The Ordovician period is the second of the six (seven in %20%20%20%20%20%20%20World%20map%20showing%20location%20of%20North%20America%20%20%20%20%20%20%20A%20satellite%20composite%20image%20of%20North%20America%20%20%20North%20America%20is%20the%20third%20largest%20%20%20continent%20in%20area%20and%20in%20population%20after%20%20%20Eurasia%20and%20%20%20Africa.%20... North America) periods of the %20%20%20The%20Palaeozoic%20is%20a%20major%20division%20of%20the%20geologic%20timescale%2C%20one%20of%20four%20geologic%20eras.%20... Paleozoic %20%20%20ERA%20is%20an%20abbreviation%20for%20several%20different%20things%2C%20including%3A%20%20the%20Equal%20Rights%20Amendment%2C%20a%20proposed%2C%20but%20unratified%2C%20Constitutional%20amendment%20in%20United%20States%20Earned%20run%20average%2C%20a%20baseball%20statistic%20Engine%20Room%20Artificer%20Education%20Reform%20Act%20Engineering%20Research%20Associates%2C%20a%20pioneering%20computer%20firm%20from%20the%201950s%20Academy%20of%20European%20Law%20Trier%20Explosive... era. The Ordovician follows the %20%20%20%20%20The%20Cambrian%20is%20a%20major%20division%20of%20the%20geologic%20timescale%20that%20begins%20about%20542%20million%20years%20before%20the%20present%20(BP)%20at%20the%20end%20of%20the%20Proterozoic%20eon_geology%20and%20ended%20about%20490%20million%20years%20BP%20with%20the%20beginning%20of%20the%20Ordovician%20period.%20... Cambrian period and is followed by the %20%20%20%20%20Alternate%20use%3A%20The%20Silurians%2C%20a%20reptilian%20race%20from%20the%20science%20fiction%20series%20%5CDoctor%20Who%5C.%20%20The%20Silurian%20is%20a%20major%20division%20of%20the%20geologic%20timescale%20that%20extends%20from%20the%20end%20of%20the%20Ordovician%20period%2C%20about%20439%20million%20years%20before%20the%20present%20(BP)%2C%20to%20the%20beginning%20of%20the%20Devonian%20period%2C%20about%20408.5%20million%20years%20BP.%20As%20with%20other%20geologic%20periods%2C%20the%20rock%20beds%20that%20define%20the%20period%5Cs%20start%20and%20end%20are%20well%20identified%2C%20but%20the%20exact%20dates%20are%20uncertain%20by%205-10%20million%20years.%20The%20base%20of%20the%20Silurian%20is%20set%20at%20a%20major%20extinction%20event%20where%2060%5C%25%20of%20marine%20species%20were%20wiped%20out.%20See%20Ordovician-Silurian%20extinction%20events.%20The%20Silurian%20system%20was%20first%20identified%20by%20Sir%20Roderick%20Murchison%2C%20who%20was%20examining%20fossil-bearing%20sedimentary%20rock%20strata%20in%20south%20Wales%20in%20the%20early%201830s.%20He%20named%20the%20sequences%20for%20a%20Celtic%20tribe%20of%20Wales%26%238212%3B%20the%20Silures%2C%20extending%20the%20convention%20his%20friend%20Adam%20Sedgwick%20had%20established%20for%20the%20%5Ccambrian%5C.%20In%201835%20the%20two%20men%20presented%20a%20joint%20paper%2C%20under%20the%20title%20On%20the%20Silurian%20and%20Cambrian%20Systems%2C%20Exhibiting%20the%20Order%20in%20which%20the%20Older%20Sedimentary%20Strata%20Succeed%20each%20other%20in%20England%20and%20Wales%2C%20which%20was%20the%20germ%20of%20the%20the%20modern%20geological%20time%20scale.%20The%20%5CSilurian%5C%20series%20quickly%20came%20to%20overlap%20Sedgwick%5Cs%20Cambrian%20sequence%2C%20provoking%20furious%20disagreements%20that%20ended%20the%20friendship.%20Charles%20Lapworth%20eventually%20resolved%20the%20conflict%20by%20defining%20a%20new%20Ordovician%20system%20including%20the%20contended%20beds.%20%20%20Silurian%20subdivisions%20The%20Silurian%20is%20usually%20broken%20into%20lower%20(Llandovery%20and%20Wenlock)%20and%20upper%20(Ludlow%20and%20Pridoli)%20subdivisions%20(epochs).%20Nevertheless%2C%20some%20schemes%20use%20a%20lower%20(Llandovery)%2C%20middle%20(Wenlock)%20and%20upper%20(Ludlow%20and%20Pridoli)%20breakdown.%20These%20faunal%20stages%20are%20characterized%20by%20their%20index%20fossils%2C%20new%20species%20of%20colonial%20marine%20Graptolites%20that%20appeared%20in%20each.