出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2015/11/21 03:34:49」(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 |
ペルム紀(ペルムき、Permian period)は、今から約2億9,900万年前から約2億5,100万年前までを指す地質時代である。ただし開始と終了の時期はそれぞれ数百万年の誤差がある。以前はドイツの地層(上下二分される)名から二畳紀(にじょうき)と呼ばれることが多かったが、近年はペルム紀と呼ばれることが多い。石炭紀の後、三畳紀(トリアス紀)の前の紀である。また、古生代の最後の紀であり、ペルム紀が終わると中生代となる。ペルム紀という名前は、ロシアのペルミという都市から名付けられた。
ペルム紀には、様々な植物、巨大な両生類や爬虫類が生息していた。その中には、恐竜や鳥類、現生爬虫類の祖先となる双弓類もいた。哺乳類の祖先に当たる単弓類(哺乳類型爬虫類)も繁栄し、陸上には豊かな生態系が築かれていた。昆虫から完全変態の種族が進化したのも、この頃であった。ペルム紀の浅い海の堆積物からは、豊富な軟体動物、棘皮動物、腕足動物の化石が産出する。三葉虫なども繁栄していた。
植物では、シダ植物に加え、イチョウ類やソテツ類といった裸子植物も繁栄を始めた。
また、特異な例であるが、この時代の微生物の培養が報告されている。約2億5000万年前に形成された岩塩層から、結晶内部に封じ込められていた古細菌と真正細菌の培養に成功し[1]、古細菌の方はハロバクテリウム科の新属新種 Halosimplex carlsbadense として記載された。
ペルム紀の初期には、赤道付近に存在していたユーラメリカ大陸と、南半球から北上してきたゴンドワナ大陸が衝突し、パンゲア大陸と呼ばれる超大陸が形成されていた。北半球にはシベリア大陸が存在していたが、やがてシベリア大陸もパンゲア大陸と衝突し、ウラル山脈が形成され、ほぼ全ての陸地が1つの超大陸としてまとまることとなった。パンゲア大陸は赤道を挟み三日月状(Cの字)の形をとった。大陸の周囲はパンサラッサと呼ばれる大洋が囲んでおり、大陸の東側(三日月形の内側)には古テチス海と呼ばれる海が広がり、シベリア大陸からゴンドワナ大陸に、小大陸や島が点々と連なっていた。
ペルム紀の初期には、ゴンドワナ大陸が南極地域にあり、大規模な氷床が発達していたため、気候は寒冷だった。しかしゴンドワナ大陸が北上して南極地域を脱したことから、氷床は融解しはじめ、気温は上昇に転じた。ペルム紀の末期には激しい気温上昇が起こり、地球の平均気温は23℃にも達した。これは、6億年前から現在まででもっとも高い気温である。
ペルム紀の終わり(P-T境界)に、地球史上最大規模とも言われる大量絶滅が起こった。このとき絶滅した種の割合は、海洋生物のうちの96%。全ての生物種の90%から95%に達すると言われる。原因はまだよくわかっていないが、スーパープルームにより地球史上もっとも激しい火山活動が起き(この火山活動が現在のシベリア・トラップを形成したとされる。噴出した溶岩の量は、富士山が過去一万年間で噴出した溶岩の量の10万倍である)、それによる気候変動がメタンハイドレートを融解させて更なる気候変動が起こるなどの大規模な環境変化が発生し、大量絶滅に繋がったとする説がある。
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ウィキメディア・コモンズには、ペルム紀に関連するカテゴリがあります。 |
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Permian Period 298.9–252.17 million years ago PreЄ
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Mean atmospheric O 2 content over period duration |
ca. 23 vol %[1] (115 % of modern level) |
Mean atmospheric CO 2 content over period duration |
ca. 900 ppm[2] (3 times pre-industrial level) |
Mean surface temperature over period duration | ca. 16 °C[3] (2 °C above modern level) |
Sea level (above present day) | Relatively constant at 60 m (200 ft) in early Permian; plummeting during the middle Permian to a constant −20 m (−66 ft) in the late Permian.[4] |
The Permian is a geologic period and system which extends from 298.9 to 252.17 million years ago.[5] It is the last period of the Paleozoic, following the Carboniferous and preceding the Triassic of the Mesozoic. The concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the ancient kingdom of Permia.
The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles, lepidosaurs and archosaurs. The world at the time was dominated by a single supercontinent known as Pangaea, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior.[6] Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors.
