|Time span formality||Formal|
|Lower boundary definition||FAD of the Graptolite Monograptus uniformis|
|Lower boundary GSSP||Klonk, Czech Republic|
|Lower GSSP ratified||1972|
|Upper boundary definition||FAD of the Conodont Siphonodella sulcata (discovered to have biostratigraphic issues as of 2006).|
|Upper boundary GSSP||La Serre, Montagne Noire, France|
|Upper GSSP ratified||1990|
|Atmospheric and climatic data|
|Sea level above present day||Relatively steady around 189 m, gradually falling to 120 m through period|
The Devonian (
The first significant
The first ammonites, a subclass of molluscs, appeared. Trilobites, the mollusc-like brachiopods, and the great coral reefs were still common. The Late Devonian extinction which started about 375 million years ago severely affected marine life, killing off all placodermi, and all trilobites, save for a few species of the order Proetida.
The period is named after Devon, a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found distributed throughout the county was eventually resolved by the definition of the Devonian Period in the geological timescale. The Great Devonian Controversy was a long period of vigorous argument and counter-argument between the main protagonists of Roderick Murchison with Adam Sedgwick against Henry De la Beche supported by George Bellas Greenough. Murchison and Sedgwick won the debate and named the period they proposed as the Devonian System.[a]
In 19th-century texts the Devonian has been called the "Old Red Age", after the red and brown terrestrial deposits known in the United Kingdom as the Old Red Sandstone in which early fossil discoveries were found. Another common term is "Age of the Fishes", referring to the evolution of several major groups of fish that took place during the period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian, Breconian, and Farlovian stages, the latter three of which are placed in the Devonian.
The Devonian has also erroneously been characterised as a "greenhouse age", due to sampling bias: most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America, which at the time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid. In fact the climate in the Devonian differed greatly during its epochs and between geographic regions. For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, Australia, North America, and China, but Africa and South America had a warm temperate climate. In the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common.
The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to those epochs are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System.
- Early Devonian
- Middle Devonian
The Middle Devonian comprised two subdivisions: first the
- Late Devonian
Finally, the Late Devonian started with the Frasnian, to , during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing Famennian subdivision, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian, .
The Devonian was a relatively warm period, and probably lacked any glaciers for much of the period. The temperature gradient from the equator to the poles was not as large as it is today. The weather was also very arid, mostly along the equator where it was the driest. Reconstruction of tropical sea surface temperature from conodont apatite implies an average value of 30 °C (86 °F) in the Early Devonian. CO2 levels dropped steeply throughout the Devonian Period. The newly evolved forests drew carbon out of the atmosphere, which were then buried into sediments. This may be reflected by a Mid-Devonian cooling of around 5 °C (9 °F). The Late Devonian warmed to levels equivalent to the Early Devonian; while there is no corresponding increase in CO2 concentrations, continental weathering increases (as predicted by warmer temperatures); further, a range of evidence, such as plant distribution, points to a Late Devonian warming. The climate would have affected the dominant organisms in reefs; microbes would have been the main reef-forming organisms in warm periods, with corals and stromatoporoid sponges taking the dominant role in cooler times. The warming at the end of the Devonian may even have contributed to the extinction of the stromatoporoids. At the terminus of the Devonian, Earth rapidly cooled into an icehouse, marking the beginning of the Late Palaeozoic Ice Age.
The Devonian world involved many continents and ocean basins of various sizes. The largest continent,
Sea levels were high worldwide, and much of the land lay under shallow seas, where tropical reef organisms lived. The enormous "world ocean", Panthalassa, occupied much of the Northern Hemisphere as well as wide swathes east of Gondwana and west of Laurussia. Other minor oceans were the Paleo-Tethys Ocean and Rheic Ocean.