%20The%20series%20and%20stages%20from%20youngest%20to%20oldest%20are%3A%20%20Pridoli%20Series%20-%20no%20stages%20defined%20(Upper%20Silurian)%20%20%20Ludlow%20Series%20divided%20into%20%20Ludfordian%20(late%20Ludlow%20-%20Upper%20Silurian)%20Gorstian%20(early%20Ludlow%20-%20Upper%20Silurian)%20%20%20%20%20Wenlock%20Series%20divided%20into%20%20Homerian%20(late%20Wenlock%20-%20Lower%20Silurian%20Sheinwoodian%20(early%20Wenlock%20-%20Lower%20Silurian%20%20%20%20%20Llandovery%20Series%20divided%20into%20%20Telychian%20(late%20Llandovery%20-%20Lower%20Silurian)%20Aeronian%20(mid%20Llandovery%20-%20Lower%20Silurian)%20Rhuddanian%20(early%20Llandovery%20-%20Lower%20Silurian)%20%20%20%20In%20North%20America%20a%20different%20series%20of%20regional%20stages%20is%20used%3A%20%20Cayugan%20(Late%20Silurian%20-%20Ludlow)%20Lockportian%20(Middle%20Silurian%20-%20Wenlock)%20Tonawandan%20(Middle%20Silurian%20-%20Wenlock)%20Ontarian%20(Lower%20Silurian%20-%20Llandovery)%20Alexandrian%20(Lower%20Silurian%20-%20Llandovery)%20%20%20Silurian%20paleogeography%20During%20the%20Silurian%2C%20Gondwana%20continued%20a%20slow%20southward%20drift%20to%20high%20southern%20latitudes%2C%20but%20there%20is%20evidence%20that%20the%20Silurian%20icecaps%20were%20less%20extensive%20than%20those%20of%20the%20late%20Ordovician%20glaciation.%20The%20melting%20of%20icecaps%20and%20glaciers%20contributed%20to%20a%20rise%20in%20sea%20level.%20Other%20cratons%20and%20continent%20fragments%20drifted%20together%20near%20the%20equator%2C%20starting%20the%20formation%20of%20a%20second%20supercontinent%20known%20as%20Laurasia.%20This%20is%20the%20time%20of%20the%20Caledonian%20orogeny%2C%20a%20spate%20of%20mountainbuilding%20in%20conjoined%20Europe%20and%20Greenland.%20During%20this%20period%2C%20the%20Earth%20entered%20a%20long%20warm%20greenhouse%20phase%2C%20and%20warm%20shallow%20seas%20covered%20much%20of%20the%20equatorial%20land%20masses.%20The%20period%20witnessed%20a%20relative%20stabilization%20of%20the%20Earth%5Cs%20general%20climate%2C%20ending%20the%20previous%20pattern%20of%20erratic%20climatic%20fluctuations.%20%20Silurian%20fauna%20Silurian%20high%20sea%20levels%20and%20warm%20shallow%20continental%20seas%20provided%20a%20hospitable%20environment%20for%20marine%20life%20of%20all%20kinds.%20Silurian%20beds%20are%20oil%20and%20gas%20producers%20in%20some%20areas.%20Extensive%20beds%20of%20Silurian%20hematite%20--%20an%20iron%20ore%20--%20in%20eastern%20North%20America%20were%20important%20to%20the%20early%20American%20colonial%20economy.%20Coral%20reefs%20made%20their%20first%20appearance%20during%20this%20time.%20A%20few%20arthropods%20seem%20to%20have%20invaded%20the%20land%20during%20the%20Silurian.%20Fish%20reached%20considerable%20diversity%20and%20developed%20movable%20jaws.%20A%20diverse%20fauna%20of%20Eurypterus%20(Sea%20Scorpions)%20--%20some%20of%20them%20several%20meters%20in%20length%20--%20graced%20the%20shallow%20Silurian%20seas%20of%20North%20America.%20Many%20of%20their%20fossils%20have%20been%20found%20in%20the%20state%20of%20New%20York.%20Brachiopods%2C%20bryozoa%2C%20mollusks%2C%20and%20trilobites%20were%20abundant%20and%20diverse.%20The%20first%20fossil%20records%20of%20vascular%20plants%2C%20that%20is%2C%20land%20plants%20with%20tissues%20that%20carry%20food%2C%20appeared%20in%20the%20Silurian%20period.%20Rhyniophytes%2C%20primitive%20lycopods%2C%20and%20myriapods%20became%20the%20first%20proper%20terrestrial%20organisms.%20The%20terrestrial%20ecosystems%20included%20the%20first%20multicellular%20animals%20that%20have%20been%20identified%2C%20relatives%20of%20modern%20spiders%20and%20centipedes%20whose%20fossils%20were%20discovered%20in%20the%201990s.%20%20External%20links%20%20Paleos%3A%20Silurian%20UCMP%20Berkeley%3A%20The%20Silurian%20%20%20%20Categories%3A%20Geologic%20timescale%20%20%20%20%20%20%20%20... Silurian period. The Ordovician, named after the %20%20%20%20Welsh%20redirects%20here%2C%20and%20this%20article%20describes%20the%20Welsh%20language.%20... Welsh tribe of the %20%20%20The%20Ordovices%20were%20one%20of%20the%20Celtic%20tribes%20living%20in%20the%20British%20Islands%2C%20before%20the%20Roman%20invasion%20of%20Britain.%20... Ordovices, was defined by %20%20%20Charles%20Lapworth%20(September%2020%2C%201842_March%2013%2C%201920)%20was%20a%2019th%20century%20English%20geologist.%20... Charles Lapworth in 1879 to resolve a situation where followers of %20%20%20Adam%20Sedgwick%20(March%2022nd%2C%201785%20-%20January%2027%2C%201873)%20was%20one%20of%20the%20founders%20of%20modern%20geology.%20... Adam Sedgwick and %20%20%20%20%20%20%20Sir%20Roderick%20Murchison%20%20%20Sir%20Roderick%20Impey%20Murchison%20(February%2019%2C%201792%20%26%238211%3B%20October%2022%2C%201871)%2C%20was%20a%20powerful%20Scottish%20geologist%20who%20first%20described%20and%20investigated%20the%20Silurian%20era.%20... Roderick Murchison were placing the same %20%20%20Rock%20is%20a%20naturally%20occurring%20aggregate%20of%20minerals%20and%2For%20mineraloids.