The Late Devonian Mass Extinction, occurring roughly between 375 and 359 million years ago, was one of Earth’s “Big Five” mass extinctions. This event significantly reshaped marine ecosystems globally.
The Devonian World and the Extinction Event
The Devonian Period (419 to 359 million years ago) featured widespread shallow seas with diverse marine life. Extensive reef systems, built by corals and stromatoporoids, flourished in these warm waters. On land, early vascular plants formed the first forests. The first tetrapods also appeared towards the end of this period.
The Late Devonian extinction was a series of pulses over several million years. Two major phases are recognized: the Frasnian-Famennian boundary event (Kellwasser Event) around 372 million years ago, and the end-Devonian Hangenberg Event around 359 million years ago. These events led to a significant loss of marine life, particularly in warm, shallow marine environments.
Leading Theories for the Extinction
Scientists propose several hypotheses for the Late Devonian extinction, suggesting a complex interplay of environmental factors. One theory involves global climate change, with evidence of abrupt cooling linked to Southern Hemisphere glaciers and sea level drops. Rapid warming pulses are also considered.
Ocean anoxia, a lack of oxygen in oceans, is another widely discussed cause. This condition likely resulted from increased nutrient runoff from continents, due to early land plants. Deep-rooted plants would have enhanced rock weathering, releasing nutrients into the oceans. This influx could have triggered massive algal blooms, which, upon decomposition, consumed vast amounts of oxygen, creating oceanic “dead zones.”
Extensive volcanism is also considered a potential contributor, with large volcanic provinces suggested as sources. Volcanic eruptions release large quantities of greenhouse gases into the atmosphere. Such emissions could have caused rapid global warming, ocean acidification, and deoxygenation, leading to widespread marine extinctions. Mercury anomalies in Late Devonian strata globally also suggest a link to extensive volcanism.
Other hypotheses include extraterrestrial impacts. While some evidence of impacts exists, a direct causal link to the extinction pulses remains debated. Rapid sea-level fluctuations also played a role, with some rises associated with anoxic conditions, further stressing marine ecosystems. The Late Devonian extinction was likely the result of a combination of these environmental stressors, rather than a single isolated event.
Biological Impact and Recovery
The Late Devonian extinction disproportionately affected marine life, particularly in warm, shallow waters. Reef-building organisms suffered severe losses, with corals and stromatoporoids nearly disappearing. This led to a significant reduction in reef-building activity. Other marine invertebrates also experienced substantial declines. Around 70-85% of all marine species may have perished, including about 75% of all fish species.
Among vertebrates, placoderm fish, the armored jawed fish that were dominant marine predators, saw around half of their families go extinct. Most jawless fish groups also disappeared by the end of the Frasnian age. This significant reduction in biodiversity created ecological bottlenecks, leading to simplified ecosystems for a prolonged period.
The impact on terrestrial life was less severe, though early land plants and arthropods experienced some losses. Following the extinction, ecosystems underwent a long period of recovery and restructuring. The subsequent Carboniferous period saw the adaptive radiation of surviving groups, with new dominant life forms emerging. This allowed for the diversification of lineages like sharks, ray-finned fishes, and early tetrapods.
Unraveling the Past Through Evidence
Scientists reconstruct the events of the Late Devonian extinction by examining various lines of geological and paleontological evidence. The fossil record provides direct insights, revealing the sudden disappearance of numerous marine species across geological strata. Analysis of fossil assemblages helps identify which groups were most affected and the timing of their declines. The near-total loss of reef-building organisms is clearly visible in the fossil record.
Geological evidence plays a significant role in understanding environmental changes. Widespread black shales, distinctive dark, organic-rich sedimentary layers, indicate periods of ocean anoxia or low-oxygen conditions. These black shales are found globally and correlate with extinction pulses, suggesting a link between deoxygenated waters and species loss. Changes in isotopic ratios in ancient rocks and fossils offer clues about shifts in ocean chemistry and global carbon cycles, indicative of climate fluctuations and biological productivity changes.
Further insights are provided by the analysis of microscopic fossils and palynology. Microfossil analysis helps date geological layers and correlate extinction events. Palynology, the study of fossil spores and pollen, offers information about terrestrial plant communities and environmental shifts. These diverse forms of evidence collectively help scientists piece together the complex narrative of the Late Devonian Mass Extinction.