Devonianpalaeogeography was dominated by thesupercontinentGondwana to the south, the small continent ofSiberia to the north, and the medium-sized continent ofLaurussia to the east. Major tectonic events include the closure of theRheic Ocean, the separation ofSouth China from Gondwana, and the resulting expansion of thePaleo-Tethys Ocean. The Devonian experienced several major mountain-building events as Laurussia and Gondwana approached; these include theAcadian Orogeny in North America and the beginning of theVariscan Orogeny in Europe. These early collisions preceded the formation of the single supercontinentPangaea in the Late Paleozoic.
The rocks of Lummaton Quarry inTorquay inDevon played an early role in defining the Devonian Period
The period is named afterDevon, a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found throughout the county was resolved by adding the Devonian Period to the geological timescale.The Great Devonian Controversy was a lengthy debate betweenRoderick Murchison,Adam Sedgwick andHenry De la Beche over the naming of the period. Murchison and Sedgwick won the debate and named it the Devonian System.[14][15][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 theOld Red Sandstone in which early fossil discoveries were found. Another common term is "Age of the Fishes",[20] referring to the evolution of several major groups offish 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.[21]
The Devonian has also erroneously been characterised as a "greenhouse age", due tosampling bias: most of the early Devonian-age discoveries came from thestrata ofWestern Europe and easternNorth America, which at the time straddled theEquator as part of the supercontinent of Euramerica wherefossil signatures of widespread reefs indicate tropicalclimates that were warm and moderately humid. In fact, the climate in the Devonian differed greatly during itsepochs 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 andSouth America had a warmtemperate climate. In the Late Devonian, by contrast, arid conditions were less prevalent across the world andtemperate climates were more common.[citation needed]
The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to thoseepochs are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System.
TheEarly Devonian lasted from 419.62 to 393.47 Ma. It began with theLochkovian Stage from 419.62 to 413.02 Ma, which was followed by thePragian from 413.02 to 410.62 Ma and then by theEmsian, which lasted until the Middle Devonian began at 393.47 Ma.[22]During this time, the firstammonoids appeared, descending frombactritoidnautiloids. Ammonoids during this time period were simple and differed little from their nautiloid counterparts. These ammonoids belong to the orderAgoniatitida, which in later epochs evolved to new ammonoid orders, for exampleGoniatitida andClymeniida. This class ofcephalopod molluscs would dominate the marine fauna until the beginning of theMesozoic Era.
TheMiddle Devonian comprised two subdivisions: first theEifelian, which then gave way to theGivetian at 387.95 Ma. During this time, the jawlessagnathan fishes began to decline in diversity in freshwater and marine environments partly due to drastic environmental changes and partly due to the increasing competition, predation, and diversity ofjawed fishes. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs and the ability to crawl out of the water and onto the land for short periods of time.[23]
A map of Earth as it appeared 370 million years ago during the Late Devonian Epoch
Finally, the Late Devonian started with theFrasnian, from 382.31 to 372.15 Ma, during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuingFamennian subdivision, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian at 358.86 Ma.[22]
The Devonian was a relatively warm period, although significantglaciers may have existed during the Early and Middle Devonian.[24] 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.[25] Reconstruction of tropicalsea surface temperature fromconodontapatite implies an average value of 30 °C (86 °F) in the Early Devonian.[25] Early Devonian mean annual surface temperatures were approximately 16 °C.[26] 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).[25] 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.[25] The climate would have affected the dominant organisms inreefs;microbes would have been the main reef-forming organisms in warm periods, with corals andstromatoporoid 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 anicehouse, marking the beginning of theLate Paleozoic icehouse.[27][28]
The Devonian world involved many continents and ocean basins of various sizes. The largest continent,Gondwana, was located entirely within theSouthern Hemisphere. It corresponds to modern daySouth America,Africa,Australia,Antarctica, andIndia, as well as minor components ofNorth America andAsia. The second-largest continent, Laurussia, was northwest of Gondwana, and corresponds to much of modern-dayNorth America andEurope. Various smaller continents,microcontinents, andterranes were present east of Laurussia and north of Gondwana, corresponding to parts of Europe and Asia. The Devonian Period was a time of greattectonic activity, as the major continents ofLaurussia andGondwana drew closer together.[29][30]
Sea levels were high worldwide, and much of the land lay under shallow seas, where tropicalreef organisms lived. The enormous "world ocean",Panthalassa, occupied much of theNorthern Hemisphere as well as wide swathes east of Gondwana and west of Laurussia. Other minor oceans were thePaleo-Tethys Ocean andRheic Ocean.[29][30]