%20... rock beds in the %20%20%20%20%20The%20Cambrian%20is%20a%20major%20division%20of%20the%20geologic%20timescale%20that%20begins%20about%20542%20million%20years%20before%20the%20present%20(BP)%20at%20the%20end%20of%20the%20Proterozoic%20eon_geology%20and%20ended%20about%20490%20million%20years%20BP%20with%20the%20beginning%20of%20the%20Ordovician%20period.%20... Cambrian and %20%20%20%20%20Alternate%20use%3A%20The%20Silurians%2C%20a%20reptilian%20race%20from%20the%20science%20fiction%20series%20%5CDoctor%20Who%5C.%20%20The%20Silurian%20is%20a%20major%20division%20of%20the%20geologic%20timescale%20that%20extends%20from%20the%20end%20of%20the%20Ordovician%20period%2C%20about%20439%20million%20years%20before%20the%20present%20(BP)%2C%20to%20the%20beginning%20of%20the%20Devonian%20period%2C%20about%20408.5%20million%20years%20BP.%20As%20with%20other%20geologic%20periods%2C%20the%20rock%20beds%20that%20define%20the%20period%5Cs%20start%20and%20end%20are%20well%20identified%2C%20but%20the%20exact%20dates%20are%20uncertain%20by%205-10%20million%20years.%20The%20base%20of%20the%20Silurian%20is%20set%20at%20a%20major%20extinction%20event%20where%2060%5C%25%20of%20marine%20species%20were%20wiped%20out.%20See%20Ordovician-Silurian%20extinction%20events.%20The%20Silurian%20system%20was%20first%20identified%20by%20Sir%20Roderick%20Murchison%2C%20who%20was%20examining%20fossil-bearing%20sedimentary%20rock%20strata%20in%20south%20Wales%20in%20the%20early%201830s.%20He%20named%20the%20sequences%20for%20a%20Celtic%20tribe%20of%20Wales%26%238212%3B%20the%20Silures%2C%20extending%20the%20convention%20his%20friend%20Adam%20Sedgwick%20had%20established%20for%20the%20%5Ccambrian%5C.%20In%201835%20the%20two%20men%20presented%20a%20joint%20paper%2C%20under%20the%20title%20On%20the%20Silurian%20and%20Cambrian%20Systems%2C%20Exhibiting%20the%20Order%20in%20which%20the%20Older%20Sedimentary%20Strata%20Succeed%20each%20other%20in%20England%20and%20Wales%2C%20which%20was%20the%20germ%20of%20the%20the%20modern%20geological%20time%20scale.%20The%20%5CSilurian%5C%20series%20quickly%20came%20to%20overlap%20Sedgwick%5Cs%20Cambrian%20sequence%2C%20provoking%20furious%20disagreements%20that%20ended%20the%20friendship.%20Charles%20Lapworth%20eventually%20resolved%20the%20conflict%20by%20defining%20a%20new%20Ordovician%20system%20including%20the%20contended%20beds.%20%20%20Silurian%20subdivisions%20The%20Silurian%20is%20usually%20broken%20into%20lower%20(Llandovery%20and%20Wenlock)%20and%20upper%20(Ludlow%20and%20Pridoli)%20subdivisions%20(epochs).%20Nevertheless%2C%20some%20schemes%20use%20a%20lower%20(Llandovery)%2C%20middle%20(Wenlock)%20and%20upper%20(Ludlow%20and%20Pridoli)%20breakdown.%20These%20faunal%20stages%20are%20characterized%20by%20their%20index%20fossils%2C%20new%20species%20of%20colonial%20marine%20Graptolites%20that%20appeared%20in%20each.%20The%20series%20and%20stages%20from%20youngest%20to%20oldest%20are%3A%20%20Pridoli%20Series%20-%20no%20stages%20defined%20(Upper%20Silurian)%20%20%20Ludlow%20Series%20divided%20into%20%20Ludfordian%20(late%20Ludlow%20-%20Upper%20Silurian)%20Gorstian%20(early%20Ludlow%20-%20Upper%20Silurian)%20%20%20%20%20Wenlock%20Series%20divided%20into%20%20Homerian%20(late%20Wenlock%20-%20Lower%20Silurian%20Sheinwoodian%20(early%20Wenlock%20-%20Lower%20Silurian%20%20%20%20%20Llandovery%20Series%20divided%20into%20%20Telychian%20(late%20Llandovery%20-%20Lower%20Silurian)%20Aeronian%20(mid%20Llandovery%20-%20Lower%20Silurian)%20Rhuddanian%20(early%20Llandovery%20-%20Lower%20Silurian)%20%20%20%20In%20North%20America%20a%20different%20series%20of%20regional%20stages%20is%20used%3A%20%20Cayugan%20(Late%20Silurian%20-%20Ludlow)%20Lockportian%20(Middle%20Silurian%20-%20Wenlock)%20Tonawandan%20(Middle%20Silurian%20-%20Wenlock)%20Ontarian%20(Lower%20Silurian%20-%20Llandovery)%20Alexandrian%20(Lower%20Silurian%20-%20Llandovery)%20%20%20Silurian%20paleogeography%20During%20the%20Silurian%2C%20Gondwana%20continued%20a%20slow%20southward%20drift%20to%20high%20southern%20latitudes%2C%20but%20there%20is%20evidence%20that%20the%20Silurian%20icecaps%20were%20less%20extensive%20than%20those%20of%20the%20late%20Ordovician%20glaciation.%20The%20melting%20of%20icecaps%20and%20glaciers%20contributed%20to%20a%20rise%20in%20sea%20level.%20Other%20cratons%20and%20continent%20fragments%20drifted%20together%20near%20the%20equator%2C%20starting%20the%20formation%20of%20a%20second%20supercontinent%20known%20as%20Laurasia.