominated the warm shallow seas of the Late Devonian. They were not built by the stony corals we know today. Instead, calcite-based reef-builders did the work: stromatoporoid sponges alongside rugose and tabulate corals. These organisms thrived only in low-nutrient water, and they shaped some of the largest reef systems Earth had yet seen. The Kellwasser event hit these reef-builders harder than any other group. Communities of beloceratids and manticoceratids were left devastated. In the Famennian that followed, reefs looked utterly different. They were dominated by siliceous sponges and calcifying bacteria, which produced structures called oncolites and stromatolites. There is evidence this shift began even before the Frasnian-Famennian boundary. The collapse was so complete that reefs would not regain their Middle Devonian extent until the Mesozoic. The scleractinian, or stony, corals that anchor modern reefs had not yet evolved and would only appear in the Triassic. The Devonian reef-builders are entirely gone today. Stromatoporoids died out in the end-Devonian Hangenberg event, while rugose and tabulate corals held on until the Permian-Triassic extinction.
Trilobites were profoundly affected by the crisis, losing three entire orders: 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. Only two trilobite orders survived into the Famennian: Phacopida and Proetida. The survivors tended to favor deep environments and tropical latitudes. A few small groups even thrived afterward, namely phacopids and cyrtosymboline phillipsiids, though these warm-water specialists would later suffer in the cold snap of the Hangenberg event. Trilobite anatomy records the stress in remarkable detail. In the run-up to the Kellwasser event, trilobites evolved smaller eyes, with eye size increasing again afterward. This hints that vision mattered less around the event, perhaps because of deeper or murkier water. The brims at the rims of their heads also expanded across this period. Those brims are thought to have aided respiration, and the spreading anoxia of the seas drove an increase in brim area. Other arthropods told a similar story. Among ostracods, no families went extinct, yet smaller taxonomic units were severely battered. Around 80 percent of ostracod species died out worldwide, with the rate reaching 91 percent in Eastern Europe. Those that could tolerate oxygen stress survived more easily, and endemic deep marine species diversified quickly in the aftermath.
Brachiopods, ammonites, conodonts, acritarchs, and graptolites were all starkly affected, and the cystoids disappeared entirely. Among the survivors, clear morphological trends emerge. Atrypid and strophomenid brachiopods grew rarer, replaced in many niches by productids. The spiny shells of the productids made them more resistant to predation and environmental disturbance. As in most extinction events, specialists clinging to small niches fared worse than generalists. Vertebrates came through the Kellwasser event relatively well, though not unscathed. Around half of placoderm families died out, mostly species-poor bottom-feeding groups. More diverse placoderm families survived this event only to perish later in the Hangenberg event. Jawless fish fared worse. Most lingering agnathan groups, including osteostracans, galeaspids, and heterostracans, vanished by the end of the Frasnian. The jawless thelodonts barely held on, then succumbed early in the Famennian. The fate of one fish lineage carries special weight for our own ancestry. Among the tetrapodomorphs, the tetrapod-like elpistostegalians, such as Tiktaalik, disappeared at the Frasnian-Famennian boundary. True tetrapods, the four-limbed vertebrates with digits, survived and radiated after the Kellwasser extinction, though their fossils stay rare until the mid-to-late Famennian.
The late Devonian crash in biodiversity ran deeper than the famous event that closed the Cretaceous. A survey by McGhee in 1996 estimates that 22 percent of all families of marine animals, largely invertebrates, were eliminated. The family is a sweeping unit, and losing so many points to a profound erosion of ecosystem diversity. On a finer scale, 57 percent of genera and at least 75 percent of species failed to cross into the Carboniferous. These species figures deserve caution. They rest on surveys of Devonian marine taxa that may not be well enough known to assess true rates of loss. Differential preservation and sampling bias make the picture hard to pin down. The timing is just as slippery. Estimates for the event's span range from 500,000 to 25 million years, reaching from the mid-Givetian to the end-Famennian. Some researchers count 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. Extinction rates stayed above background for the last 20 to 25 million years of the Devonian. Within that long interval, about eight to ten distinct events can be seen, of which the Kellwasser and the Hangenberg stand out as most severe. The Kellwasser itself may have struck in two pulses, separated by roughly 800,000 years, with the second pulse more severe than the first.
