The Late Devonian mass extinction was not caused by a single catastrophic event but by a cascade of environmental changes that played out over millions of years, roughly 383 to 359 million years ago. Unlike the asteroid strike that ended the dinosaurs, this crisis involved the interplay of expanding land plants, ocean oxygen loss, volcanic eruptions, climate cooling, and glaciation, all reinforcing one another in ways researchers are still untangling. The result was one of the five worst biodiversity crises in Earth’s history, yet its drawn-out, pulsed nature makes it one of the hardest to pin down.
A Crisis Spread Across Millions of Years
One reason the Late Devonian extinction resists simple explanation is that it was not a single moment of destruction. The crisis unfolded in distinct pulses, the most studied of which are the Lower and Upper Kellwasser events near the Frasnian–Famennian boundary (around 372 million years ago) and the Hangenberg Crisis at the very end of the Devonian (around 359 million years ago). High-resolution dating of sedimentary cycles suggests the two Kellwasser carbon-isotope excursions are separated by roughly 600,000 years, pointing to astronomical pacing of Earth’s climate as a possible rhythm-keeper for these extinction pulses.1PubMed Central. Timing and pacing of the Late Devonian mass extinction event regulated by eccentricity and obliquity Regional analysis of Appalachian marine faunas shows that the major period of extinction was spread over at least seven million years, with roughly 70% of macroinvertebrate species vanishing across that interval.2GeoScienceWorld. The Frasnian-Famennian extinction event: A preliminary analysis of Appalachian marine ecosystems
This matters for understanding causes. A single bolt from the sky would produce a sharp, geologically instantaneous die-off. A multi-million-year stagger suggests ongoing environmental deterioration punctuated by acute crises, which is exactly what the geological record shows.
How the Spread of Trees Helped Poison the Sea
The Devonian period is sometimes called the “Age of Fishes,” but it was also the age in which vascular plants first colonized land in a major way. By the Late Devonian, the earliest forests had taken root, and their expansion had far-reaching consequences for ocean chemistry. Tree roots broke apart rock and soil at rates far exceeding what barren landscapes could manage. That weathering released phosphorus, a nutrient that washed into rivers and eventually the sea. The enhanced transfer of terrestrial phosphorus to marine systems via weathering and erosion is well documented in ancient lake and soil records and has long been linked to hypotheses about marine eutrophication during the Devonian.3GSA Bulletin. Enhanced terrestrial nutrient release during the Devonian emergence and expansion of forests: Evidence from lacustrine phosphorus and geochemical records
Eutrophication is what happens when excess nutrients trigger runaway algal growth. When those algae die and decompose, the process consumes dissolved oxygen, creating dead zones. Modeling work scaled from Devonian-era phosphorus data shows that the increase in riverine phosphorus flux promoted oceanic eutrophication and deoxygenation. For the Lower Kellwasser event, the enhanced burial of organic matter in the ocean led to a drop in atmospheric CO₂ and associated climate cooling of about 0.5 to 1.5 °C, accompanied by a positive shift in carbon isotopes.4Communications Earth & Environment. The expansion of land plants during the Late Devonian contributed to the marine mass extinction In other words, the very success of terrestrial life helped starve the oceans of oxygen and cool the planet, both of which hammered marine ecosystems.
Oceans Starved of Oxygen
Ocean anoxia, the loss of dissolved oxygen in seawater, is the most consistently documented kill mechanism across the Late Devonian extinction pulses. Marine organisms that depend on oxygen-rich water, which is most of them, face a lethal squeeze when oxygen disappears. The evidence for widespread anoxia comes from multiple geochemical proxies preserved in rocks of this age.
During the Hangenberg Crisis at the end of the Devonian, uranium isotope records and trace-metal enrichments indicate that anoxic conditions expanded to cover more than 5% of the continental shelf seafloor area.5Earth and Planetary Science Letters. Extensive marine anoxia associated with the Late Devonian Hangenberg Crisis Five percent might not sound like much, but continental shelves are where the overwhelming majority of marine biodiversity lives. Losing oxygen across that fraction of productive seafloor is devastating.
