part_of silurian
The Silurian is a geologic period and ___MASK13___ that extends from the end of the Ordovician Period , at 443 . 8 million years ago ( mya ) , to the beginning of the Devonian Period . As with other geologic periods , the rock beds that define the period ' s start and end are well identified , but the exact dates are uncertain by several million years . The base of the Silurian is set at a major extinction event when 60 % of marine species were wiped out . See Ordovician - Silurian extinction events . A significant evolutionary milestone during the Silurian was the diversification of jawed and bony fish . Life also began to appear on land in the form of small , moss - like , vascular plants that grew beside lakes , streams , and coastlines , and also in the form of small terrestrial arthropods . However , terrestrial life would not greatly diversify and affect the landscape until the Devonian . The Ordovician is a geologic period and ___MASK13___ , the second of six periods of the ___MASK33___ Era . The Ordovician spans 41 . 2 million years from the end of the Cambrian Period million years ago ( Mya ) to the start of the Silurian Period Mya . The Ordovician , named after the Celtic tribe of the Ordovices , was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison , who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods , respectively . Lapworth , recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods , and placed them in a period of their own . It received international sanction in 1960 , when it was adopted as an official period of the ___MASK33___ Era by the International Geological Congress . The Carboniferous is a geologic period and ___MASK13___ that spans 60 million years from the end of the Devonian Period million years ago ( Mya ), to the beginning of the Permian Period , Mya . The name " Carboniferous " means " coal - bearing " and derives from the Latin words " carb " (" coal ") and " fer " (" I bear , I carry "), and was coined by geologists William Conybeare and William Phillips in 1822 . Based on a study of the British rock succession , it was the first of the modern ' ___MASK13___ ' names to be employed , and reflects the fact that many coal beds were formed globally during that time . The Carboniferous is often treated in North America as two geological periods , the earlier Mississippian and the later Pennsylvanian . The Devonian is a geologic period and ___MASK13___ of the ___MASK33___ , spanning 60 million years from the end of the Silurian , million years ago ( Mya ), to the beginning of the Carboniferous , 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 Fish ". The first ray - finned and lobe - finned bony fish appeared , while the placodermi began dominating almost every known aquatic environment . A ___MASK68___ is any member of a group of organisms that consist of all gill - bearing aquatic craniate animals that lack limbs with digits . They form a sister group to the tunicates , together forming the olfactores . Included in this definition are the living hagfish , lampreys , and cartilaginous and bony ___MASK68___ as well as various extinct related groups . Tetrapods emerged within lobe - finned fishes , so cladistically they are ___MASK68___ as well . However , traditionally ___MASK68___ are rendered obsolete or paraphyletic by excluding the tetrapods ( i . e ., the amphibians , reptiles , birds and mammals which all descended from within the same ancestry ). Because in this manner the term " ___MASK68___ " is defined negatively as a paraphyletic group , it is not considered a formal taxonomic grouping in systematic biology . The traditional term pisces ( also ichthyes ) is considered a typological , but not a phylogenetic classification . Vascular plants ( from Latin " vasculum ": duct ), also known as tracheophytes ( from the equivalent Greek term " trachea ") and also higher plants , form a large group of plants ( ___MASK28___ . 308 , 312 accepted known species ) that are defined as those land plants that have lignified tissues ( the xylem ) for conducting water and minerals throughout the plant . They also have a specialized non - lignified tissue ( the phloem ) to conduct products of photosynthesis . Vascular plants include the clubmosses , horsetails , ferns , gymnosperms ( including conifers ) and angiosperms ( flowering plants ). Scientific names for the group include Tracheophyta and Tracheobionta . The ___MASK53___ Period ( or ) was the first ___MASK44___ of the ___MASK33___ Era , of the Phanerozoic Eon . The ___MASK53___ lasted 55 . 6 million years from the end of the preceding Ediacaran Period 541 million years ago ( mya ) to the beginning of the Ordovician Period mya . Its subdivisions , and its base , are somewhat in flux . The period was established ( as  ___MASK53___ series ) by Adam Sedgwick , who named it after Cambria , the Latinised form of " Cymru ", the Welsh name for Wales , where Britain ' s ___MASK53___ rocks are best exposed . The ___MASK53___ is unique in its unusually high proportion of lagerstätte sedimentary deposits , sites of exceptional preservation where " soft " parts of organisms are preserved as well as their more resistant shells . As a result , our understanding of the ___MASK53___ biology surpasses that of some later periods . In ___MASK73___ , adaptive radiation is a process in which organisms diversify rapidly from an ancestral ___MASK46___ into a multitude of new forms , particularly when a change in the environment makes new resources available , creates new challenges , or opens new environmental niches . Starting with a recent single ancestor , this process results in the speciation and phenotypic adaptation of an array of ___MASK46___ exhibiting different morphological and physiological traits . An example of adaptive radiation would be the avian ___MASK46___ of the Hawaiian honeycreepers . Via natural selection , these birds adapted rapidly and converged based on the different environments of the ___MASK27___ . The Ordovices were one of the Celtic tribes living in Great Britain before the Roman invasion . Their tribal lands were located in present - ___MASK62___ North Wales and England between the Silures to the south and the Deceangli to the north - east . The Ordovices were conquered by the Roman governor Gnaeus Julius Agricola in the campaign of AD 77  78 . ___MASK52___ ( short for " Freedom Organisation for the Right to Enjoy Smoking Tobacco ") is a United Kingdom political pressure group which campaigns against tobacco control activity . Adam Sedgwick ( 22 ___MASK51___ 1785  27 ___MASK26___ 1873 ) was one of the founders of modern geology . He proposed the Devonian period of the geological timescale . Later , he proposed the ___MASK53___ period , based on work which he did on Welsh ___MASK79___ strata . Placodermi ( from the Greek  = plate and  = ___MASK34___ , literally " plate - skinned ") is an extinct class of armoured prehistoric fish , known from fossils , which lived from the Silurian to the end of the Devonian Period . Their ___MASK98___ and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked , depending on the species . Placoderms were among the first jawed fish ; their jaws likely evolved from the first of their gill arches . Placoderms are paraphyletic , and consist of several distinct outgroups or sister taxa to all living jawed vertebrates , which originated among their ranks . This is illustrated by a 419 - million - year - old fossil , " Entelognathus ", from ___MASK58___ , which is the only known placoderm with a type of bony jaw like that found in modern bony fishes . This includes a dentary bone , which is found in humans and other tetrapods . The jaws in other placoderms were simplified and consisted of a single bone . Placoderms were also the first fish to develop pelvic fins , the precursor to hindlimbs in tetrapods , as well as true teeth . 380 - million - year - old fossils of three other genera , " Incistoscutum ", " Materpiscis " and " Austroptyctodus ", represent the oldest known examples of live birth . Charles Lapworth , FRS , FGS ( 20 ___MASK45___ 1842  13 March 1920 ) was an English geologist who pioneered faunal analysis using index fossils and identified the Ordovician period . FOSSIL is a standard protocol for allowing serial ___MASK21___ for telecommunications programs under the DOS ___MASK56___ . FOSSIL is an acronym for " Fido Opus SEAdog Standard Interface Layer ". " Fido " refers to FidoBBS , " Opus " refers to Opus - CBCS BBS , and " SEAdog " refers to a Fidonet compatible mailer . The standards document that defines the FOSSIL protocol is maintained by the Fidonet Technical Standards Committee . The ___MASK33___ ( or Palaeozoic ) Era ( from the Greek " palaios " (  ), " old " and " zoe " (  ), " life ", meaning " ancient life ") is the earliest of three geologic eras of the Phanerozoic , from . It is the longest of the Phanerozoic eras , and is subdivided into six geologic periods ( from oldest to youngest ): the Cambrian , Ordovician , Silurian , Devonian , Carboniferous , and Permian . The ___MASK33___ comes after the Neoproterozoic era of the Proterozoic and is followed by the Mesozoic . A continent is one of several very large landmasses on ___MASK96___ . Generally identified by convention rather than any strict criteria , up to seven regions are commonly regarded as continents . Ordered from largest in size to smallest , they are : ___MASK30___ , Africa , North America , South America , Antarctica , Europe , and Australia . In geology , areas of continental crust include regions covered with water . Devon ( archaically known as Devonshire ) is a county of ___MASK9___ , reaching from the Bristol Channel in the north to the ___MASK3___ in the south . It is ___MASK2___ of South West ___MASK9___ , bounded by Cornwall to the west , Somerset to the northeast , and Dorset to the east . The City of Exeter is the county town ; seven other districts of East Devon , Mid Devon , North Devon , South Hams , Teignbridge , Torridge , and West Devon are under the jurisdiction of Devon County Council ; Plymouth and Torbay are each a ___MASK2___ of Devon but administered as unitary authorities . Combined as a ceremonial county , Devon ' s area is and its population is about 1 . 1 million . ___MASK9___ is a country that is part of the United Kingdom . It shares land borders with Scotland to the north and Wales to the west . The Irish ___MASK42___ lies northwest of ___MASK9___ and the Celtic ___MASK42___ lies to the southwest . ___MASK9___ is separated from continental Europe by the North ___MASK42___ to the east and the English Channel to the south . The country covers five - eighths of the island of Great Britain ( which lies in the North Atlantic ) in its centre and south ; and includes over 100 smaller islands such as the Isles of Scilly , and the Isle of Wight . Roderick Impey Murchison , 1st Baronet KCB DCL FRS FRSE FLS PRGS PBA MRIA ( 22 ___MASK7___ 1792  22 October 1871 ) was a Scottish geologist who first described and investigated the Silurian ___MASK13___ . An arthropod ( from Greek " arthro -", ___MASK14___ + " podos ", foot ) is an invertebrate animal having an exoskeleton ( external ___MASK76___ ), a segmented body , and jointed appendages ( paired appendages ). Arthropods form the phylum Arthropoda , which includes the insects , arachnids , myriapods , and crustaceans . Arthropods are characterized by their jointed limbs and cuticle made of chitin , often mineralised with calcium carbonate . The arthropod body plan consists of segments , each with a pair of appendages . The rigid cuticle inhibits growth , so arthropods replace it periodically by moulting . Their versatility has enabled them to become the most species - rich members of all ecological guilds in most environments . They have over a million described species , making up more than 80 % of all described living animal species , some of which , unlike most animals , are very successful in dry environments .