Black shale is the chief geological fingerprint of the Kellwasser event, forming in stable, oxygen-starved seawater. This anoxia correlates with biotic crises more closely than cooling does, which suggests it may have played the leading role in the dying. Many sites record a rapid rise in organic carbon burial and widespread anoxia in deep bottom waters. Some shallow waters suffered photic zone euxinia, where toxic sulfides built up even in sunlit surface layers. Negative δ238U excursions coincide with both the Lower and Upper Kellwasser events, offering direct evidence of rising anoxia. The trigger may have come from dry land. During the Silurian-Devonian Terrestrial Revolution, plants assisted by fungi transformed the continents. Their maximum height climbed from 30 centimeters at the start of the Devonian to 30-meter archaeopterid trees by its end. Advanced vascular systems made this possible, allowing complex branches and roots, while seeds let plants spread into areas that were not waterlogged. Archaeopteris forests expanded rapidly, and their deep roots broke up bedrock and built soil layers that could reach several meters thick. Earlier Devonian plants bore only rhizoids and rhizomes reaching a few centimeters down. Soil promotes weathering, the chemical breakdown of rock, which released phosphates and other nutrients. As nutrients flooded river water, the result may have been eutrophication, harmful algal blooms, and anoxia. Dead algae sank and decomposed so fast that they used up the oxygen, suffocating bottom-dwelling fish. The stromatoporoid and coral reefs of the Frasnian thrived only in low-nutrient water, so this nutrient surge may itself have driven extinction. A site in Greenland, tracking inland lake deposits, supports the link between plant-led weathering, phosphorus runoff, and global cooling.
A positive δ18O excursion appears across the Frasnian-Famennian boundary in brachiopods from North America, Germany, Spain, Morocco, Siberia, and China. It signals the removal of atmospheric carbon dioxide and a global cooling event, and it shows up in time-equivalent strata in South China and the western Palaeotethys, marking a globally synchronous shift. The spread of land plants pulled CO2 levels down over the long term, dropping the gas from about 15 to three times present levels through silicate weathering and organic burial. This drawdown may have begun the Late Palaeozoic Ice Age during the Famennian, a possible cause of the Hangenberg event. Volcanism offers a competing or complementary culprit, first proposed in 2002. The Viluy Large igneous province, in the Vilyuysk region on the Siberian Craton, produced volcanic rocks, dyke belts, and sills covering more than 320,000 square kilometers, with over 1 million cubic kilometers of magmatic material. Argon dating has tied the Viluy traps to the Kellwasser extinction, with one volcanic phase agreeing with the 372.2 million year age proposed for the event. Coronene and mercury enrichment in Kellwasser deposits points toward volcanism, since coronene is known only with large igneous province emissions and extraterrestrial impacts, and no impact has been confirmed here. Other suspects are weaker. The Siljan Ring impact in Sweden either just preceded the Kellwasser event or coincided with it, but modelling has ruled out a single impact as inconsistent with the evidence. A near-Earth supernova has been floated for the Hangenberg event, based on signs of ultraviolet damage to pollen and spores over thousands of years. Detecting the radioisotopes 146Sm or 244Pu in end-Devonian strata would confirm a supernova origin, but no direct evidence exists yet.
Common questions
What was the Late Devonian mass extinction?
The Late Devonian mass extinction, also known as the Kellwasser event, was a mass extinction that occurred around 372.15 million years ago at the boundary between the Frasnian and Famennian ages. It is ranked among the Big Five most severe mass extinctions in Earth's history, with likely around 40 percent of marine species going extinct.
When did the Late Devonian mass extinction happen?
The Kellwasser event of the Late Devonian mass extinction occurred around 372.15 million years ago. A second extinction, the Hangenberg event, followed about 13 million years later around 358.86 million years ago, bringing an end to the Devonian period.
What caused the Late Devonian mass extinction?
The causes remain unclear, but leading hypotheses include ocean anoxia marked by black shale, global cooling, and oceanic volcanism such as the Viluy Large igneous province. The spread of deep-rooted land plants may have driven weathering, nutrient runoff, algal blooms, and the resulting anoxia.
How many species died in the Late Devonian mass extinction?
Likely around 40 percent of marine species went extinct in the Late Devonian mass extinction. A 1996 survey estimates 22 percent of marine animal families were eliminated, while 57 percent of genera and at least 75 percent of species did not survive into the Carboniferous.
What animals went extinct in the Late Devonian mass extinction?
Hard-hit groups included reef-building stromatoporoids and rugose and tabulate corals, along with brachiopods, trilobites, ammonites, conodonts, and graptolites. Three trilobite orders vanished, cystoids disappeared, and most jawless agnathan fish died out by the end of the Frasnian.
Why is the Late Devonian mass extinction called the Kellwasser event?
It is named for its type locality, the Kellwassertal in Lower Saxony, Germany. The Kellwasser was the first pulse to be detected from the marine invertebrate record and was the most severe of the Late Devonian extinction crises.
All sources
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