In some settings, anoxia progressed into something even more toxic: euxinia, where hydrogen sulfide, a gas poisonous to most animals, builds up in the water column. Biomarker evidence from the Frasnian–Famennian boundary in Poland reveals spikes in isorenieratane, a chemical signature left by green sulfur bacteria that thrive only where hydrogen sulfide reaches the sunlit upper ocean.6PubMed Central. Coincidence of photic zone euxinia and impoverishment of arthropods in the aftermath of the Frasnian-Famennian biotic crisis When sulfidic water reaches the photic zone, it poisons not just bottom-dwellers but organisms throughout the water column. The coincidence of these euxinic pulses with sharp declines in arthropod diversity at the same sites makes a strong case that hydrogen sulfide was an active killer, not just a passive byproduct of stagnant water.
Cooling, Glaciation, and Falling Seas
The Late Devonian is increasingly recognized as a period of significant climate cooling, which contrasts with the popular image of ancient mass extinctions as heat-driven catastrophes. Oxygen isotope records from conodont fossils, tiny tooth-like structures from extinct eel-like animals that serve as excellent paleothermometers, record substantial temperature drops in tropical surface waters during the Kellwasser events. One study translates those isotope shifts into cooling of roughly 5 to 7 °C in low-latitude surface waters.7Geology. Conodont apatite δ18O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction A higher-resolution record from South China finds cooling of about 3 °C during the Lower Kellwasser event and about 6 °C during the Upper Kellwasser event.8Earth and Planetary Science Letters. Did climate changes trigger the Late Devonian Kellwasser Crisis? Evidence from a high-resolution conodont δ18OPO4 record from South China A tropical ocean losing several degrees is a seismic shift for marine life adapted to warm conditions.
This cooling was severe enough to trigger glaciation on the supercontinent Gondwana. Ice sheets developed in what is now South America during the Late Devonian, causing global sea-level drops that exposed and eroded carbonate platforms across North America, central Europe, and southern China.9Palaeogeography, Palaeoclimatology, Palaeoecology. Late Devonian–earliest Mississippian glaciation in Gondwanaland and its biogeographic consequences Sea-level regression shrinks the shallow marine habitats where most species live, compounding the stress from anoxia and cooling. The onset of glaciation also brought increased clastic sedimentation, reduced biochemical deposition, and widespread unconformities in the geological record.10Palaeogeography, Palaeoclimatology, Palaeoecology. Late Devonian glaciation in South America
The association of the Late Devonian extinction with cooling rather than warming fits a broader pattern: some of Earth’s great mass extinctions, including the Late Ordovician event, are linked to climate cooling rather than the greenhouse crises that drove the end-Permian and end-Triassic catastrophes.11PubMed Central. Selectivity of mass extinctions: Patterns, processes, and future directions
Volcanism and Mercury
Large igneous provinces, the massive outpourings of magma that have been implicated in several mass extinctions, were active during the Late Devonian. Three major volcanic provinces overlap the 380-to-360-million-year-ago window: the Yakutsk-Viluy province in Siberia, the Kola-Dnieper province in what was then Baltica, and the Magdalen silicic province in Laurussia.12PubMed Central. Volcanic related methylmercury poisoning as the possible driver of the end-Devonian Mass Extinction These eruptions would have pumped enormous quantities of carbon dioxide and sulfur into the atmosphere, potentially driving both warming and acid rain, but one of the more distinctive findings from the Devonian record is the mercury signal.
Mercury anomalies in sedimentary rocks spanning the extinction interval point to massive volcanic mercury emissions. Mercury itself is toxic, but the more insidious pathway involves methylmercury, the bioaccumulative form that concentrates up food chains. The same study identifies these volcanic provinces and the contemporaneous Almaden mercury deposit (one of the largest geochemical anomalies on Earth) as plausible sources for the extreme mercury loading observed in latest Devonian sediments.12PubMed Central. Volcanic related methylmercury poisoning as the possible driver of the end-Devonian Mass Extinction Whether methylmercury poisoning was a primary driver or an aggravating factor layered on top of anoxia and cooling is still debated, but the chemical fingerprint is there.