The Permian (along with the Paleozoic) ended with the largest mass extinction in Earth's history, in which nearly 90% of marine species and 70% of terrestrial species died out.[7] It would take well into the Triassic for life to recover from this catastrophe.[8] Recovery from the Permian-Triassic extinction event was protracted; on land, ecosystems took 30 million years to recover.[9]
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The term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil.[10][11] The region now lies in the Perm Krai of Russia.
Official ICS 2013 Subdivisions of the Permian System,[12] from most recent to most ancient rock layers are:
Sea levels in the Permian remained generally low, and near-shore environments were limited by the collection of almost all major landmasses into a single continent – Pangaea. This could have in part caused the widespread extinctions of marine species at the end of the period by severely reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean ("Panthalassa", the "universal sea"), and the Paleo-Tethys Ocean, a large ocean that was between Asia and Gondwana. The Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic Era. Large continental landmasses create climates with extreme variations of heat and cold ("continental climate") and monsoon conditions with highly seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea. Such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores. The first modern trees (conifers, ginkgos and cycads) appeared in the Permian.
Three general areas are especially noted for their extensive Permian deposits – the Ural Mountains (where Perm itself is located), China, and the southwest of North America, where the Permian Basin in the U.S. state of Texas is so named because it has one of the thickest deposits of Permian rocks in the world.
The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an Ice Age, which began in the Carboniferous. Glaciers receded around the mid-Permian period as the climate gradually warmed, drying the continent's interiors.[14] In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles.[14]
Permian marine deposits are rich in fossil mollusks, echinoderms, and brachiopods.[15] Fossilized shells of two kinds of invertebrates are widely used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist that is one of the foraminiferans, and ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct.
Terrestrial life in the Permian included diverse plants, fungi, arthropods, and various types of tetrapods. The period saw a massive desert covering the interior of the Pangaea. The warm zone spread in the northern hemisphere, where extensive dry desert appeared.[15] The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals died out or became marginal elements.
The Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began. The swamp-loving lycopod trees of the Carboniferous, such as Lepidodendron and Sigillaria, were progressively replaced in the continental interior by the more advanced seed ferns and early conifers. At the close of the Permian, lycopod and equisete swamps reminiscent of Carboniferous flora were relegated to a series of equatorial islands in the Paleotethys Sea that later would become South China.[16]
The Permian saw the radiation of many important conifer groups, including the ancestors of many present-day families. Rich forests were present in many areas, with a diverse mix of plant groups. The southern continent saw extensive seed fern forests of the Glossopteris flora. Oxygen levels were probably high there. The ginkgos and cycads also appeared during this period.
From the Pennsylvanian Subperiod of the Carboniferous period until well into the Permian, the most successful Insects were primitive relatives of cockroaches. Six fast legs, four well-developed folding wings, fairly good eyes, long, well-developed antennae (olfactory), an omnivorous digestive system, a receptacle for storing sperm, a chitin-based exoskeleton that could support and protect, as well as a form of gizzard and efficient mouth parts, gave it formidable advantages over other herbivorous animals. About 90% of insects at the start of the Permian were cockroach-like insects ("Blattopterans").[17]
Primitive forms of dragonflies (Odonata) were the dominant aerial predators and probably dominated terrestrial insect predation as well. True Odonata appeared in the Permian,[18][19] and all are effectively semi-aquatic insects (aquatic immature stages, and terrestrial adults), as are all modern odonates. Their prototypes are the oldest winged fossils,[20] go back to the Devonian, and are different in several respects from the wings of other insects.[21] Fossils suggest they may have possessed many modern attributes even by the late Carboniferous, and it is possible that they captured small vertebrates, for at least one species had a wing span of 71 centimetres (28 in).[22] Several other insect groups appeared during the Permian, including the Coleoptera (beetles) and Hemiptera (true bugs).
Early Permian terrestrial faunas were dominated by pelycosaurs, diadectes and amphibians,[23][24] the middle Permian by primitive therapsids such as the dinocephalia, and the late Permian by more advanced therapsids such as gorgonopsians and dicynodonts. Towards the very end of the Permian the first archosaurs appeared, a group that would give rise to the crurotarsans and the dinosaurs in the following period. Also appearing at the end of the Permian were the first cynodonts, which would go on to evolve into mammals during the Triassic. Another group of therapsids, the therocephalians (such as Lycosuchus), arose in the Middle Permian.[25][26] There were no aerial vertebrates (with the exception of gliding lizards, the avicephalans).
The Permian period saw the development of a fully terrestrial fauna and the appearance of the first large herbivores and carnivores. It was the high tide of the anapsids in the form of the massive Pareiasaurs and host of smaller, generally lizard-like groups. A group of small reptiles, the diapsids started to abound. These were the ancestors to most modern reptiles and the ruling dinosaurs as well as pterosaurs and crocodiles.
