Late Devonian Extinction: The Kellwasser and Hangenberg Events

The Late Devonian extinction was a series of mass extinction pulses that struck life in the oceans between about 372 and 359 million years ago, at the end of the Devonian Period. Its two main blows, the Kellwasser and Hangenberg events, wiped out an estimated three-quarters of marine species, destroyed the largest reef systems the planet had yet seen and ended the age of armoured fishes. It is counted among the “Big Five” mass extinctions and reshaped the animals that would go on to colonise land and sea.

Background: the Devonian world

The Devonian Period lasted from about 419 to 359 million years ago. It is often called the “Age of Fishes”, because jawed fishes diversified dramatically in its seas and rivers. Armoured placoderms, early sharks, ray-finned fishes and lobe-finned fishes all flourished, alongside many jawless fishes with bony head shields.

Warm, shallow seas covered large parts of the continents. In them grew enormous reefs built not by modern corals but by stromatoporoid sponges together with tabulate and rugose corals. Devonian reefs are thought to have covered a far larger area than reefs do today, stretching across what are now western Canada, Europe, North Africa and Australia.

On land, the Devonian was a time of revolution. Plants evolved roots, wood and seeds, and by the Late Devonian the first true forests had appeared, dominated by trees such as Archaeopteris. Towards the end of the period the first four-limbed vertebrates, or tetrapods, were living in shallow waters. The extinction would cut through this flourishing world in several stages.

What happened: a crisis in pulses

Unlike the sudden asteroid impact that ended the age of dinosaurs, the Late Devonian extinction was drawn out. Palaeontologists recognise several separate events spread over millions of years, of which two stand out.

The Kellwasser event

The first and most damaging pulse came at the boundary between two stages of the Late Devonian, the Frasnian and the Famennian, about 372 million years ago. It is therefore often called the Frasnian–Famennian extinction. It is also named the Kellwasser event after two dark, organic-rich limestone and shale beds, the Lower and Upper Kellwasser horizons, first studied in the Kellwasser valley in the Harz Mountains of Germany.

These black layers record times when sea floors over wide areas were starved of oxygen. Similar horizons are found in Morocco, North America, China and elsewhere, showing that the disturbance was global. Chemical signals in the rocks, including shifts in carbon isotopes, point to major changes in the carbon cycle and to cooling of tropical seas at the same time.

The victims were overwhelmingly marine and tropical. The great stromatoporoid–coral reefs collapsed and never returned on the same scale. Brachiopods, which were among the most common shelled animals of the time, suffered heavy losses, as did trilobites, ammonoids, conodonts (small eel-like animals known mainly from their tooth-like parts) and many fishes.

The Hangenberg event

The second major pulse, the Hangenberg event, closed the Devonian Period about 359 million years ago. It takes its name from the Hangenberg Shale, another black shale in the Sauerland region of western Germany. This time the oceans again lost oxygen, and there is good evidence for glaciers spreading across the southern supercontinent of Gondwana, in what is now South America and Africa.

The Hangenberg event hit vertebrates especially hard. The placoderms, including giant predators such as Dunkleosteus, disappeared entirely. Many lineages of lobe-finned fishes, jawless fishes and early tetrapods were also lost. Ammonoids were almost wiped out, and the surviving trilobites were reduced to a single order.

A crisis of births as well as deaths

A distinctive feature of the Late Devonian crisis is that the rate at which new species appeared fell sharply. Studies of the fossil record suggest that low origination, not only high extinction, explains much of the drop in diversity. In other words, life did not simply die out faster; it also failed to replace what was lost.

Causes: a long-running debate

No single cause is accepted by all researchers. Most explanations focus on the oceans losing oxygen, but they differ on what drove that change.

Forests and nutrient runoff

One influential idea links the extinction to the spread of land plants. As deep-rooted trees colonised the continents, they broke up rock and soil far more effectively than before. This could have washed large quantities of nutrients such as phosphorus into the seas, causing algal blooms. When the algae died and decayed, they used up oxygen in the water, creating “dead zones” similar to those caused by fertiliser runoff today.

The spread of forests would also have drawn carbon dioxide out of the atmosphere, cooling the climate. This hypothesis fits the timing of the black shales and explains why tropical shallow-water life suffered most, but it is difficult to prove directly.

