The Devonian extinction was one of the five worst mass extinctions in Earth’s history, wiping out roughly 70-80% of all species over a prolonged period near the end of the Devonian period, between 374 and 359 million years ago. Unlike the asteroid strike that killed the dinosaurs in a geological instant, this extinction played out in at least two major pulses separated by about 15 million years, making it one of the most drawn-out biodiversity crises the planet has ever experienced.
Two Extinction Pulses, Not One
The Devonian extinction is better understood as a series of crises rather than a single event. The first and more famous pulse, called the Kellwasser event, struck at the boundary between the Frasnian and Famennian stages around 374 million years ago. This is the event that earns the Devonian extinction its spot among the “Big Five” mass extinctions. It hit tropical marine life especially hard, devastating reef ecosystems and wiping out many families of ocean-dwelling organisms.
The second pulse, the Hangenberg event, arrived roughly 15 million years later at around 359 million years ago, right at the boundary between the Devonian and Carboniferous periods. For a long time, scientists considered the Hangenberg event relatively minor. But more recent analysis of vertebrate diversity tells a different story: the Hangenberg event caused a 32% loss in genus-level diversity among vertebrates, compared to about 19% for the Kellwasser event. In other words, the supposedly lesser crisis was actually more devastating for backboned animals, creating a bottleneck that shaped the early evolution of modern jawed vertebrates.
What Caused It
There is no single smoking gun for the Devonian extinction, which is part of what makes it so unusual among mass extinctions. Instead, multiple environmental disruptions appear to have piled on top of each other over millions of years.
Massive Volcanic Eruptions
At least three large igneous provinces were actively erupting during the Late Devonian interval between 380 and 360 million years ago. The Yakutsk-Viluy system in Siberia covered an area of about 800,000 square kilometers. The Kola-Dnieper province in what is now Eastern Europe was even larger, at roughly 3 million square kilometers. A third, the Magdalen province, was also active during this window. These weren’t single eruptions but sustained volcanic episodes that poured gases and toxic metals into the atmosphere over long stretches of time.
Mercury concentrations in marine sediments from this period provide direct chemical evidence of this volcanism. In rock sections from the Carnic Alps (straddling modern Italy and Austria), researchers found mercury spikes 12 to 100 times higher than background levels, with peak concentrations exceeding 20,000 parts per billion. Volcanic and hydrothermal activity are the primary sources of such extreme mercury enrichment in sedimentary rocks. Once this mercury reached the ocean, microbes could have converted it into methylmercury, an organic form that is highly toxic to aquatic life and accumulates up the food chain. This methylmercury poisoning has been proposed as a direct killing mechanism during the Hangenberg event.
Ocean Suffocation
Widespread ocean deoxygenation was a recurring feature of the Late Devonian. Chemical signatures in marine carbonates show that oxygen-depleted zones expanded into shallower waters during the extinction pulses, suffocating organisms that depended on well-oxygenated environments. The pattern is similar to other major anoxic events in Earth’s history, including several during the Cretaceous period. For marine life already stressed by other factors, losing oxygen in their habitat was often the final blow.
The Spread of Land Plants
One of the more surprising contributors to this marine extinction was happening on land. The Devonian was the period when plants first developed deep root systems and spread across continents in large numbers. As these early forests expanded, their roots broke down rock through chemical weathering at accelerating rates, releasing nutrients like phosphorus into rivers and eventually the ocean. This nutrient flood promoted massive algal blooms in shallow seas. When those algae died and decomposed, the process consumed enormous amounts of dissolved oxygen, reinforcing the anoxic conditions that were already killing marine life. In essence, the greening of the land helped poison the sea.
Rapid Global Cooling
On top of everything else, the Late Devonian saw a dramatic shift in global climate. Glacial deposits from this period have been found not only in high-latitude regions of South America (which was part of the supercontinent Gondwana) but also in the Appalachian basin of the eastern United States, which was at a much lower latitude. At 36 separate localities in Appalachia, researchers identified features best explained by glacial processes, including deposits of glacial debris overlain by lake sediments with dropstones (rocks dropped from melting ice). This cooling coincided with a sharp drop in global sea level, which would have exposed and destroyed shallow marine habitats where much of ocean life was concentrated. The cooling episode is thought to mark the end of a warm period that had persisted since the middle of the Silurian, tens of millions of years earlier.
What Died
The Devonian extinction reshaped life on Earth in ways that are still visible in the fossil record. Its victims were overwhelmingly marine, and the losses were staggering in certain groups.
Reef ecosystems suffered the most dramatic collapse. During the Middle Devonian, shallow seas hosted vast reef complexes that stretched from roughly 50 degrees south to 60 degrees north of the equator. Many of these reef systems were far larger in area than today’s Great Barrier Reef. They were built primarily by rugose and tabulate corals along with stromatoporoids, a type of calcified sponge. These reef communities also supported brachiopods, bryozoans, and large single-celled organisms called foraminifera. The extinction pulses collapsed and nearly eliminated these reef ecosystems entirely. About 50% of coral genera and 47% of stromatoporoid genera disappeared during the Taghanic event alone, which preceded the two main pulses. By the end of the Devonian, stromatoporoid-coral reef systems had vanished. Reef ecosystems of comparable scale would not return for roughly 100 million years.
Placoderms, the armored fish that dominated Devonian oceans, were driven to complete extinction by the Hangenberg event. Trilobites, already in decline, lost most of their remaining diversity and never recovered, limping along as a shadow of their former selves until their final extinction in the Permian. Many families of brachiopods, which had been among the most common shelled animals on the seafloor, also disappeared.
How It Reshaped Vertebrate Life
Perhaps the most consequential legacy of the Devonian extinction is what it did to vertebrates. Statistical comparisons of fossil sites show that vertebrate communities before and after the Hangenberg event were dramatically different. When researchers measured how similar animal communities were on either side of this boundary, the difference was stark: faunas from the Famennian stage (the last stage of the Devonian) and the Tournaisian stage (the first stage of the Carboniferous) scored as highly dissimilar. By contrast, the same analysis could barely distinguish between vertebrate communities from before and after the earlier Kellwasser event.
This means the Hangenberg event, not the more famous Kellwasser event, was the true turning point for vertebrate evolution. The extinction of placoderms and the severe reduction of other Devonian fish groups created ecological space for new lineages. Ray-finned fish, sharks, and the ancestors of all land-living vertebrates diversified in the aftermath. The bottleneck created by the Hangenberg event essentially cleared the evolutionary stage and set the trajectory for the vertebrate world that followed, including, eventually, the rise of tetrapods on land.
Why It Took So Long to Understand
The Devonian extinction has always been the hardest of the Big Five to pin down. Its drawn-out, multi-pulse nature made it difficult to study compared to the sharp boundary left by the asteroid impact that ended the Cretaceous. For decades, the Kellwasser event received nearly all the attention while the Hangenberg event was dismissed as minor. Only with more detailed fossil databases and geochemical tools, like mercury concentration analysis and oxygen-level proxies in marine carbonates, have scientists begun to appreciate the full scope of both events and the complex interplay of causes behind them. The picture that has emerged is of an extinction driven not by any single catastrophe but by a perfect storm of volcanism, ocean chemistry changes, climate shifts, and even the evolution of plants, all converging over millions of years to fundamentally restructure life on Earth.