出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/05/24 13:05:44」(JST)
| 顕生代 | 新生代 | 第四紀 |
| 新第三紀 | ||
| 古第三紀 | ||
| 中生代 | 白亜紀 | |
| ジュラ紀 | ||
| 三畳紀 | ||
| 古生代 | ペルム紀 | |
| 石炭紀 | ||
| デボン紀 | ||
| シルル紀 | ||
| オルドビス紀 | ||
| カンブリア紀 | ||
| 原生代 | ||
| 始生代 | ||
| 冥王代 | ||
デボン紀（デボンき、Devonian period）は、地質時代の区分のひとつである。古生代の中ごろ、シルル紀の後、石炭紀の前で、約4億1600万年前から約3億5920万年前までの時期を指す[1][注釈 1]デヴォン紀と記載されることもある。イギリス南部のデヴォン州に分布するシルル紀の地層と石炭紀の地層にはさまれる地層をもとに設定された地質時代である。デボン紀は、魚類の種類や進化の豊かさと、出現する化石の量の多さから、「魚の時代」とも呼ばれている。
デボン紀の開始時期にあたる約4億2000万年前、複数の陸塊同士（ローレンシア大陸やバルティカ大陸など）が衝突し、ユーラメリカ大陸が赤道直下に誕生した。現在の北アメリカ東海岸、グリーンランド、スコットランドがユーラメリカ大陸の一部であった。ユーラメリカ大陸には、陸塊の衝突時にできた巨大な山脈があった。その山脈が大気の流れを大きく遮り、恒常的な降雨を周辺地域にもたらしていた。そのため長大な河川が出現し、この河川に沿って動植物が大陸内部まで活動範囲を拡げていくことが可能となった。
前代のシルル紀には既に植物の陸棲化は開始していたが、デボン紀には河川に沿って大規模に植生域が拡大していったアーケオプテリス（またはアルカエオプテリス、Archaeopteris）などのシダ状の葉を持つ樹木状植物が誕生したことにより、最古の森林が形成されていった。この森林の拡大にしたがい湿地帯も同時に形成されていった。[2][3]
河川と森林そして湿地帯の存在が生物種の進化を支え、さらに大陸内部の気候は、乾季や、時には大規模な乾燥期もあったため、後述する昆虫類や両生類など、より乾燥に強い生物種の誕生を促した。
海洋では河川から流れてくる栄養もあり、コケムシやサンゴ[注釈 2]が大規模なコロニー（個体群）を形成していった。このコロニーに、腕足類、ウミユリ、三葉虫、甲殻類、直角殻のオウムガイなどが生息し、豊かな海を形成していた。アンモナイトもこの時代に誕生した。この豊かな海の時代に、板皮類などの古いタイプの魚類が繁栄を極めていた。サメなどの軟骨魚類もこの時代の海に出現した。
現世の魚類の大部分が属する硬骨魚類もデボン紀に登場した。大陸河川域で棘魚類から分岐、進化したと考えられている。[注釈 3]乾季などで気候が乾燥する時期には、水中の酸素濃度（溶存酸素）が低い環境にあるため、ハイギョやシーラカンスなどの肺を持った肉鰭類が登場した。さらにデボン紀後期には、ハイギョ類のエウステノプテロンか近傍の種[注釈 4]から、アカントステガやイクチオステガといった両生類が出現した[注釈 5]。
ちなみにアジやタイなど、現世の大部分の硬骨魚類が属する条鰭類の真骨類には肺がないのは、遊泳力向上のために肺が浮き袋に変化したからである。そのため、デボン紀の硬骨魚類は条鰭類であっても肺があり、空気呼吸をしていたと思われる。実際、デボン紀に登場し現生する条鰭類ポリプテルス目、ガー目、アミア目は、空気呼吸ができる。
前代のシルル紀には、既にダニ（鋏角類）や、ムカデなどが属する多足類が陸上に出現しており、節足動物の陸棲化は脊椎動物よりも進んでいた。さらに約4億年前のデボン紀前期には、昆虫類が誕生した。
この昆虫類を含む六脚類の起源は、先行して上陸していた多足類である、と以前は考えられていた。しかし、遺伝子解析から昆虫類は、カニやエビが属する甲殻類や、ミジンコやフジツボが属する鰓脚類が、六脚類により近いと判明している。この結果から、昆虫類は現生の淡水のミジンコとの共通の祖先種から、後期シルル紀の淡水域において生息していたと考えられる。その祖先種から、前述の河川と陸上の境界域で進化を重ね、陸棲化したのが昆虫だと考えられる。実際、出現当初の昆虫類の化石は、淡水域と陸上であった場所でしか発見されておらず、また現生の昆虫のほとんどが陸棲である。
デボン紀の昆虫は、現在発見されている化石からは翅の獲得はみられず、原始的な形態であった。現在の昆虫類は、動物種の大半を占めるほど多種であるが、その多様な進化は石炭紀以降で顕著になったと思われる。
サメなどの軟骨魚類は、前期デボン紀には存在していた[4] 。ただし歯の化石[注釈 6]には、それよりも古いシルル紀末期のものもあるため、厳密に言えば、起源は前代のシルル紀にあると考えられる。
サメの祖先は不詳であるが、板皮類に求める説が強い。例えば2008年には、現生のサメが持つ胎生能力を板皮類も持っていたことが発見され、共通の起源が示唆されている。