%20This%20is%20the%20time%20of%20the%20Caledonian%20orogeny%2C%20a%20spate%20of%20mountainbuilding%20in%20conjoined%20Europe%20and%20Greenland.%20During%20this%20period%2C%20the%20Earth%20entered%20a%20long%20warm%20greenhouse%20phase%2C%20and%20warm%20shallow%20seas%20covered%20much%20of%20the%20equatorial%20land%20masses.%20The%20period%20witnessed%20a%20relative%20stabilization%20of%20the%20Earth%5Cs%20general%20climate%2C%20ending%20the%20previous%20pattern%20of%20erratic%20climatic%20fluctuations.%20%20Silurian%20fauna%20Silurian%20high%20sea%20levels%20and%20warm%20shallow%20continental%20seas%20provided%20a%20hospitable%20environment%20for%20marine%20life%20of%20all%20kinds.%20Silurian%20beds%20are%20oil%20and%20gas%20producers%20in%20some%20areas.%20Extensive%20beds%20of%20Silurian%20hematite%20--%20an%20iron%20ore%20--%20in%20eastern%20North%20America%20were%20important%20to%20the%20early%20American%20colonial%20economy.%20Coral%20reefs%20made%20their%20first%20appearance%20during%20this%20time.%20A%20few%20arthropods%20seem%20to%20have%20invaded%20the%20land%20during%20the%20Silurian.%20Fish%20reached%20considerable%20diversity%20and%20developed%20movable%20jaws.%20A%20diverse%20fauna%20of%20Eurypterus%20(Sea%20Scorpions)%20--%20some%20of%20them%20several%20meters%20in%20length%20--%20graced%20the%20shallow%20Silurian%20seas%20of%20North%20America.%20Many%20of%20their%20fossils%20have%20been%20found%20in%20the%20state%20of%20New%20York.%20Brachiopods%2C%20bryozoa%2C%20mollusks%2C%20and%20trilobites%20were%20abundant%20and%20diverse.%20The%20first%20fossil%20records%20of%20vascular%20plants%2C%20that%20is%2C%20land%20plants%20with%20tissues%20that%20carry%20food%2C%20appeared%20in%20the%20Silurian%20period.%20Rhyniophytes%2C%20primitive%20lycopods%2C%20and%20myriapods%20became%20the%20first%20proper%20terrestrial%20organisms.%20The%20terrestrial%20ecosystems%20included%20the%20first%20multicellular%20animals%20that%20have%20been%20identified%2C%20relatives%20of%20modern%20spiders%20and%20centipedes%20whose%20fossils%20were%20discovered%20in%20the%201990s.%20%20External%20links%20%20Paleos%3A%20Silurian%20UCMP%20Berkeley%3A%20The%20Silurian%20%20%20%20Categories%3A%20Geologic%20timescale%20%20%20%20%20%20%20%20... Silurian periods respectively. %20%20%20Charles%20Lapworth%20(September%2020%2C%201842_March%2013%2C%201920)%20was%20a%2019th%20century%20English%20geologist.%20... Charles Lapworth simply took all the conflicting %20%20%20%20%20Interstate%20road%20cut%20through%20limestone%20and%20shale%20strata%20in%20eastern%20Tennessee%20%20%20In%20geology%20and%20related%20fields%2C%20a%20stratum%20(plural%3A%20strata)%20is%20a%20layer%20of%20rock%20or%20soil%20with%20internally%20consistent%20characteristics%20that%20distinguishes%20it%20from%20contiguous%20layers.%20... strata and placed them in the new Ordovician period.

Ordovician dating

The Ordovician period started at a minor %20%20%20An%20extinction%20event%20(also%20extinction-level%20event%2C%20ELE)%20is%20a%20period%20in%20time%20when%20a%20large%20number%20of%20species%20die%20out.%20... extinction event, possibly caused by a %20%20%20In%20astronomy%2C%20gamma-ray%20bursts%20(GRBs)%20are%20flashes%20of%20gamma%20rays%20that%20last%20from%20seconds%20to%20hours%2C%20the%20longer%20ones%20being%20followed%20by%20several%20days%20of%20X-ray%20afterglow.%20... gamma ray burst, some time 490 million years ago (%20%20%20Mya%20has%20more%20than%20one%20meaning%3A%20%20Mya%2C%20an%20American%20R%26amp%3BB%20singer.%20... mya) and lasted for about 50-80 million years. It ended with a major extinction event 443.5 mya that wiped out 60% of marine %20%20%20%20See%20genus%20(mathematics)%20for%20the%20use%20of%20the%20term%20in%20mathematics.%20... genera. The dates given are recent radiometric dates and vary slightly from those used in other sources.This is the second period of the Paleozoic Era.

Ordovician rocks contain abundant %20%20%20FOSSIL%20is%20a%20standard%20for%20allowing%20serial%20communication%20for%20telecommunications%20programs%20under%20DOS.%20... fossils and contain major %20%20%20Oil%20is%20a%20generic%20term%20for%20organic%20%20%20liquids%20that%20are%20not%20%20%20miscible%20with%20%20%20water.%20... oil and %20%20%20Gas%20(actually%20as%2C%20part%20of%20the%20Gnu%20Binutils%20package)%20is%20the%20default%20Gcc%20Back-end.%20... gas reservoirs in some regions.

