The Devonian Period represents an ancient geological era spanning millions of years. This distant past was marked by significant and profound extinction events. Understanding what might have led to these widespread disappearances of life remains a compelling area of scientific investigation.
The Devonian Period and its Extinction Events
The Devonian Period, often called the “Age of Fishes,” saw remarkable diversification of marine life, including various fish species, and the initial spread of vascular plants onto land. This 60-million-year interval (419 to 359 million years ago) was not a single, abrupt extinction. Instead, the “Devonian extinction” refers to a series of biodiversity declines over millions of years, particularly affecting tropical marine species. The most severe, the Frasnian-Famennian extinction (around 372 million years ago), resulted in the loss of a substantial portion of Earth’s species. This prolonged crisis profoundly affected marine ecosystems, leading to widespread disappearances.
Leading Scientific Theories for the Devonian Extinction
Global cooling and widespread glaciation are prominent hypotheses for the Devonian extinction. Glacial deposits in ancient Gondwana indicate large ice sheet formation. Extensive ice formation would have drawn vast water from oceans, substantially dropping global sea levels. This sea-level fall reduced shallow marine habitats, stressing these diverse ecosystems.
Widespread ocean anoxia, or oxygen depletion, is another theory, possibly driven by increased nutrient runoff. Rapid land plant proliferation during the Devonian could have accelerated rock weathering, releasing nutrients like phosphorus into oceans. This influx might have fueled massive algal blooms; as these died and decomposed, they consumed vast dissolved oxygen. Such conditions created extensive “dead zones” where most marine life could not survive.
Large-scale volcanic activity, especially Large Igneous Provinces (LIPs), explains environmental disruptions. The Viluy Traps in Siberia, for example, were active around the Frasnian-Famennian event. These eruptions released enormous greenhouse gases, potentially causing warming, or aerosols that could have caused cooling by blocking sunlight. Such volatile atmospheric changes could destabilize global climates, contributing to ecosystem collapse.
Though less favored as a primary cause, asteroid impact has been investigated, similar to the end-Cretaceous extinction. Some formations, like the Siljan impact structure in Sweden, have been considered, but their dating relative to extinction pulses remains debated. While an impact could cause localized devastation and environmental stress, the prolonged Devonian extinction suggests multiple, interacting factors rather than a single catastrophic strike.
Environmental Changes and Biological Impacts
Proposed causes of the Devonian extinction led to cascading environmental changes, directly impacting marine ecosystems. Global cooling and glaciation, for instance, caused a significant sea-level drop, reducing shallow, warm water environments where many organisms thrived. This habitat reduction placed immense pressure on species adapted to these conditions. Periods of warming, possibly linked to volcanism, also altered ocean temperatures, further stressing temperature-sensitive marine life.
Widespread anoxic conditions in oceans are evident in the geological record by extensive black shales from this period. These dark, organic-rich sediments form in oxygen-depleted waters where organic matter does not fully decompose. The proliferation of anoxic zones suffocated vast seafloor areas, eliminating habitats for bottom-dwelling organisms and disrupting marine food webs. Changes in ocean chemistry, including acidification or nutrient shifts, further compounded these challenges.
Biological impacts were severe, particularly on marine invertebrates foundational to Devonian ecosystems. Reef-building organisms, including rugose and tabulate corals, suffered catastrophic declines, collapsing extensive reef systems of the earlier Devonian. Brachiopods, a type of shelled marine invertebrate, also experienced significant losses, as did trilobites (already in decline). Certain fish groups, including placoderms, also faced substantial reductions in diversity and abundance, reshaping the marine vertebrate fauna of the subsequent Carboniferous Period.