中期デボン紀には、クラドセラケ[注釈 7][注釈 8]が登場した。捕食生物であり、7対の鰓[注釈 9]を有し、硬い歯、背びれ、尾びれの形状と、現生のサメと変わらない形態をしていた。
デボン紀後期から石炭紀初期は、5大大量絶滅の一時期であり、特に前述のサンゴ礁を作る赤道域の浅海域で選択的に絶滅が起こっている[5]。この大絶滅により、海洋生物種の82%が絶滅した。その中には、デボン紀に繁栄を極めたダンクルオステウス[注釈 10]などの板皮類[注釈 11]や、原始的な脊椎動物である無顎類の大部分[注釈 12]や、プロエタス目を除いた三葉虫の大部分[注釈 13]が含まれる。
炭素、酸素、ストロンチウムなどの同位体測定や、元素分析による古環境解析から、気候の急激な寒暖の変化、海水面の後退、乾燥化、低酸素化、などの大きな環境変化がデボン紀後期に繰返し発生し、おそらくこれらの環境変化が大量絶滅の要因だとは考えられている[5][6]。
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| ウィキメディア・コモンズには、デボン紀に関連するカテゴリがあります。 |
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| Devonian Period 419.2–358.9 million years ago PreꞒ
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| Mean atmospheric O 2 content over period duration |
ca. 15 vol %[1] (75 % of modern level) |
| Mean atmospheric CO 2 content over period duration |
ca. 2200 ppm[2] (8 times pre-industrial level) |
| Mean surface temperature over period duration | ca. 20 °C[3] (6 °C above modern level) |
| Sea level (above present day) | Relatively steady around 189m, gradually falling to 120m through period[4] |
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 419.2 ± 3.2 Mya (million years ago), to the beginning of the Carboniferous Period, about 358.9 ± 0.4.[5] It is named after Devon, England, where rocks from this period were first studied. The Devonian period experienced the first significant adaptive radiation of terrestrial life. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods also became well-established. Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the "Age of Fish". The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment.
The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolving into legs.[6] In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician. The first ammonite mollusks appeared. Trilobites, the mollusk-like brachiopods and the great coral reefs, were still common. The Late Devonian extinction which started about 375 million years ago,[7] severely affected marine life, killing off all placoderms, and all trilobites, save for a few species of the order Proetida.
The paleogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, and the early formation of the small continent of Euramerica in between.