Ordovician subdivisions

The Ordovician is usually broken into Lower (Tremadoc and Arenig), Middle (Caradoc, Llanvirn, Llandeilo) and Upper (Ashgill) subdivisions. The Faunal stages from youngest to oldest are:

• Hirnantian/Gamach (Upper-Ashgill)
• Rawtheyan/Richmond (Upper-Ashgill)
• Cautleyan/Richmond (Upper-Ashgill)
• Pusgillian/Maysville/Richmond (Upper-Ashgill)

• Trenton (Middle-Caradoc)
• Onnian/Maysville/Eden (Middle-Caradoc)
• Actonian/Eden (Middle-Caradoc)
• Marshbrookian/Sherman (Middle-Caradoc)
• Longvillian/Sherman (Middle-Caradoc)
• Soundleyan/Kirkfield (Middle-Caradoc)
• Harnagian/Rockland (Middle-Caradoc)
• Costonian/Black River (Middle-Caradoc)
• Chazy (Middle-Llandeilo)
• Llandeilo (Middle-Llandeilo)
• Whiterock (Middle-Llanvirn)
• Llanvirn (Middle-Llanvirn)

• Cassinian (Lower-Arenig)
• Arenig/Jefferson/Castleman (Lower-Arenig)
• Tremadoc/Deming/Gaconadian (Lower-Tremadoc)

Ordovician paleogeography

During the Ordovician, the southern continents were collected into a single continent called %20%20%20%20This%20article%20is%20about%20the%20ancient%20supercontinent%20of%20Gondwana.%20... Gondwana. Gondwana started the period in %20%20%20%20%20%20%20%20%20%20In%20geography%2C%20the%20equator%20is%20an%20imaginary%20line%20drawn%20around%20a%20planet%2C%20halfway%20between%20the%20poles%2C%20where%20the%20surface%20of%20the%20roughly%20spherical%20planet%20is%20parallel%20to%20the%20axis%20of%20rotation.%20... equatorial %20%20%20Latitude%2C%20denoted%20%26%23966%3B%2C%20gives%20the%20location%20of%20a%20place%20on%20Earth%20north%20or%20south%20of%20the%20Equator.%20... latitudes and, as the period progressed, drifted toward the %20%20%20%20%20%20%20The%20ceremonial%20South%20Pole.%20... South Pole. The early Ordovician was thought to be quite warm, at least in the tropics. As with North America and %20%20%20%20%20%20%20World%20map%20showing%20location%20of%20Europe%20%20%20%20%20%20%20A%20satellite%20composite%20image%20of%20Europe%20%20%20Europe%20is%20%20%20geologically%20and%20%20%20geographically%20a%20%20%20peninsula%2C%20forming%20the%20westernmost%20part%20of%20%20%20Eurasia.%20... Europe, Gondwana was largely covered with shallow seas during the Ordovician. By the end of the period, Gondwana had neared or approached the pole and was largely %20%20%20%20%20%20%20%20Austria%5Cs%20longest%20glacier%2C%20the%20Pasterze%2C%20winds%20its%208%20km%20(5%20mile)%20route%20at%20the%20foot%20of%20Austria%5Cs%20highest%20mountain%2C%20the%20Grossglockner%20%20%20A%20glacier%20is%20a%20large%2C%20long-lasting%20river%20of%20ice%20that%20is%20formed%20on%20land%20and%20moves%20in%20response%20to%20gravity.%20... glaciated.

Ordovician rocks are chiefly %20%20%20%20Two%20types%20of%20sedimentary%20rock%3A%20limey%20shale%20overlaid%20by%20limestone.%20... sedimentary. Because of the restricted area and low elevation of solid land, which set limits to %20%20%20%20%20%20%20Severe%20soil%20erosion%20in%20a%20wheat%20field%20near%20Washington%20State%20University%2C%20USA.%20%20%20... erosion, marine %20%20%20Sediment%20is%20any%20particulate%20matter%20that%20can%20be%20transported%20by%20fluid%20flow%20and%20which%20eventually%20is%20deposited%20as%20a%20layer%20of%20solid%20particles%20on%20the%20bed%20or%20bottom%20of%20a%20body%20of%20water%20or%20other%20liquid.%20... sediments that make up a large part of the Ordovician system consist chiefly of %20%20%20%20%20Limey%20shale%20overlaid%20by%20limestone.%20... limestone. %20%20%20%20%20%20%20Shale%20%20%20Shale%20is%20a%20fine-grained%20sedimentary%20rock%20whose%20original%20constituents%20were%20clays%20or%20muds.%20... Shale and %20%20%20%20%20%20%20Red%20Sandstone%20in%20Wyoming%20%20%20%20%20%20%20Layered%20sandstone%20%20%20Sandstone%20is%20an%20arenaceous%20sedimentary%20rock%20composed%20mainly%20of%20feldspar%20and%20quartz%20and%20varies%20in%20colour%20(in%20a%20similar%20way%20to%20sand)%2C%20through%20grey%2C%20yellow%2C%20red%2C%20and%20white.%20... sandstone are less conspicuous.