Volcanism

Large volcanic provinces were active in the Late Devonian, notably the Viluy Traps in eastern Siberia. Some studies have dated parts of these eruptions close to the Kellwasser event. Volcanic gases could have caused warming, acid rain and ocean chemistry changes, but the precise timing and scale remain under study.

Impacts and other ideas

In his presidential address to the Paleontological Society, published in 1970, the Canadian palaeontologist Digby McLaren proposed that an asteroid impact caused the Frasnian–Famennian extinction. Several impact craters of roughly Late Devonian age are known, including Siljan in Sweden and the Alamo impact structure in Nevada. However, none has been convincingly dated to the extinction pulses, and most researchers no longer see impacts as the main cause.

For the Hangenberg event, researchers have also pointed to glaciation and sea-level fall, and to evidence of malformed plant spores that may indicate damage to the ozone layer. A more speculative suggestion is that a nearby supernova raised ultraviolet radiation. These ideas remain debated.

Key people and research

The Late Devonian extinction was recognised gradually as palaeontologists compiled the fossil record. In 1982 David Raup and Jack Sepkoski identified it as one of five great mass extinctions in the marine fossil record, giving rise to the idea of the “Big Five”.

In the 1980s and 1990s German geologists, notably Otto Walliser, developed the concept of named global “events” recorded in rock layers, including the Kellwasser and Hangenberg horizons. The American palaeontologist George McGhee wrote the standard books on the subject, The Late Devonian Mass Extinction (1996) and When the Invasion of Land Failed (2013).

In 2010 Lauren Sallan and Michael Coates argued that the Hangenberg event was a bottleneck for jawed vertebrates. They concluded that the groups that dominate today, such as ray-finned fishes, sharks and tetrapods, owe their success partly to having survived it.

By the numbers

Figures vary between studies because they depend on how the fossil record is counted and which intervals are included.

Consequences and legacy

The loss of the Devonian reefs had lasting effects. Large reef systems built by animals remained rare for many millions of years, and the next great reef builders, the modern stony corals, did not appear until the Triassic. Brachiopods, trilobites and ammonoids all recovered in reduced or altered forms.

Among vertebrates, the extinction cleared the way for new winners. With the placoderms gone, sharks and ray-finned fishes diversified in the early Carboniferous. On land, the Devonian tetrapods that had evolved from fishes such as Tiktaalik were reduced to a few surviving lineages. A poorly known interval of about 15 million years in the early Carboniferous, known as “Romer’s gap”, followed before tetrapods appear again in abundance.

The extinction also matters for understanding the present. Rapid environmental change, ocean warming or cooling, nutrient pollution and the spread of low-oxygen water are all concerns for modern seas. The Late Devonian shows how such stresses, acting together over long periods, can push ecosystems past breaking point.

Myths and facts

A common assumption is that all mass extinctions were sudden catastrophes. The Late Devonian crisis was the opposite: a slow-burning series of pulses separated by millions of years.

It is also sometimes said that the extinction killed most life on Earth. In fact land plants and land animals were relatively little affected compared with marine life, especially tropical shallow-water communities. The scale of loss was severe, but smaller than that of the later Permian–Triassic extinction, the worst in Earth’s history. It followed the earlier Late Ordovician mass extinction, the first of the Big Five.

Timeline

• c. 419 million years ago: The Devonian Period begins.
• c. 385 million years ago: The first forests spread on land, and smaller extinction events affect marine life in the Middle Devonian.
• c. 372 million years ago: The Kellwasser event at the Frasnian–Famennian boundary devastates reefs and tropical marine life.
• c. 365–360 million years ago: Early tetrapods live in shallow waters while placoderms such as Dunkleosteus still hunt in the seas.
• c. 359 million years ago: The Hangenberg event ends the Devonian; the placoderms die out.
• 1970: Digby McLaren’s published address proposes an asteroid impact as the cause of the Frasnian–Famennian extinction.
• 1982: Raup and Sepkoski identify the Late Devonian crisis as one of the five great mass extinctions.
• 2010: Sallan and Coates describe the Hangenberg event as a bottleneck in vertebrate evolution.

Why it matters

The Late Devonian extinction destroyed the most extensive reefs in Earth’s history and ended the reign of the armoured fishes. By deciding which groups of fishes and early tetrapods survived, it helped shape the ancestry of most living vertebrates, including humans. It remains a key case study of how long-term environmental stress, rather than a single disaster, can trigger a mass extinction.