The Asteroid Question
Given how tightly the end-Cretaceous extinction is linked to an asteroid impact, it is natural to ask whether an impact caused the Late Devonian crisis too. There is some physical evidence: glass spherules resembling microtektites have been found near the Frasnian–Famennian boundary in Belgium, displaying teardrop and dumbbell shapes consistent with impact-generated debris. The Siljan Ring in Sweden and the Charlevoix structure in Quebec are candidate craters of approximately the right age.13Science. Microtektites and Mass Extinctions: Evidence for a Late Devonian Asteroid Impact
A “multiple impacts” hypothesis proposes that several Late Devonian impacts, including the Alamo event in Nevada, the Siljan impact, and the Flynn Creek impact, occurred within a narrow window and collectively disrupted climate by first warming and then rapidly cooling the planet.14Palaeogeography, Palaeoclimatology, Palaeoecology. The ‘multiple impacts hypothesis’ for mass extinction: a comparison of the Late Devonian and the late Eocene The idea is intriguing, but the impact hypothesis has never gained the dominance it holds for the end-Cretaceous event. None of the known Late Devonian craters are large enough, individually, to cause a global mass extinction. And the drawn-out, pulsed character of the die-off does not fit the sudden, one-two punch expected from a single large impact. Most researchers treat impacts as possible contributors rather than the primary cause.
Wilder Possibilities
Two more speculative hypotheses have attracted attention. One involves a nearby supernova explosion that could have stripped the ozone layer through a sustained barrage of cosmic rays, potentially lasting tens of thousands of years. Researchers have proposed that end-Devonian extinctions were triggered by supernovae at a distance somewhat beyond the “kill distance” that would have precipitated a full mass extinction, offering a partial explanation for the ozone damage seen in the record.15PubMed Central. Supernova triggers for end-Devonian extinctions
The ozone damage itself is not speculative. Malformed land plant spores discovered in east Greenland demonstrate that elevated UV-B radiation, consistent with ozone layer reduction, coincided with the Devonian–Carboniferous boundary extinction.16PubMed Central. UV-B radiation was the Devonian-Carboniferous boundary terrestrial extinction kill mechanism What caused that ozone depletion is the open question. A supernova is one possibility. Volcanic halogens from the large igneous provinces are another. Both remain difficult to test directly.
The atmospheric picture also includes evidence of elevated oxygen levels and widespread wildfires. Fossilized charcoal at the Frasnian–Famennian boundary in New York State indicates wildfire activity, and the charcoal record suggests atmospheric oxygen levels of roughly 23 to 25%, above today’s 21%.17Geological Society of America Bulletin. Evidence of wildfires and elevated atmospheric oxygen at the Frasnian–Famennian boundary in New York (USA): Implications for the Late Devonian mass extinction Elevated oxygen would have made terrestrial ecosystems more combustible, and fire-driven soil erosion could have further boosted the nutrient runoff that was already choking the oceans.
What the Extinction Actually Destroyed
The biological toll of the Late Devonian crisis was not spread evenly. Marine ecosystems bore the brunt, and within them, reef-building organisms were hit hardest. Stromatoporoids, the sponge-like animals that had been the dominant reef builders for tens of millions of years, and tabulate corals survived into the latest Famennian but were finally terminated during the Hangenberg Crisis.18Palaeogeography, Palaeoclimatology, Palaeoecology. The last episode of coral-stromatoporoid reef and faunal changes across a Devonian-Carboniferous Boundary section of South China The collapse of these reef ecosystems left a gap that lasted millions of years. The low point in abundance and distribution of shallow-water reefs occurred during the latest Famennian and early Tournaisian, with buildups constructed mainly by microbial mats and stromatolites rather than complex animal frameworks.19SEPM Society for Sedimentary Geology. Latest Devonian and Early Carboniferous Reefs: Depressed Reef Building After the Middle Paleozoic Collapse Coral-dominated reefs of the kind that had flourished throughout the Middle Devonian did not truly recover until the Mesozoic, over 100 million years later.