By the early Devonian, the continent Laurussia (also known as
Most of Laurussia was located south of the equator, but in the Devonian it moved northwards and began to rotate counterclockwise towards its modern position. While the most northern parts of the continent (such as
In the Early and Middle Devonian, the west coast of Laurussia was a passive margin with broad coastal waters, deep silty embayments, river deltas and estuaries, found today in Idaho and Nevada. In the Late Devonian, an approaching volcanic island arc reached the steep slope of the continental shelf and began to uplift deep water deposits. This minor collision sparked the start of a mountain-building episode called the Antler orogeny, which extended into the Carboniferous. Mountain building could also be found in the far northeastern extent of the continent, as minor tropical island arcs and detached Baltic terranes re-join the continent. Deformed remnants of these mountains can still be found on Ellesmere Island and Svalbard. Many of the Devonian collisions in Laurussia produce both mountain chains and foreland basins, which are frequently fossiliferous.
Gondwana was by far the largest continent on the planet. It was completely south of the equator, although the northeastern sector (now Australia) did reach tropical latitudes. The southwestern sector (now South America) was located to the far south, with
The northern rim of Gondwana was mostly a passive margin, hosting extensive marine deposits in areas such as northwest Africa and Tibet. The eastern margin, though warmer than the west, was equally active. Numerous mountain building events and granite and kimberlite intrusions affected areas equivalent to modern day eastern Australia, Tasmania, and Antarctica.
While the South China-Annamia continent was the newest addition to the Asian microcontinents, it was not the first. North China and the Tarim Block (now northwesternmost China) were located westward and continued to drift northwards, powering over older oceanic crust in the process. Further west was a small ocean (the Turkestan Ocean), followed by the larger microcontinents of Kazakhstania, Siberia, and Amuria. Kazakhstania was a volcanically active region during the Devonian, as it continued to assimilate smaller island arcs.
Siberia was located just north of the equator as the largest landmass in the Northern Hemisphere. At the beginning of the Devonian, Siberia was inverted (upside down) relative to its modern orientation. Later in the period it moved northwards and began to twist clockwise, though it was not near its modern location. Siberia approached the eastern edge of Laurussia as the Devonian progressed, but it was still separated by a seaway, the
Similar volcanic activity also affected the nearby microcontinent of Amuria (now Manchuria, Mongolia and their vicinities). Though certainly close to Siberia in the Devonian, the precise location of Amuria is uncertain due to contradictory paleomagnetic data.
Closure of the Rheic Ocean
The Rheic Ocean, which separated Laurussia from Gondwana, was wide at the start of the Devonian, having formed after the drift of Avalonia away from Gondwana. It steadily shrunk as the period continued, as the two major continents approached near the equator in the early stages of the assembly of Pangaea. The closure of the Rheic Ocean began in the Devonian and continued into the Carboniferous. As the ocean narrowed, endemic marine faunas of Gondwana and Laurussia combined into a single tropical fauna. The history of the western Rheic Ocean is a subject of debate, but there is good evidence that Rheic oceanic crust experienced intense subduction and metamorphism under Mexico and Central America.
The closure of the eastern part of the Rheic Ocean is associated with the assemblage of central and southern Europe. In the early Paleozoic, much of Europe was still attached to Gondwana, including the terranes of
Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopods, the enigmatic hederellids, microconchids and corals. Lily-like crinoids (animals, their resemblance to flowers notwithstanding) were abundant, and trilobites were still fairly common. Bivalves became commonplace in deep water and outer shelf environments. The first ammonites also appeared during or slightly before the early Devonian Period around 400 Mya. Bactritoids make their first appearance in the Early Devonian as well; their radiation, along with that of ammonoids, has been attributed by some authors to increased environmental stress resulting from decreasing oxygen levels in the deeper parts of the water column. Among vertebrates, jawless armored fish (ostracoderms) declined in diversity, while the jawed fish (gnathostomes) simultaneously increased in both the sea and fresh water. Armored placoderms were numerous during the lower stages of the Devonian Period and became extinct in the Late Devonian, perhaps because of competition for food against the other fish species. Early cartilaginous (Chondrichthyes) and bony fishes (Osteichthyes) also become diverse and played a large role within the Devonian seas. The first abundant genus of cartilaginous fish, Cladoselache, appeared in the oceans during the Devonian Period. The great diversity of fish around at the time has led to the Devonian being given the name "The Age of Fish" in popular culture.