Continental boundary ofLaurussia (Euramerica) and its constituents, superimposed onto modern coastlines
By the early Devonian, the continent Laurussia (also known asEuramerica) was fully formed through the collision of the continentsLaurentia (modern day North America) andBaltica (modern day northern and eastern Europe). The tectonic effects of this collision continued into the Devonian, producing a string of mountain ranges along the southeastern coast of the continent. In present-day eastern North America, theAcadian Orogeny continued to raise theAppalachian Mountains. Further east, the collision also extended the rise of theCaledonian Mountains ofGreat Britain andScandinavia. As the Caledonian Orogeny wound down in the later part of the period,orogenic collapse facilitated a cluster of granite intrusions in Scotland.[29]
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 asGreenland andEllesmere Island) established tropical conditions, most of the continent was located within the natural dry zone along theTropic of Capricorn, which (as nowadays) is a result of the convergence of two great air-masses, theHadley cell and theFerrel cell. In these near-deserts, theOld Red Sandstone sedimentary beds formed, made red by the oxidised iron (hematite) characteristic of drought conditions. The abundance of red sandstone on continental land also lends Laurussia the name "the Old Red Continent".[31] For much of the Devonian, the majority of western Laurussia (North America) was covered by subtropicalinland seas which hosted a diverse ecosystem of reefs and marine life. Devonian marine deposits are particularly prevalent in themidwestern andnortheastern United States. Devonian reefs also extended along the southeast edge of Laurussia, a coastline now corresponding to southernEngland,Belgium, and other mid-latitude areas of Europe.[29]
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 inIdaho andNevada. In the Late Devonian, an approaching volcanicisland 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 theAntler orogeny, which extended into the Carboniferous.[29][32] 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 andSvalbard. Many of the Devonian collisions in Laurussia produce both mountain chains andforeland basins, which are frequently fossiliferous.[29][30]
The Early-Middle Devonian world, with major continents Gondwana (Go), Euramerica/Laurussia (Eu), and Siberia (Si)
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, withBrazil situated near theSouth Pole. The northwestern edge of Gondwana was anactive margin for much of the Devonian, and saw theaccretion of many smaller land masses and island arcs. These includeChilenia,Cuyania, andChaitenia, which now form much ofChile andPatagonia.[29][33] These collisions were associated withvolcanic activity andplutons, but by the Late Devonian the tectonic situation had relaxed and much of South America was covered by shallow seas. These south polar seas hosted a distinctive brachiopod fauna, the Malvinokaffric Realm, which extended eastward to marginal areas now equivalent to South Africa and Antarctica. Malvinokaffric faunas even managed to approach the South Pole via a tongue of Panthalassa which extended into theParaná Basin.[29]
The northern rim of Gondwana was mostly a passive margin, hosting extensive marine deposits in areas such as northwest Africa andTibet. The eastern margin, though warmer than the west, was equally active. Numerous mountain building events andgranite andkimberlite intrusions affected areas equivalent to modern day easternAustralia,Tasmania, and Antarctica.[29]
The earth at 380 Ma, centered on thePaleo-Tethys Ocean, which fully opened during the Devonian
Several island microcontinents (which would later coalesce into modern day Asia) stretched over a low-latitudearchipelago to the north of Gondwana. They were separated from the southern continent by an oceanic basin: thePaleo-Tethys. Although the western Paleo-Tethys Ocean had existed since the Cambrian, the eastern part only began to rift apart as late as the Silurian. This process accelerated in the Devonian. The eastern branch of the Paleo-Tethys was fully opened whenSouth China andAnnamia (aterrane equivalent to most ofIndochina), together as a unified continent, detached from the northeastern sector of Gondwana. Nevertheless, they remained close enough to Gondwana that their Devonian fossils were more closely related to Australian species than to north Asian species. Other Asian terranes remained attached to Gondwana, includingSibumasu (western Indochina), Tibet, and the rest of theCimmerian blocks.[29][30]
World map at 400 Ma (Early Devonian), showing continents andterranes with modern continent borders superimposed
While the South China-Annamia continent was the newest addition to the Asian microcontinents, it was not the first.North China and theTarim 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 ofKazakhstania,Siberia, andAmuria. Kazakhstania was a volcanically active region during the Devonian, as it continued to assimilate smaller island arcs.[29] The island arcs of the region, such as the Balkhash-West Junggar Arc, exhibited biological endemism as a consequence of their location.[34]