Thriving also were the early ancestors to mammals, the synapsida, which included some large members such as Dimetrodon. Reptiles grew to dominance among vertebrates, because their special adaptations enabled them to flourish in the drier climate.[23]
Permian amphibians consisted of temnospondyli, lepospondyli and batrachosaurs.
Edaphosaurus pogonias and Platyhystrix - Early Permian, North America and Europe
Dimetrodon and Eryops - Early Permian, North America
Ocher fauna, Estemmenosuchus and Ivantosaurus - Middle Permian, Ural Region
Titanophoneus and Ulemosaurus - Ural Region
The Permian ended with the most extensive extinction event recorded in paleontology: the Permian-Triassic extinction event. 90% to 95% of marine species became extinct, as well as 70% of all land organisms. It is also the only known mass extinction of insects.[8][27] Recovery from the Permian-Triassic extinction event was protracted; on land, ecosystems took 30M years to recover.[9] Trilobites, which had thrived since Cambrian times, finally became extinct before the end of the Permian. Nautiluses, a species of cephalopods, surprisingly survived this occurrence.
There is also significant evidence that massive flood basalt eruptions from magma output lasting thousands of years in what is now the Siberian Traps contributed to environmental stress leading to mass extinction. The reduced coastal habitat and highly increased aridity probably also contributed. Based on the amount of lava estimated to have been produced during this period, the worst-case scenario is an expulsion of enough carbon dioxide from the eruptions to raise world temperatures five degrees Celsius.[14]
Another hypothesis involves ocean venting of hydrogen sulfide gas. Portions of the deep ocean will periodically lose all of their dissolved oxygen allowing bacteria that live without oxygen to flourish and produce hydrogen sulfide gas. If enough hydrogen sulfide accumulates in an anoxic zone, the gas can rise into the atmosphere. Oxidizing gases in the atmosphere would destroy the toxic gas, but the hydrogen sulfide would soon consume all of the atmospheric gas available to change it. Hydrogen sulfide levels would increase dramatically over a few hundred years. Modeling of such an event indicates that the gas would destroy ozone in the upper atmosphere allowing ultraviolet radiation to kill off species that had survived the toxic gas.[28] Of course, there are species that can metabolize hydrogen sulfide.
Another hypothesis builds on the flood basalt eruption theory. Five degrees Celsius would not be enough increase in world temperatures to explain the death of 95% of life. But such warming could slowly raise ocean temperatures until frozen methane reservoirs below the ocean floor near coastlines melted, expelling enough methane, among the most potent greenhouse gases, into the atmosphere to raise world temperatures an additional five degrees Celsius. The frozen methane hypothesis helps explain the increase in carbon-12 levels midway into the Permian-Triassic boundary layer. It also helps explain why the first phase of the layer's extinctions was land-based, the second was marine-based (and starting right after the increase in C-12 levels), and the third land-based again.[citation needed]
An even more speculative hypothesis is that intense radiation from a nearby supernova was responsible for the extinctions.[29]
In 2006, a group of American scientists from The Ohio State University reported evidence for a possible huge meteorite crater (Wilkes Land crater) with a diameter of around 500 kilometers in Antarctica.[30] The crater is located at a depth of 1.6 kilometers beneath the ice of Wilkes Land in eastern Antarctica. The scientists speculate that this impact may have caused the Permian–Triassic extinction event, although its age is bracketed only between 100 million and 500 million years ago. They also speculate that it may have contributed in some way to the separation of Australia from the Antarctic landmass, which were both part of a supercontinent called Gondwana. Levels of iridium and quartz fracturing in the Permian-Triassic layer do not approach those of the Cretaceous–Paleogene boundary layer. Given that a far greater proportion of species and individual organisms became extinct during the former, doubt is cast on the significance of a meteorite impact in creating the latter. Further doubt has been cast on this theory based on fossils in Greenland showing the extinction to have been gradual, lasting about eighty thousand years, with three distinct phases.[31]
Many scientists argue that the Permian-Triassic extinction event was caused by a combination of some or all of the hypotheses above and other factors; the formation of Pangaea decreased the number of coastal habitats and may have contributed to the extinction of many clades.[citation needed]
Wikisource has original works on the topic: Paleozoic#Permian |
Wikimedia Commons has media related to Permian. |
Preceded by Proterozoic Eon | Phanerozoic Eon | |||||||||||
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Paleozoic Era | Mesozoic Era | Cenozoic Era | ||||||||||
Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian | Triassic | Jurassic | Cretaceous | Paleogene | Neogene | 4ry |
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リンク元 | 「ペルム紀」 |
拡張検索 | 「Permian period」 |
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