The Devonian saw significant expansion in the diversity of nektonic marine life driven by the abundance of planktonic microorganisms in the free water column as well as high ecological competition in benthic habitats, which were extremely saturated; this diversification has been labeled the Devonian Nekton Revolution by many researchers. However, other researchers have questioned whether this revolution existed at all; a 2018 study found that although the proportion of biodiversity constituted by nekton increased across the boundary between the Silurian and Devonian, it decreased across the span of the Devonian, particularly during the Pragian, and that the overall diversity of nektonic taxa did not increase significantly during the Devonian compared to during other geologic periods, and was in fact higher during the intervals spanning from the Wenlock to the Lochkovian and from the Carboniferous to the Permian. The study's authors instead attribute the increased overall diversity of nekton in the Devonian to a broader, gradual trend of nektonic diversification across the entire Palaeozoic.
A now-dry barrier reef, located in present-day
By the Devonian Period, life was well underway in its colonization of the land. The moss forests and bacterial and algal mats of the Silurian were joined early in the period by primitive rooted plants that created the first stable soils and harbored arthropods like mites, scorpions, trigonotarbids and myriapods (although arthropods appeared on land much earlier than in the Early Devonian and the existence of fossils such as Protichnites suggest that amphibious arthropods may have appeared as early as the Cambrian). By far the largest land organism at the beginning of this period was the enigmatic Prototaxites, which was possibly the fruiting body of an enormous fungus, rolled liverwort mat, or another organism of uncertain affinities that stood more than 8 metres (26 ft) tall, and towered over the low, carpet-like vegetation during the early part of the Devonian. Also, the first possible fossils of insects appeared around 416 Mya, in the Early Devonian. Evidence for the earliest tetrapods takes the form of trace fossils in shallow lagoon environments within a marine carbonate platform/shelf during the Middle Devonian, although these traces have been questioned and an interpretation as fish feeding traces (Piscichnus) has been advanced.
The greening of land
The 'greening' of the continents acted as a carbon sink, and atmospheric concentrations of carbon dioxide may have dropped. This may have cooled the climate and led to a massive extinction event. (See Late Devonian extinction).
Lycopod axis (branch) from the Middle Devonian of Wisconsin
Bark (possibly from a cladoxylopsid) from the Middle Devonian of Wisconsin
Animals and the first soils
Primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed plants that characterized a recognizably modern world had its genesis in the Late Devonian Epoch. The development of soils and plant root systems probably led to changes in the speed and pattern of erosion and sediment deposition. The rapid evolution of a terrestrial ecosystem that contained copious animals opened the way for the first vertebrates to seek terrestrial living. By the end of the Devonian, arthropods were solidly established on the land.
Dunkleosteus, one of the largest armoured fish ever to roam the planet, lived during the Late Devonian
Lower jaw of Eastmanosteus pustulosus from the Middle Devonian of Wisconsin
Tooth of the lobe-finned fish Onychodus from the Middle Devonian of Wisconsin
Enrolled phacopid trilobite from the Devonian of Ohio
Tropidoleptus carinatus, an orthid brachiopod from the Middle Devonian of New York
Pleurodictyum americanum, Kashong Shale, Middle Devonian of New York
SEM image of a hederelloid from the Devonian of Michigan (largest tube diameter is 0.75 mm)
Devonian spiriferid brachiopod from Ohio which served as a host substrate for a colony of hederelloids
Late Devonian extinction
The Late Devonian extinction is not a single event, but rather is a series of pulsed extinctions at the Givetian-Frasnian boundary, the Frasnian-Famennian boundary, and the Devonian-Carboniferous boundary. Together, these are considered one of the "Big Five" mass extinctions in Earth's history. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef systems.
Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, and acritarchs, and the world saw the disappearance of an estimated 96% of vertebrates like conodonts and bony fishes, and all of the ostracoderms and placoderms. Land plants as well as freshwater species, such as our tetrapod ancestors, were relatively unaffected by the Late Devonian extinction event (there is a counterargument that the Devonian extinctions nearly wiped out the tetrapods).