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, theUral Ocean. Although Siberia's margins were generally tectonically stable and ecologically productive, rifting and deepmantle plumes impacted the continent withflood basalts during the Late Devonian. TheAltai-Sayan region was shaken by volcanism in the Early and Middle Devonian, while Late Devonian magmatism was magnified further to produce theVilyuy Traps, flood basalts which may have contributed to the Late Devonian Mass Extinction. The last major round of volcanism, the Yakutsk Large Igneous Province, continued into the Carboniferous to produce extensive kimberlite deposits.[29][30]
Similar volcanic activity also affected the nearby microcontinent of Amuria (nowManchuria,Mongolia and their vicinities). Though certainly close to Siberia in the Devonian, the precise location of Amuria is uncertain due to contradictorypaleomagnetic data.[29]
The Rheic Ocean, which separated Laurussia from Gondwana, was wide at the start of the Devonian, having formed after the drift ofAvalonia away from Gondwana. It steadily shrunk as the period continued, as the two major continents approached near theequator in the early stages of the assembly ofPangaea. 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 intensesubduction andmetamorphism under Mexico and Central America.[29][30]
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 ofIberia,Armorica (France),Palaeo-Adria (the western Mediterranean area),Bohemia,Franconia, andSaxothuringia. These continental blocks, collectively known as the Armorican Terrane Assemblage, split away from Gondwana in the Silurian and drifted towards Laurussia through the Devonian. Their collision with Laurussia leads to the beginning of theVariscan Orogeny, a major mountain-building event which would escalate further in the Late Paleozoic. Franconia and Saxothuringia collided with Laurussia near the end of the Early Devonian, pinching out the easternmost Rheic Ocean. The rest of the Armorican terranes followed, and by the end of the Devonian they were fully connected with Laurussia. This sequence of rifting and collision events led to the successive creation and destruction of several small seaways, including the Rheno-Hercynian, Saxo-Thuringian, and Galicia-Moldanubian oceans. Their sediments were eventually compressed and completely buried as Gondwana fully collided with Laurussia in the Carboniferous.[29][30][35]
Sea levels in the Devonian were generally high. Marine faunas continued to be dominated byconodonts,[37]bryozoans,[38] diverse and abundantbrachiopods,[39] the enigmatichederellids,[40]microconchids,[38] andcorals.[41][42] Lily-likecrinoids (animals, their resemblance to flowers notwithstanding) were abundant, andtrilobites were still fairly common.Bivalves became commonplace in deep water and outer shelf environments.[43] The first ammonites also appeared during or slightly before the early Devonian Period around 400 Ma.[44]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.[45] Among vertebrates, jawless armored fish (ostracoderms) declined in diversity, while the jawed fish (gnathostomes) simultaneously increased in both the sea andfresh water. Armored placoderms were numerous during the early ages 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 Fishes" in popular culture.[46]
The Devonian saw significant expansion in the diversity ofnektonic 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 theDevonian Nekton Revolution by many researchers.[47] 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.[48]
A now-dry barrier reef, located in present-dayKimberley Basin of northwestAustralia, once extended 350 km (220 mi), fringing a Devonian continent.[49] Reefs are generally built by variouscarbonate-secreting organisms that can erect wave-resistant structures near sea level. Although modern reefs are constructed mainly by corals and calcareousalgae, Devonian reefs were either microbial reefs built up mostly byautotrophiccyanobacteria or coral-stromatoporoid reefs built up by coral-likestromatoporoids and tabulate andrugose corals. Microbial reefs dominated under the warmer conditions of the early and late Devonian, while coral-stromatoporoid reefs dominated during the cooler middle Devonian.[50]
Prototaxites milwaukeensis, a large fungus, initially thought to be a marine alga, from the Middle Devonian of Wisconsin
By the Devonian Period, life was well underway in its colonization of the land. Themoss forests andbacterial and algal mats of the Silurian were joined early in the period by primitive rooted plants that created the first stablesoils and harbored arthropods likemites,scorpions,trigonotarbids[51] andmyriapods (although arthropods appeared on land much earlier than in the Early Devonian[52] and the existence of fossils such asProtichnites suggest that amphibious arthropods may have appeared as early as theCambrian). By far the largest land organism at the beginning of this period was the enigmaticPrototaxites, which was possibly the fruiting body of an enormous fungus,[53] rolled liverwort mat,[54] or another organism of uncertain affinities[55] 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 ofinsects appeared around 416 Ma, in the Early Devonian. Evidence for the earliesttetrapods takes the form of trace fossils in shallow lagoon environments within a marine carbonate platform/shelf during the Middle Devonian,[56] although these traces have been questioned and an interpretation as fish feeding traces (Piscichnus) has been advanced.[57]
The Devonian Period marks the beginning of extensive land colonization byplants. With large land-dwellingherbivores not yet present, large forests grew and shaped the landscape.
