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DEVONIAN PERIOD (Fourth period of the Paleozoic Eon | 419.2 to 358.9 MYA)
TRILOBITE
AMMONITE
DUNKLEOSTEUS

Climate

The Devonian period is known for its sudden changes in the climate, often having sudden drawdowns of atmospheric carbon dioxide, massive volcanism, and rapid sea-level fluctuations. Many global events, such as the Devonian black shale events, were linked with severe oxygen depletion. The greening of land through the spreading and diversification of early plants also created new soil types and influenced Earth’s geochemical cycles and atmospheric climate. This greening was in part caused by the tectonic movement reshaping the land. All in all, the Devonian is a period of climatic chaos, with constantly fluctuating oxygen and carbon dioxide levels leading to the extinction of 75%-80% of all species.

Major Geologic Events

  • First Order Events:
  • Kellwasser Crisis: An extinction event that occurred at the Frasnian–Famennian boundary and led to the loss of important geologic groups and entire ecosystems, such as the metazoan reefs.
  • Hangenberg Crisis: An extinction event and the Devonian–Carboniferous boundary which was marked by a stepped mass extinction that wiped out majot ecosystems, including forests.
  • Second-Order Events: Characterized by sudden extinctions across many groups and ecosystems, resulting in the complete disappearance of several widespread orders and families, the basal Emsian atopus Event (where planktonic graptolites died out), the Taghanic Crisis, Frasnes events, and the Lower Kellwasser Event, all characterize these second-order Events.
  • Third-Order Events: Show globally elevated extinction rates, often at lower taxonomic levels (genera and species) across many clades and ecosystems, some examples include the Silurian–Devonian boundary Klonk Event, Daleje Event, Chotěc Event, Kačák Event, Condroz Event, and Annulata events.
  • Fourth-Order Events: Involve the sudden disappearance of relatively fewer, but widespread, groups due to a global trigger, some examples include the Lochkovian–Pragian boundary event, Chebbi Event, Upper Zlíchov Event, Middlesex Event, and Dasberg Event.

The Late-Devonian Mass Extinction

The Devonian ended in a mass extinction, where 22% of all marine families disappeared. The cause of the extinction is unknown, though some speculate that it was caused by global cooling from glaciations on Gondwana or an extraterrestrial impact. The development of forests might have also helped the extinction along by releasing an overabundance of nutrients into the water.

Major Evolutionary Developments

The Devonian period was also a time of evolution for marine and plant life. This era saw the proliferation of free-swimming cephalopods, including but not limited to some of Earth’s oldest ammonoids, as well as the rapid expansion of fish life, including toothed sharks and Giant Placoderms, a kind of heavily armored predatory fish. Evolution was not restrained to sea, as on land a massive diversification of plant life occurred, giving us our planet’s oldest forests, and plants with underground rhizomes (a kind of ancient root system), which reshaped the landscapes, forming new soil types. Plants were not alone on land during this period, however, as it showed the emergence of the earliest four-limbed land animals. The Devonian was not just a time of extinction, but immense evolutionary growth both on land and sea.

Significance

The Devonian period was greatly significant due to the increase of marine and plant diversity which prospered during it, as well as the advent of the first Tetrapods. This period gave, and at times took away, vast amounts of marine life, making it incredibly influential for life in our seas. The greening of the Earth also occurred during this time, forever shaping our land’s soil and plant life. Finally, the Devonian gave us the first ever vertebrates to step out of water, giving rise to the earliest tetrapods. The evolutionary and geologic significance of this period truly cannot be understated.

Sources

Becker, Ralph Thomas, Peter Königshof, and Carlton Elliot Brett. "Devonian climate, sea level and evolutionary events: an introduction." (2016).

Young, Gavin C. "Placoderms (armored fish): dominant vertebrates of the Devonian period." Annual Review of Earth and Planetary Sciences 38.1 (2010): 523-550.

Yang, Jiangnan, et al. "Fern-like plants establishing the understory of the late devonian xinhang lycopsid forest." Life 14.5 (2024): 602.

Jarvik, Erik. "The devonian tetrapod Ichthyostega." The Devonian tetrapod Ichthyostega. (1996). 1-214.