The history of life on Earth is marked by five major biological crises, or mass extinctions. The Late Devonian extinction is arguably the most complex due to its protracted nature. This episode of profound biodiversity loss did not occur in a single, sudden moment. Instead, the crisis unfolded as a series of distinct extinction pulses spread over millions of years, making the question of “when” a complicated one. Paleontological evidence reveals a prolonged interval of environmental instability and biological decline that reshaped marine ecosystems worldwide.
Defining the Late Devonian Extinction Event
The overall timeframe for the Late Devonian extinction spans the latter half of the Devonian Period, an extended interval of environmental turmoil. This period of heightened extinction rates began around 375 million years ago (Ma), continuing through to the end of the Devonian at approximately 359 Ma. While the entire interval saw significant species loss, the most devastating phase is precisely dated to the boundary between the Frasnian and Famennian geological ages.
This critical juncture, known as the Frasnian/Famennian (F/F) boundary, is constrained by modern dating techniques to about 372 million years ago. The severity of the event at this boundary is so pronounced that it is often referenced as the Late Devonian extinction itself. Geologists refer to the most intense phase of this crisis as the Kellwasser Event complex, named after distinct layers of black shale found in Germany. The presence of these specific rock layers provides a global marker for the most widespread marine collapse during the Late Devonian.
The total duration of the prolonged crisis is difficult to pin down, with estimates ranging up to 25 million years. This extended span across the Late Devonian makes it an unusual mass extinction, contrasting sharply with the relatively abrupt crises of other geological periods. The main pulse at the F/F boundary represents the single most significant drop in biodiversity within this extended period of faunal decline.
The Chronology of the Extinction Pulses
The protracted nature of the Late Devonian event is best understood by examining its two most distinct extinction pulses, which occurred millions of years apart. The first and most severe is the Kellwasser Event, tightly correlated with the Frasnian/Famennian boundary around 372 Ma. This pulse is characterized in the rock record by two distinct, globally synchronous layers of anoxic black shale, referred to as the Lower and Upper Kellwasser horizons. The widespread deposition of these black shales indicates a sudden and massive depletion of dissolved oxygen in the world’s oceans, devastating marine life, particularly tropical reef-building organisms.
This initial pulse inflicted heavy losses upon organisms living in shallow, warm seas, including trilobites, brachiopods, and the coral species that constructed the massive Devonian reefs. The collapse of these complex reef ecosystems was a major consequence of the Kellwasser Event, resulting in a long-term hiatus in reef building that persisted for millions of years. The subsequent Famennian age, which followed the Kellwasser crisis, was a period of continued environmental stress rather than smooth recovery.
The second major pulse, known as the Hangenberg Event, occurred much later, marking the very end of the Devonian Period at the boundary with the Carboniferous, approximately 359 million years ago. This event represents a distinct, second wave of collapse, separated from the Kellwasser crisis by a gap of about 13 million years. The Hangenberg Event is also defined by a black shale layer and is associated with a rapid, widespread sea level fall.
While less severe than the Kellwasser Event in terms of overall species loss, the Hangenberg pulse caused a final, deep cut into the surviving Devonian fauna. It notably impacted different groups, leading to the final extinction of key Devonian groups like the armored placoderm fish. Analyzing the distinct timing and different biological targets of the Kellwasser and Hangenberg pulses is crucial for understanding why the Late Devonian extinction cannot be assigned a single, simple date.
Leading Hypotheses for the Environmental Collapse
The complexity of the Late Devonian extinction suggests that multiple, possibly interacting, environmental changes drove the successive extinction pulses. One of the most common explanations, particularly for the Kellwasser Event, is the widespread onset of global anoxia, or oxygen depletion, in the oceans. Geochemical signatures and the deposition of organic-rich black shales provide strong evidence that deep and shallow waters became euxinic, containing high levels of toxic hydrogen sulfide. This anoxia may have been triggered by massive nutrient runoff from the continents.
This runoff was possibly linked to the rapid expansion of rooted land plants that enhanced soil formation and weathering. Another major hypothesis points to large-scale volcanism as a potential environmental trigger for the crisis. The eruption of vast continental flood basalts, such as the Viluy Traps in Siberia, could have injected enormous volumes of sulfur dioxide and carbon dioxide into the atmosphere.
Initial greenhouse warming from the carbon dioxide may have been followed by a rapid cooling phase caused by sulfur aerosols blocking sunlight, leading to significant climatic disruption. The dating of the first phase of the Viluy Traps volcanic activity shows a correlation with the timing of the Kellwasser Event.
A third prominent theory, particularly relevant to the later Hangenberg Event, is the role of rapid global cooling and glaciation. Evidence suggests a shift toward colder conditions during the Late Devonian, culminating in the growth of ice sheets on the supercontinent Gondwana. This glaciation would have caused a substantial eustatic sea level drop, draining shallow continental seas where much of the diverse marine life lived. The loss of these extensive shallow-water habitats provides a strong mechanism for the distinct second wave of extinction.