ManyEarly Devonian plants did not have true roots or leaves like extant plants, although vascular tissue is observed in many of those plants. Some of the early land plants such asDrepanophycus likely spread by vegetative growth and spores.[58] The earliest land plants such asCooksonia consisted of leafless,dichotomous axes with terminal sporangia and were generally very short-statured, and grew hardly more than a few centimetres tall.[59] Fossils ofArmoricaphyton chateaupannense, about 400 million years old, represent the oldest known plants withwoody tissue.[60] By the Middle Devonian, shrub-like forests of primitive plants existed:lycophytes,horsetails,ferns, andprogymnosperms evolved. Most of these plants had true roots and leaves, and many were quite tall. The earliest-known trees appeared in the Middle Devonian.[61] These included a lineage of lycopods and another arborescent, woody vascular plant, thecladoxylopsids and progymnospermArchaeopteris.[62] Thesetracheophytes were able to grow to large size on dry land because they had evolved the ability to biosynthesizelignin, which gave them physical rigidity and improved the effectiveness of their vascular system while giving them resistance to pathogens and herbivores.[63] InEifelian age, cladoxylopsid trees formed the first forests in Earth history.[64] By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of many plant groups and growth forms has been referred to as the Devonian Explosion or the Silurian-Devonian Terrestrial Revolution.[65]
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 oferosion and sediment deposition. The rapid evolution of a terrestrial ecosystem that contained copious animals opened the way for the firstvertebrates to seek terrestrial living. By the end of the Devonian, arthropods were solidly established on the land.[66]
The Late Devonian is characterised by three episodes of extinction ("Late D")
TheLate 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.[67] Together, these are considered one of the "Big Five" mass extinctions in Earth's history.[68] 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.[69]
Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, andacritarchs, and the world saw the disappearance of an estimated 96% of vertebrates likeconodonts andbony fishes, and all of the ostracoderms and placoderms.[67][70] 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[71]).
The reasons for the Late Devonian extinctions are still unknown, and all explanations remain speculative.[72][73][74][75] Canadian paleontologistDigby McLaren suggested in 1969 that the Devonian extinction events were caused by an asteroid impact. However, while there were Late Devonian collision events (see theAlamo bolide impact), little evidence supports the existence of a large enough Devonian crater.[76]
^Sedgwick and Murchison coined the term "Devonian system" in 1840:[16] "We propose therefore, for the future, to designate these groups collectively by the nameDevonian 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:[17] "Again, Mr. Lonsdale, after an extensive examination of the fossils of South Devon, had pronounced them, more than a year since, to forma 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:[18] "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".
^Kaufmann, B.; Trapp, E.; Mezger, K. (2004). "The numerical age of the Upper Frasnian (Upper Devonian) Kellwasser horizons: A new U-Pb zircon date from Steinbruch Schmidt(Kellerwald, Germany)".The Journal of Geology.112 (4):495–501.Bibcode:2004JG....112..495K.doi:10.1086/421077.
^Algeo, T. J. (1998). "Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events".Philosophical Transactions of the Royal Society B: Biological Sciences.353 (1365):113–130.doi:10.1098/rstb.1998.0195.
^Barclay, W.J. (1989).Geology of the South Wales Coalfield Part II, the country around Abergavenny. Memoir for 1:50,000 geological sheet (England and Wales) (3rd ed.). pp. 18–19.ISBN0-11-884408-3.
^Clack, Jennifer (13 August 2007)."Devonian climate change, breathing, and the origin of the tetrapod stem group".Integrative and Comparative Biology.47 (4):510–523.doi:10.1093/icb/icm055.PMID21672860.Estimates of oxygen levels during this period suggest that they were unprecedentedly low during the Givetian and Frasnian periods. At the same time, plant diversification was at its most rapid, changing the character of the landscape and contributing, via soils, soluble nutrients, and decaying plant matter, to anoxia in all water systems. The co-occurrence of these global events may explain the evolution of air-breathing adaptations in at least two lobe-finned groups, contributing directly to the rise of the tetrapod stem group.
^Zhang, Ying-ying; Xue, Jin-Zhuang; Liu, Le; Wang, De-ming (2016). "Periodicity of reproductive growth in lycopsids: An example from the Upper Devonian of Zhejiang Province, China".Paleoworld.25 (1):12–20.doi:10.1016/j.palwor.2015.07.002.
^Gonez, Paul; Gerrienne, Philippe (2010). "A new definition and a lectotypification of the genusCooksonia Lang 1937".International Journal of Plant Sciences.171 (2):199–215.doi:10.1086/648988.S2CID84956576.
^Hogan, C. Michael (2010)."Fern". In Basu, Saikat; Cleveland, C. (eds.).Encyclopedia of Earth. Washington DC: National Council for Science and the Environment.
^McGhee, George R. (2013).When the invasion of land failed: The legacy of the Devonian extinctions. New York: Columbia University Press.ISBN9780231160568.
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