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 defining of the Devonian period in the geological timescale. The Great Devonian Controversy is a classic case of how the foundations of our present-day geological knowledge and classification of the rock record and geological timescale was socially as well as scientifically constructed. After 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'.[8]
While the rock beds that define the start and end of the Devonian period are well identified, the exact dates are uncertain. According to the International Commission on Stratigraphy (Ogg, 2004), the Devonian extends from the end of the Silurian Period 419.2 ± 3.2 Mya, to the beginning of the Carboniferous Period 358.9 ± 0.4 Mya (in North America, the beginning of the Mississippian subperiod of the Carboniferous.[5]
In nineteenth-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",[9] 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.[10]
The Devonian has also erroneously been characterized 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 but in fact the climate in the Devonian differed greatly between epochs and 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.
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The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to these epochs are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System.
The Early Devonian lasts from 419.2 ± 2.8 to 393.3 ± 2.5 and begins with the Lochkovian stage, which lasts until the Pragian. This spans from 410.8 ± 2.8 to 407.6 ± 2.5, and is followed by the Emsian, which lasts until the Middle Devonian begins, 393.3± 2.7 million years ago. The Middle Devonian comprises two subdivisions, the Eifelian giving way to the Givetian 387.7± 2.7 million years ago. During this time the armoured jawless ostracoderm fish were declining in diversity; the jawed fish were thriving and increasing in diversity in both the oceans and freshwater. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop essential characteristics such as well developed lungs, and the ability to crawl out of the water and onto the land for short periods of time.
Finally, the Late Devonian starts with the Frasnian, 382.7 ± 2.8 to 372.2 ± 2.5, during which the first forests were taking shape on land. The first tetrapods appear 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, 358.9± 2.5 million years ago.[citation needed]
The Devonian was a relatively warm period, and probably lacked any glaciers. 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.[13] Reconstruction of tropical sea surface temperature from conodont apatite implies an average value of 30 °C (86 °F) in the Early Devonian.[13] CO2 levels dropped steeply throughout the Devonian period as the burial of the newly evolved forests drew carbon out of the atmosphere into sediments; this may be reflected by a Mid-Devonian cooling of around 5 °C (9 °F).[13] 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 Late Devonian warming.[13] 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.
The Devonian period was a time of great tectonic activity, as Euramerica and Gondwana drew closer together.
The continent Euramerica (or Laurussia) was created in the early Devonian by the collision of Laurentia and Baltica, which rotated into the natural dry zone along the Tropic of Capricorn, which is formed as much in Paleozoic times as nowadays by the convergence of two great air-masses, the Hadley cell and the Ferrel cell. In these near-deserts, the Old Red Sandstone sedimentary beds formed, made red by the oxidized iron (hematite) characteristic of drought conditions.
Near the equator, the plate of Euramerica and Gondwana were starting to meet, beginning the early stages of assembling Pangaea. This activity further raised the northern Appalachian Mountains and formed the Caledonian Mountains in Great Britain and Scandinavia.
The west coast of Devonian North America, by contrast, was a passive margin with deep silty embayments, river deltas and estuaries, in today's Idaho and Nevada; an approaching volcanic island arc reached the steep slope of the continental shelf in Late Devonian times and began to uplift deep water deposits, a collision that was the prelude to the mountain-building episode of Mississippian times called the Antler orogeny.[14]
Sea levels were high worldwide, and much of the land lay under shallow seas, where tropical reef organisms lived. The deep, enormous Panthalassa (the "universal ocean") covered the rest of the planet. Other minor oceans were Paleo-Tethys, Proto-Tethys, Rheic Ocean, and Ural Ocean (which was closed during the collision with Siberia and Baltica).
Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by bryozoa, diverse and abundant brachiopods, the enigmatic hederelloids, microconchids and corals. Lily-like crinoids (animals, their resemblance to flowers notwithstanding) were abundant, and trilobites were still fairly common. Among vertebrates, jaw-less 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 shark, Cladoselache, appeared in the oceans during the Devonian Period. The great diversity of fish around at the time, have led to the Devonian being given the name "The Age of Fish" in popular culture.
The first ammonites also appeared during or slightly before the early Devonian Period around 400 Mya.[16]
A now dry barrier reef, located in present day Kimberley Basin of northwest Australia, once extended a thousand kilometers, fringing a Devonian continent. Reefs in general are built by various carbonate-secreting organisms that have the ability to erect wave-resistant frameworks close to sea level. The main contributors of the Devonian reefs were unlike modern reefs, which are constructed mainly by corals and calcareous algae. They were composed of calcareous algae and coral-like stromatoporoids, and tabulate and rugose corals, in that order of importance.[clarification needed]]]
Dunkleosteus, one of the largest armoured fish to ever roam the planet, lived during the late Devonian
Early shark Cladoselache, several lobe-finned fishes, including Eusthenopteron that was an early marine tetrapod, and the placoderm Bothriolepis on a painting from 1905
Enrolled phacopid trilobite from the Devonian of Ohio
The common tabulate coral Aulopora from the Middle Devonian of Ohio – view of colony origin encrusting a brachiopod valve
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
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 and myriapods (although arthropods appeared on land much earlier than in the Early Devonian and the existence of fossils such as Climactichnites suggest that land arthropods may have appeared as early as the Cambrian period). Also the first possible fossils of insects appeared around 416 Mya in the Early Devonian. The first tetrapods, evolving from lobe-finned fish, appeared in the coastal water no later than middle Devonian, and gave rise to the first Amphibians.[17]
Early Devonian plants did not have roots or leaves like the plants most common today and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall. By far the largest land organism was Prototaxites, the fruiting body of an enormous fungus that stood more than 8 meters tall, towering over the low, carpet-like vegetation. By the Middle Devonian, shrub-like forests of primitive plants existed: lycophytes, horsetails, ferns, and progymnosperms had evolved. Most of these plants had true roots and leaves, and many were quite tall. The earliest known trees, from the genus Wattieza, appeared in the Late Devonian around 385 Ma.[18] In the Late Devonian, the tree-like ancestral Progymnosperm Archaeopteris which had conifer-like true wood and fern-like foliage and the cladoxylopsids grew.[19] (See also: lignin.) These are the oldest known trees of the world's first forests. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion".
The 'greening' of the continents acted as a carbon dioxide sink, and atmospheric levels of this greenhouse gas may have dropped. This may have cooled the climate and led to a massive extinction event. See Late Devonian extinction.
Primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed-plants that characterizes a recognizably modern world had its genesis in the Late Devonian period. 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 containing copious animals opened the way for the first vertebrates to seek out a terrestrial living. By the end of the Devonian, arthropods were solidly established on the land.[20]
A major extinction occurred at the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 372.2 ± 1.6 Mya, when all the fossil agnathan fishes, save for the psammosteid heterostracans, suddenly disappeared. A second strong pulse closed the Devonian period. The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota, more drastic than the familiar extinction event that closed the Cretaceous.
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, conodonts, and acritarchs, as well as jawless fish, and all placoderms. Land plants as well as freshwater species, such as our tetrapod ancestors, were relatively unaffected by the Late Devonian extinction event.
The reasons for the Late Devonian extinctions are still unknown, and all explanations remain speculative.[21] 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.
| Wikisource has original works on the topic: Paleozoic#Devonian |
| Wikimedia Commons has media related to Devonian. |
| Preceded by Proterozoic Eon | Phanerozoic Eon | |||||||||||
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| Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian | Triassic | Jurassic | Cretaceous | Paleogene | Neogene | 4ry | |
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| リンク元 | 「デボン紀」 |
| 拡張検索 | 「Devonian period」 |