Among vertebrates, the extinction was paradoxically creative. The previously dominant armored fishes (placoderms), lobe-finned fishes, and acanthodians suffered catastrophic losses and never meaningfully recovered. In their place, groups that had been minor players during the Devonian, including ray-finned fishes, sharks, and early tetrapods, underwent major diversification events and went on to dominate all subsequent vertebrate faunas.20PubMed Central. End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates The extinction essentially reset the vertebrate playing field, and the winners of that reset are our ancestors.
A Speciation Shutdown, Not Just a Kill Spike
One of the more surprising findings from quantitative analysis of the Late Devonian fossil record challenges the standard framing of mass extinctions. During the Frasnian biodiversity crisis, extinction rates were actually not unusually high compared to the preceding Middle Devonian. Instead, biodiversity collapsed because speciation rates plummeted to anomalously low levels.21PubMed Central. Invasive Species and Biodiversity Crises: Testing the Link in the Late Devonian In plain terms, species were not dying off faster than normal. They were just not being replaced. New species stopped forming at anywhere near the rate needed to keep pace with background extinction.
This distinction matters for understanding causation. A sudden kill mechanism like an asteroid strike or a volcanic winter produces a spike in extinction rates. A prolonged environmental deterioration that homogenizes habitats, reduces ecological niches, and suppresses geographic isolation would instead suppress the origination of new species, exactly the pattern seen in the Late Devonian. The spread of invasive species across newly connected marine environments, driven by sea-level changes, has been proposed as one mechanism for this speciation suppression. When barriers between previously isolated marine basins disappeared, cosmopolitan species displaced regional ones without generating new diversity.
The World After
The aftermath of the Late Devonian extinction shaped vertebrate life for tens of millions of years. Following the end-Devonian crisis, vertebrates experienced persistent reductions in body size lasting at least 36 million years. The post-extinction world was dominated by small, fast-breeding ray-finned fishes, sharks, and tetrapods, most under a meter in length. Intriguingly, this global shrinkage was not related to oxygen levels or temperature, suggesting that ecological factors, such as the collapse of food webs and the loss of large predators, were the key drivers of the size trend.22PubMed. Body-size reduction in vertebrates following the end-Devonian mass extinction
The reef gap is another long shadow. With stromatoporoid-coral reef ecosystems gone, the Early Carboniferous oceans hosted microbial buildups that were structurally simpler and far less biodiverse than the complex reefs they replaced.19SEPM Society for Sedimentary Geology. Latest Devonian and Early Carboniferous Reefs: Depressed Reef Building After the Middle Paleozoic Collapse Complex metazoan reef ecosystems took staggeringly long to rebuild. The Devonian reef collapse is a reminder that some ecological structures, once dismantled, do not bounce back on human-relevant timescales, or even on timescales that would feel short to a geologist.
Why One Cause Will Never Be Enough
The search for “the” cause of the Late Devonian extinction keeps circling back to the same frustrating truth: the crisis was too long, too geographically variable, and too ecologically complex for any single mechanism. The plant-driven eutrophication hypothesis explains the anoxia and the carbon-isotope excursions beautifully but does not account for the cooling or the ozone damage. Volcanism explains the mercury anomalies and could have contributed to both warming and cooling at different phases, but the eruptions were spread over 20 million years and cannot be precisely tied to individual extinction pulses. Glaciation and sea-level fall explain the loss of shallow marine habitats but not the euxinia that poisoned deeper waters. Impacts remain possible contributors but lack a smoking-gun crater of sufficient size.
What the evidence increasingly supports is a cascading model: the expansion of forests increased nutrient runoff, which fertilized the oceans, which triggered anoxia, which buried organic carbon, which drew down CO₂, which cooled the climate, which eventually triggered glaciation, which dropped sea levels, which destroyed reef habitat. Volcanism and possibly impacts injected additional stress at various points. Ozone depletion, whether from volcanic emissions or astrophysical sources, added a terrestrial kill mechanism. Each stressor amplified the others. The Late Devonian extinction was, in a sense, Earth’s systems responding to an evolutionary revolution on land, the rise of forests, in ways that catastrophically destabilized the oceans. That no single cause is sufficient is not a failure of the science. It is the answer.