Devonian

The Devonian (/dɛˈvoʊn.i.ən/ deh-VOH-nee-ən) is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, 419.2 million years ago (Mya), to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, England, where rocks from this period were first studied.Dunkleosteus, one of the largest armoured fish ever to roam the planet, lived during the late DevonianEarly shark Cladoselache, several lobe-finned fishes, including Eusthenopteron that was an early marine tetrapod, and the placoderm Bothriolepis in a painting from 1905Enrolled phacopid trilobite from the Devonian of OhioThe common tabulate coral Aulopora from the Middle Devonian of Ohio – view of colony encrusting a brachiopod valveTropidoleptus carinatus, an orthid brachiopod from the Middle Devonian of New York.Pleurodictyum americanum, Kashong Shale, Middle Devonian of New YorkSEM 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 The Devonian (/dɛˈvoʊn.i.ən/ deh-VOH-nee-ən) is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, 419.2 million years ago (Mya), to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, England, where rocks from this period were first studied. The first significant adaptive radiation of life on dry land occurred during the Devonian. 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 Fishes.' The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment. The ancestors of all four-limbed vertebrates (tetrapods) began adapting to walking on land, as their strong pectoral and pelvic fins gradually evolved into legs. In the oceans, primitive sharks became more numerous than in the Silurian and Late Ordovician. The first ammonites, species 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 palaeogeography 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 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. 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, the Devonian extends from the end of the Silurian 419.2 Mya, to the beginning of the Carboniferous 358.9 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', 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 but 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.