The reasons for the Late Devonian extinctions are still unknown, and all explanations remain speculative. Canadian paleontologist Digby McLaren suggested in 1969 that the Devonian extinction events were caused by an asteroid impact. However, while there were Late Devonian collision events (see the Alamo bolide impact), little evidence supports the existence of a large enough Devonian crater.
- Falls of the Ohio State Park – State park in Indiana, United States. One of the largest exposed Devonian fossil beds in the world.
- Geologic time scale – System that relates geologic strata to time
- List of Early Devonian land plants
- List of fossil sites (with link directory)
- Phacops rana– Extinct species of trilobite, a Devonian trilobite
- [[::Category:Devonian plants|:Category:Devonian plants]]
- Sedgwick and Murchison coined the term "Devonian system" in 1840: "We propose therefore, for the future, to designate these groups collectively by the name Devonian system". Sedgwick and Murchison acknowledged William Lonsdale's role in proposing, on the basis of fossil evidence, the existence of a Devonian stratum between those of the Silurian and Carboniferous periods: "Again, Mr. Lonsdale, after an extensive examination of the fossils of South Devon, had pronounced them, more than a year since, to form a group intermediate between those of the Carboniferous and Silurian systems". William Lonsdale stated that in December 1837 he had suggested the existence of a stratum between the Silurian and Carboniferous ones: "Mr. Austen's communication [was] read December 1837 ... . It was immediately after the reading of that paper ... that I formed the opinion relative to the limestones of Devonshire being of the age of the old red sandstone; and which I afterwards suggested first to Mr. Murchison and then to Prof. Sedgwick".
- "Chart/Time Scale". www.stratigraphy.org. International Commission on Stratigraphy.
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- "Devonian". Dictionary.com Unabridged (Online). n.d.
- Amos, Jonathan. "Fossil tracks record 'oldest land-walkers'". BBC News. Retrieved 24 December 2016.
- Newitz, Annalee (13 June 2013). "How do you have a mass extinction without an increase in extinctions?". The Atlantic.
- Gradstein, Ogg & Smith (2004)
- Sedgwick, Adam; Murchison, Roderick Impey (1840). "On the physical structure of Devonshire, and on the subdivisions and geological relations of its older stratified deposits, etc. Part I and Part II". Transactions of the Geological Society of London. Second series. Vol. 5 part II. p. 701.
- Sedgwick & Murchison 1840, p. 690.
- Lonsdale, William (1840). "Notes on the age of limestones from south Devonshire". Transactions of the Geological Society of London. Second series. Vol. 5 part II. p. 724.
- Gradstein, Ogg & Smith 2004.
- Farabee, Michael J. (2006). "Paleobiology: The Late Paleozoic: Devonian". The Online Biology Book. Estrella Mountain Community College.
- . Retrieved 16 November 2022.
- "Devonian Period". Encyclopedia Britannica. geochronology. Retrieved 15 December 2017.
- Blakey, Ron C. "Devonian Paleogeography, Southwestern US". jan.ucc.nau.edu. Northern Arizona University. Archived from the original on 15 April 2010.
- Kazlev, M. Alan (28 May 1998). "Palaeos Paleozoic: Devonian: The Devonian Period – 1". Palaeos. Retrieved 24 January 2019.
- MacPherson, C. (28 August 2019). "Analyzing the World's Oldest Woody Plant Fossil". Canadian Light Source. Retrieved 19 May 2021.
- Smith, Lewis (19 April 2007). "Fossil from a forest that gave Earth its breath of fresh air". The Times. London. Retrieved 1 May 2010.
- Hogan, C. Michael (2010). "Fern". In Basu, Saikat; Cleveland, C. (eds.). Encyclopedia of Earth. Washington DC: National Council for Science and the Environment.
- After a Mass Extinction, Only the Small Survive | Carl Zimmer
- "Devonian". Devonian Times. Archived from the original on 11 February 2010.
- "Devonian life". UC Berkeley. – site introduces the Devonian
- "Geologic Time Scale". International Commission on Stratigraphy (ICS). 2004. Retrieved 19 September 2005.
- "Examples of Devonian Fossils".
- "Devonian chronostratigraphy scale".
- "Devonian". Palaeos. Archived from the original on 28 October 2007.
- "Museum". Age of Fishes.