Ordovician fauna

In North America and Europe, the Ordovician was a time of shallow continental seas rich in life. %20%20%20%20%20%20Orders%20%20%20%20Agnostida%20Redlichiida%20Corynexochida%20Lichida%20Nektaspida%3F%20Phacopida%20Proetida%20Asaphida%20Harpetida%20Ptychopariida%20%20%20%20Trilobites%20are%20extinct%20arthropods%20in%20the%20class%20Trilobita.%20... Trilobites and %20%20%20%20%20%20%20Classes%20%20%20%20Lingulata%20Paterinata%20(extinct)%20Craniforma%20Chileata%20(extinct)%20Obolellata%20(extinct)%20Kutorginata%20(extinct)%20Strophomenata%20(extinct)%20Rhynchonellata%20%20%20%20Brachiopods%20(from%20Latin%20bracchium%2C%20arm%20%2B%20New%20Latin%20-poda%2C%20foot)%20make%20up%20one%20of%20the%20major%20animal%20phyla%2C%20Brachiopoda.%20... brachiopods in particular were rich and diverse. The first %20%20%20%20%20%20%20Fossilized%20Bryozoa%2C%20Ordovician%20limestone%2C%20Batavia%2C%20Ohio%20%20%20Bryozoans%20(%5Cmoss%20animals%5C)%20are%20tiny%20colonial%20animals%20that%20generally%20build%20stony%20skeletons%20of%20calcium%20carbonate%2C%20superficially%20similar%20to%20coral.%20... bryozoa appear in the Ordovician as do the first %20%20%20%20%20%20%20%20%20%20%20Some%20of%20the%20%20%20biodiversity%20of%20a%20coral%20reef.%20%20%20... coral reefs. Solitary %20%20%20%20%20%20%20%20%20Orders%20%20%20%20see%20%20%20Anthozoa%20%20%20%20zsnobordinkid505%40aol.... corals date back to at least the %20%20%20%20%20The%20Cambrian%20is%20a%20major%20division%20of%20the%20geologic%20timescale%20that%20begins%20about%20542%20million%20years%20before%20the%20present%20(BP)%20at%20the%20end%20of%20the%20Proterozoic%20eon_geology%20and%20ended%20about%20490%20million%20years%20BP%20with%20the%20beginning%20of%20the%20Ordovician%20period.%20... Cambrian. %20%20%20%20%20%20Classes%20%20%20%20Caudofoveata%20Aplacophora%20Polyplacophora%20-%20Chitons%20Monoplacophora%20Bivalvia%20-%20Bivalves%20Scaphopoda%20-%20Tusk%20shells%20Gastropoda%20-%20Snails%20and%20Slugs%20Cephalopoda%20-%20Squids%2C%20Octopuses%2C%20etc.%20%20%20%20%20... Molluscs, which had also appeared during the Cambrian, become common and varied, especially %20%20%20%20%20%20Orders%20%20%20%20%20Subclass%20Protobranchia%20%20Solemyoida%20Nuculoida%20%20%20Subclass%20Pteriomorphia%20-%20oysters%20%20Arcoida%20Mytiloida%20Pterioida%20%20%20Subclass%20Paleoheterodonta%20-%20mussels%20%20Trigoinoida%20Unionoida%20%20%20Subclass%20Heterodonta%20-%20clams%2C%20zebra%20mussels%20%20Veneroida%20Myoida%20%20%20Subclass%20Anomalosdesmata%20%20Pholadomyoida%20%20%20%20%20%20%20Animals%20of%20the%20Class%20Bivalvia%20are%20known%20as%20bivalves%20because%20they%20typically%20have%20two-part%20shells%2C%20with%20both%20parts%20being%20more%20or%20less%20symmetrical.%20The%20Class%20has%208%2C000%20species%2C%20including%20scallops%2C%20clams%2C%20oysters%20and%20mussels.%20The%20name%20is%20also%20spelled%20Bivalva.%20An%20old%20name%20for%20the%20Class%20is%20Pelecypoda.%20Bivalves%20are%20exclusively%20aquatic%3B%20they%20include%20both%20marine%20and%20freshwater%20forms.%20Bivalves%20lack%20a%20radula%20and%20feed%20by%20siphoning%20and%20filtering%20small%20particles%20from%20water.%20Some%20bivalves%20are%20epifaunal%3A%20that%20is%2C%20they%20attach%20themselves%20to%20surfaces%20in%20the%20water%2C%20by%20means%20of%20a%20byssus.%20Others%20are%20infaunal%3A%20they%20bury%20themselves%20in%20sand%20or%20other%20sediments.%20These%20forms%20typically%20have%20a%20strong%20digging%20foot.%20Some%20bivalves%20can%20swim.%20%20Palaeontology%20of%20the%20Bivalves%20The%20oldest%20known%20fossil%20bivalves%20are%20500%20million%20years%20old.%20Some%20are%20now%20extinct%2C%20such%20as%20the%20Jurassic%20Gryphaea%2C%20also%20known%20as%20%5Cdevil%5Cs%20toenails%5C.%20Examples%20of%20fossil%20freshwater%20bivalves%20are%20the%20large%20mussel%20Archanodon%2C%20found%20in%20the%20Old%20Red%20Sandstone%20formation%20of%20the%20Devonian%20Period%20in%20the%20United%20Kingdom.%20The%20non-marine%20bands%20of%20Carboniferous%20Coal%20Measures%20contain%20%5Cmussel%20bands%5C%20which%20are%20crowded%20with%20specimens%20such%20as%20Carbonicola%20and%20Naiadites.%20These%20freshwater%20bivalves%20prove%20useful%20as%20zone%20or%20index%20fossils%20for%20subdividing%20the%20strata%20of%20the%20Coal%20Measures.%20Most%20fossil%20bivalves%20are%20relatively%20small%2C%20typically%20being%20a%20few%20centimetres%20(about%20an%20inch)%20long.%20However%2C%20a%20few%20giant%20forms%20are%20known%2C%20such%20as%20Inoceramus%20from%20the%20Cretaceous-period%20Chalk%20Formation%20of%20Europe%2C%20which%20grew%20to%20well%20over%20a%20meter%20(3.3%20feet)%20in%20length.%20The%20shells%20of%20bivalves%20are%20composed%20of%20calcium%20carbonate%20and%20are%20quite%20strong%20in%20most%20species%2C%20and%20hence%20easily%20become%20fossilized.