Devonian

Devonian
Name	Devonian
Color	Devonian
Top boundary def	FAD of the conodont Polygnathus costatus partitus
Top gssp location	Klonk, Czech Republic
Top gssp coords	49, 51, 18, N...
Top gssp acceptance	1972
Bottom boundary def	FAD of the graptolite Monograptus uniformis
Bottom gssp location	Klonk, Czech Republic
Bottom gssp coords	49, 51, 18, N...
Bottom gssp acceptance	1972
Time scale	ICS
Period	Devonian
Epoch	Late Devonian
Age	Famennian
Celestial body	earth
Usage	Global (ICS)
Timescales used	ICS Time Scale
Chrono unit	Period
Stratigraphic unit	System
Proposed by	Sedgwick & Murchison, 1839
Lower boundary def	FAD of the graptolite Monograptus uniformis
Lower gssp location	Klonk, Czech Republic
Lower gssp acceptance	1972
Upper boundary def	FAD of the conodont Siphonodella sulcata (discovered to have biostratigraphic issues as of 2006)
Upper boundary gssp location	La Serre, Montagne Noire, France
Upper boundary gssp acceptance	1990

Devonian. Spanning from approximately 419 to 359 million years ago, it is the fourth period of the Paleozoic Era, following the Silurian and preceding the Carboniferous. Often called the "Age of Fishes," it witnessed a dramatic diversification of marine life and the crucial colonization of land by plants and early tetrapods. The period concluded with a series of significant extinction pulses that reshaped global ecosystems.

Etymology and history of study

The period was formally named in 1839 by British geologists Adam Sedgwick and Roderick Murchison after the county of Devon in England, where its characteristic marine rocks were first studied. Their work built upon earlier observations in regions like the Rhineland and the Ardennes. The subdivision of the period into Early Devonian, Middle Devonian, and Late Devonian epochs was later refined through the identification of key index fossils, particularly graptolites and conodonts. The base of the period is now defined by a Global Boundary Stratotype Section and Point located at Klonk in the Czech Republic.

Geology

The stratigraphy is characterized by widespread deposition of both marine and terrestrial sediments. Major rock sequences from this interval include the Old Red Sandstone of the Anglo-Welsh Basin, the Hunsrück Slate of Germany, and the extensive reef complexes of the Canning Basin in Australia. These formations provide a rich fossil record. The period is also marked by significant orogenic activity, most notably the Acadian orogeny in Appalachia and the early phases of the Variscan orogeny in Europe, which contributed to mountain building.

Paleogeography

The paleogeographic configuration was dominated by the supercontinent Gondwana in the southern hemisphere, which included the future South America, Africa, India, Australia, and Antarctica. To the north, the continents Laurentia, Baltica, and Siberia were converging, with the closure of the Iapetus Ocean and the opening of the Rheic Ocean. This assembly created the smaller supercontinent Euramerica (or Laurussia), and vast, warm, shallow epicontinental seas covered much of its interior, such as the area that would become the Midwestern United States.

Climate

The climate is generally considered to have been warm and relatively stable for much of its duration, with no evidence of polar ice caps. High levels of atmospheric carbon dioxide contributed to a pronounced greenhouse climate. Evidence from evaporite deposits in regions like the Michigan Basin and paleobotanical data suggest seasonal aridity in continental interiors. Toward the end of the period, a significant cooling trend occurred, potentially linked to the spread of land plants and a drawdown of CO₂, which may have contributed to the Late Devonian extinction.

Life

Marine ecosystems were dominated by diverse placoderms, lobe-finned fish like Eusthenopteron, and early sharks such as Cladoselache. Extensive reef systems, built by tabulate corals, rugose corals, and stromatoporoid sponges, thrived in shallow seas. On land, this period saw a revolutionary transformation: vascular plants, including early lycophytes, trimerophytes, and the first Archaeopteris forests, colonized continents. This greening of the land facilitated the evolution of the first fully terrestrial arthropods and, by the late stages, the earliest tetrapods like Tiktaalik and Ichthyostega.

Extinction events

The period ended with a series of extinction pulses, collectively known as the Late Devonian extinction, one of the "Big Five" mass extinctions in Earth's history. The most severe phases are identified as the Kellwasser Event and the Hangenberg Event. These crises disproportionately affected marine life, devastating tropical reef communities, numerous brachiopod families, ammonoids, and all remaining placoderms. Proposed causes include widespread ocean anoxia, global cooling triggered by plant evolution, and potential extraterrestrial impacts, such as the suspected Siljan Ring crater in Sweden.

Category:Devonian Category:Geological periods