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The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about Mya (million years ago), to the beginning of the Carboniferous Period, about . 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.〔("Fossil tracks record 'oldest land-walkers'" ), BBC News.〕 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,〔(How Do You Have a Mass Extinction Without an Increase in Extinctions? )〕 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. ==History== 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.〔Rudwick M.S.J. 1985 ''The great Devonian controversy: the shaping of scientific knowledge among gentlemanly specialists''. Chicago: University of Chicago Press.〕〔Note: * Sedgwick and Murchison coined the term "Devonian system" in: Adam Sedgwick and Roderick Impey Murchison (1840) "On the physical structure of Devonshire, and on the subdivisions and geological relations of its older stratified deposits, etc.," ''Transactions of the Geological Society of London'', 2nd series, 5 (part II) : 633-687 (Part I) and 688-705 (Part II). (From p. 701: ) "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. From (Sedgwick and Murchison, 1840), p. 690: "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. See: William Lonsdale (1840) ("Notes on the age of limestones from south Devonshire," ) ''Transactions of the Geological Society of London'', 2nd series, 5 (part II) : 721-738 ; see especially pp. 724 and 727. From p. 724: " … Mr. Austen's communication () 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, … ."〕 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 Mya, to the beginning of the Carboniferous Period Mya (in North America, the beginning of the Mississippian subperiod of the Carboniferous).〔 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",〔(Age of Fishes Museum )〕 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.〔Barclay, W.J. 1989. ''Geology of the South Wales Coalfield Pt II, the country around Abergavenny'', 3rd edn. Memoir of the British Geological Survey Sheet 232 (Eng & Wales) pp18-19〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Devonian」の詳細全文を読む スポンサード リンク
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