%20However%2C%20in%20some%20bivalves%20the%20shell%20is%20made%20up%20of%20a%20form%20of%20calcium%20carbonate%20known%20as%20aragonite%2C%20which%20easily%20dissolves%20away%20when%20the%20fossil%20is%20entombed%20in%20sediment%2C%20leaving%20only%20an%20impression%20of%20the%20shells%5C%20internal%20and%20external%20surfaces%20(known%20as%20internal%20and%20external%20moulds).%20Remarkable%20bivalve%20fossils%20have%20been%20found%20in%20which%20the%20original%20colour%20banding%20of%20the%20shell%20has%20been%20preserved%2C%20such%20as%20in%20specimens%20of%20Aviculopecten%20planoradiatus%20from%20the%20Carboniferous%20Period%20of%20Derbyshire%20in%20England.%20Bivalves%20seem%20to%20have%20evolved%20at%20a%20relatively%20slow%20pace%2C%20especially%20when%20compared%20to%20some%20of%20the%20other%20molluscs%20-%20such%20as%20the%20ammonites.%20The%20bivalves%20which%20appeared%20in%20the%20early%20Ordovician%20Period%20were%20marine%2C%20and%20like%20many%20other%20groups%20of%20molluscs%20they%20radiated%20out%20to%20fill%20many%20of%20their%20niches%20(which%20they%20still%20occupy%20today)%20by%20the%20end%20of%20the%20Ordovician.%20Bivalves%20begin%20to%20become%20reasonably%20common%20fossils%20in%20the%20rocks%20of%20the%20Silurian%20and%20Devonian%20Periods.%20The%20shells%20of%20bivalves%20in%20rocks%20older%20than%20those%20of%20the%20Carboniferous%20are%20not%20generally%20preserved%3B%20these%20fossils%20largely%20consist%20of%20mere%20impressions%20in%20the%20rock.%20Freshwater%20species%20first%20appear%20at%20the%20boundary%20between%20the%20Silurian%20and%20Devonian%20Periods.%20All%20of%20the%20major%20groups%20of%20bivalves%20were%20in%20existence%20by%20the%20end%20of%20the%20Palaeozoic%20Era.%20%20%20%20%20Fossil%20bivalve%20Gryphaea%20arcuata%20from%20the%20Jurassic%20of%20England%20%20%20Bivalves%20are%20much%20more%20numerous%20and%20varied%20in%20rocks%20from%20the%20Mesozoic%20Era%2C%20and%20they%20may%20occur%20in%20great%20numbers%20at%20some%20localities.%20Many%20of%20the%20ancient%20bivalve%20forms%20became%20extinct%20during%20the%20Triassic%20Period%20and%20were%20replaced%20by%20many%20new%20forms%2C%20some%20of%20which%20have%20persisted%20to%20the%20present%20day.%20Like%20the%20gastropods%2C%20bivalves%20are%20very%20abundant%20fossils%20in%20rocks%20of%20the%20Cenozoic%20Era.%20Many%20of%20these%20bivalves%20are%20related%20to%20living%20forms%20so%20it%20is%20possible%20to%20state%20with%20some%20confidence%20the%20conditions%20in%20which%20the%20rocks%20they%20are%20found%20were%20deposited%2C%20such%20as%20shallow%20marine%20water%2C%20brackish%20or%20freshwater%2C%20under%20sub-tropical%20or%20a%20cold%20climate%2C%20etc.%20Some%20living%20forms%20of%20bivalve%20have%20a%20very%20long%20ancestry%2C%20such%20as%20the%20modern%20Nucula%2C%20which%20has%20relatives%20in%20rocks%20from%20the%20Ordovician%20that%20are%20not%20very%20different%20from%20those%20found%20living%20today.%20%20History%20of%20the%20Bivalves%20Bivalves%20appeared%20late%20in%20the%20Cambrian%20Explosion%20and%20came%20to%20dominate%20over%20brachiopods%20during%20the%20Palaeozoic%3B%20indeed%2C%20by%20the%20end_Permian%20extinction%2C%20bivalves%20were%20undergoing%20a%20huge%20radiation%20in%20numbers%20while%20brachiopods%20(along%20with%20~95%25%20of%20all%20species)%20were%20devastated.%20This%20raises%20of%20the%20question%20of%20how%20did%20bivalvia%20come%20to%20challenge%20the%20brachiopoda%20niche%20before%20the%20extinction%20event%2C%20as%20well%20as%20how%20did%20the%20bivalves%20escape%20that%20fate.%20Although%20inevitable%20biases%20exist%20in%20the%20fossil%20record%20and%20our%20documentation%20thereof%2C%20bivalves%20essentially%20appear%20to%20be%20better%20adapted%20to%20aquatic%20life.%20Far%20more%20sophisticated%20than%20the%20brachiopods%2C%20bivalves%20use%20an%20energetically_efficient%20ligament_muscle%20system%20for%20opening%20valves%2C%20and%20thus%20require%20less%20food%20to%20subsist.%20Furthermore%2C%20their%20ability%20to%20burrow%20allows%20for%20evasion%20of%20predators%3A%20buried%20bivalves%20feed%20by%20extending%20a%20siphon%20to%20the%20surface%20(indicated%20by%20the%20presence%20of%20a%20palial%20sinus%2C%20the%20size%20of%20which%20is%20proportional%20to%20the%20burrowing%20depth%2C%20and%20represented%20by%20their%20dentition).%20As%20well%2C%20bivalves%20became%20mobile%3A%20some%20developed%20spines%20for%20buoyancy%2C%20while%20others%20suck%20in%20and%20eject%20water%20to%20enable%20propulsion.%20This%20allowed%20bivalves%20to%20themselves%20become%20predators.%20With%20such%20a%20wide%20range%20of%20adaptations%20it%20is%20unsurprising%20that%20the%20shapes%20of%20bivalve%20shells%20vary%20greatly%20-%20some%20are%20rounded%20and%20globular%2C%20others%20are%20flattened%20and%20plate-like%2C%20while%20still%20others%2C%20such%20as%20the%20razor%20shell%20Enis%2C%20have%20become%20greatly%20elongated%20in%20order%20to%20aid%20burrowing.