The Hangenberg Event marks the final chapter of the Devonian Period, approximately 359 million years ago. It represents the second of two major extinction pulses that together constitute the Late Devonian mass extinction. This event unfolded over a period estimated to be between 100,000 and 300,000 years, triggering a worldwide collapse of both marine and terrestrial ecosystems. The crisis was a turning point, altering the evolutionary trajectory of life and setting the stage for the Carboniferous Period.
Planetary Upheaval: What Triggered the Crisis?
The Hangenberg crisis was not caused by a single cataclysm but by a cascade of interconnected environmental changes. A primary trigger is thought to be rapid global cooling, driven by the expansion of the first large forests. During the Devonian, plants evolved deep root systems and grew to unprecedented sizes, with some trees reaching 30 meters in height. This “greening” of the land increased the weathering of silicate rocks, a process that pulls carbon dioxide (CO2) from the atmosphere.
This large-scale removal of a greenhouse gas is believed to have plunged the planet into a brief but intense ice age. Evidence for this glaciation comes from widespread glacial deposits found in places like northern Brazil, which was located near the South Pole during the Devonian. As ice sheets formed, global sea levels fell drastically, by perhaps more than 100 meters. This regression eliminated vast areas of shallow, warm-water marine habitats.
The environmental turmoil was compounded by widespread oceanic anoxia, or the loss of dissolved oxygen in the water. The sea-level fall and changes in ocean circulation disrupted the chemical balance of the seas. Nutrient runoff from the newly forested continents may have fueled massive algal blooms, and the decay of this organic matter would have consumed oxygen, creating suffocating “dead zones.”
Life’s Great Die-Off: Devonian Species on the Edge
The environmental crisis triggered a major loss of life, with the most severe impacts felt in the planet’s oceans. Marine ecosystems in the warm, shallow seas of the tropics were heavily impacted. The placoderms, a diverse class of armored fish that had dominated the Devonian seas, were wiped out completely. These predators, some of which were the largest vertebrates of their time, could not survive the rapid changes.
Other groups suffered immense losses. Ammonoids, shelled relatives of modern squid, were nearly annihilated, with evidence suggesting that all post-Devonian ammonoids descended from a single surviving lineage. Trilobites also took a heavy blow; the phacopid trilobites, characterized by their large, complex eyes, vanished entirely. The reef systems, built by stromatoporoid sponges and tabulate-rugose corals, collapsed and would not reappear for millions of years.
The extinction was not confined to the sea, as the Hangenberg Event also affected freshwater and terrestrial ecosystems. The fossil record shows a significant extinction among early tetrapods, the four-limbed vertebrates taking their first steps on land. This loss is linked to an interval in the fossil record known as “Romer’s Gap,” a period of about 15 million years in the subsequent Carboniferous from which very few terrestrial vertebrate fossils are found.
A New Dawn: Life’s Resurgence in the Carboniferous
In the wake of the Hangenberg Event, depleted ecosystems provided opportunities for surviving lineages to diversify. The recovery period saw a restructuring of life, particularly in the oceans, paving the way for more modern-looking marine communities. With the dominant placoderms gone, other fish groups rose to prominence, including sharks and the ray-finned fishes (Actinopterygii).
This radiation involved rapid evolutionary innovation. For instance, many early Carboniferous ray-finned fishes and sharks developed powerful jaws capable of crushing hard shells, a feeding strategy known as durophagy. This allowed them to exploit the abundant shelled invertebrates like brachiopods and crinoids that had also survived. The success of the ray-finned fishes was so profound that they went from being a minor group in the Devonian to the dominant group of fishes in the Carboniferous.
The recovery also highlights a phenomenon known as “Lazarus taxa.” This term describes groups that disappear from the fossil record during an extinction event, only to reappear millions of years later. This pattern suggests that these groups survived in extremely low numbers or in isolated refuge areas, making them invisible in the fossil record until their populations could rebound. The ecosystems that emerged were permanently transformed from those of the Devonian.
Reading the Earth’s Diary: Evidence of the Hangenberg Event
Scientists reconstruct the Hangenberg Event by studying clues in rock layers. The most direct evidence is the Hangenberg Black Shale, a dark, organic-rich rock layer found globally. Its formation indicates widespread anoxia, as the lack of oxygen prevented organic matter from decaying, allowing it to accumulate on the seafloor. This layer serves as a marker bed signaling the onset of the crisis.
The fossil record shows a clear turnover in species. In strata from before the event, certain fossils are abundant, but directly above the black shale layer, they abruptly disappear. The fossils of survivor groups and new species then begin to appear in the overlying Carboniferous rocks. This sharp change is a primary indicator of a mass extinction.
Geochemical analysis offers a more detailed picture of the environmental changes. A significant positive shift in carbon isotopes (δ13C) indicates that a massive amount of organic carbon was buried, consistent with the anoxia hypothesis. Additionally, enrichments of trace metals like vanadium and uranium in the sediments act as proxies, confirming that ocean waters were depleted of oxygen.