%20%20%20External%20links%20%20Conchology%2C%20Inc.%20%5B1%5D%20-%20palaeontology%20from%20the%20University%20of%20California%2C%20Berkeley%20%20%20%20%20Categories%3A%20Molluscs%20%7C%20Fossils%20%20%20%20%20%20%20%20... bivalves, %20%20%20%20%20%20Subclass%20%20%20%20Subclass%20Eogastropoda%20%26%23160%3B%26%23160%3B%26%23160%3B%20Patellogastropoda%20Subclass%20Orthogastropoda%20%26%23160%3B%26%23160%3BSuperorder%20Cocculiniformia%20%26%23160%3B%26%23160%3BSuperorder%20Hot%20Vent%20Taxa%20%26%23160%3B%26%23160%3B%26%23160%3B%20Neomphaolida%20%26%23160%3B%26%23160%3BSuperorder%20Vetigastropoda%20%26%23160%3B%26%23160%3BSuperorder%20Neritaemorphi%20%26%23160%3B%26%23160%3B%26%23160%3B%20Neritopsina%20%26%23160%3B%26%23160%3BSuperorder%20Caenogastropoda%20%26%23160%3B%26%23160%3B%26%23160%3B%20Architaenioglossa%20%26%23160%3B%26%23160%3B%26%23160%3B%20Sorbeoconcha%20%26%23160%3B%26%23160%3BSuperorder... gastropods, and %20%20%20%20%20%20%20Orders%20%20%20%20Nautilida%20Bactrida%20%20%20%20%20Nautiloids%20are%20a%20group%20of%20marine%20animals%20which%20all%20possess%20an%20external%20shell%2C%20the%20most%20well%20known%20example%20being%20the%20modern%20nautiluses.%20... nautiloid %20%20%20%20%20%20%20%20%20Orders%20%20%20%20%20%20Sepiida%20%20%20Sepiolida%20%20%20Spirulida%20%20%20Teuthida%20%20%20%20Octopoda%20%20%20Vampyromorphida%20%20%20%20Nautilida%20%20%20%20The%20Cephalopods%20(%5Chead-foot%5C)%20are%20the%20%20%20mollusc%20%20%20class%20Cephalopoda%20characterized%20by%20bilateral%20body%20symmetry%2C%20a%20prominent%20head%2C%20and%20a%20modification%20of%20the%20mollusc%20foot%20into%20the%20form%20of%20arms%20or%20%20%20tentacles.%20... cephalopods. It was long thought that the first true %20%20%20%20%20%20Typical%20classes%20%20%20%20Petromyzontidae%20(lampreys)%20Placodermi%20-%20extinct%20Chondrichthyes%20(cartilaginous%20fish)%20Acanthodii%20-%20extinct%20Actinopterygii%20(ray-finned%20fish)%20Actinistia%20(coelacanths)%20Dipnoi%20(lungfish)%20Amphibia%20(amphibians)%20Reptilia%20(reptiles)%20Aves%20(birds)%20Mammalia%20(mammals)%20%20%20%20%20Vertebrata%20is%20a%20subphylum%20of%20chordates%2C%20specifically%2C%20those%20with%20backbones%20or%20spinal%20columns.%20... vertebrates (fish - Ostracoderms) appeared in the Ordovician, but recent discoveries in %20%20%20%20%20%20%20%20%20%20%20The%20%20%20Great%20Wall%20of%20China%2C%20stretching%20over%206%2C700%20km%2C%20was%20erected%20beginning%20in%20the%203rd%20century%20BC%20to%20guard%20the%20north%20from%20raids%20by%20men%20on%20horses.%20%20%20... China reveal that they probably originated in the early %20%20%20%20%20The%20Cambrian%20is%20a%20major%20division%20of%20the%20geologic%20timescale%20that%20begins%20about%20542%20million%20years%20before%20the%20present%20(BP)%20at%20the%20end%20of%20the%20Proterozoic%20eon_geology%20and%20ended%20about%20490%20million%20years%20BP%20with%20the%20beginning%20of%20the%20Ordovician%20period.%20... Cambrian. Now-extinct marine animals called %20%20Graptolites%20(Graptolita)%20are%20colonial%20animals%20known%20chiefly%20from%20the%20Upper%20Cambrian%20through%20the%20Mississippian%20(Lower%20Carboniferous).%20... graptolites thrived in the oceans. Some cystoids and %20%20%20%20%20%20%20Orders%20%20%20%20Articulata%20Cladida%20(extinct)%20Flexibilia%20(extinct)%20Camerada%20(extinct)%20Disparida%20(extinct)%20%20%20%20Crinoids%2C%20also%20known%20as%20%5Csea%20lilies%5C%20or%20%5Cfeather-stars%5C%2C%20are%20marine%20animals%20that%20make%20up%20the%20class%20Crinoidea%20of%20the%20%20%20echinoderms%20(phylum%20Echinodermata).%20... crinoids appeared. The first terrestrial %20%20%20%20%20%20%20%20%20%20%5CDivisions%5C%20%20%20%20%20Green%20algae%20land%20plants%20(embryophytes)%20non-vascular%20embryophytes%20%20Hepatophyta%20-%20liverworts%20Anthocerophyta%20-%20hornworts%20Bryophyta%20-%20mosses%20%20%20vascular%20plants%20(tracheophytes)%20seedless%20vascular%20plants%20%20Lycopodiophyta%20-%20clubmosses%20Equisetophyta%20-%20horsetails%20Pteridophyta%20-%20%5Ctrue%5C%20ferns%20Psilotophyta%20-%20whisk%20ferns%20Ophioglossophyta%20-%20adderstongue%20ferns%20%20%20seed%20plants%20(spermatophytes)%20%20%26%238224%3BPteridospermatophyta%20-%20seed%20ferns%20Pinophyta%20-%20conifers%20Cycadophyta%20-%20cycads%20Ginkgophyta%20-%20ginkgo%20Gnetophyta%20-%20gnetae%20Magnoliophyta... plants appeared in the form of tiny plants resembling %20%20%20%20%20%20%20Orders%20%20%20%20%20Need%20to%20be%20entered%20%20%20%20%20Liverworts%20are%20non-vascular%20plants%20in%20the%20Class%20Marchantiopsida%2C%20formerly%20known%20as%20the%20Hepaticae.%20... liverworts.