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Ordovician

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Ordovician
485.4 ± 1.9 – 443.8 ± 1.5Ma
Map of Earth as it appeared 460 million years ago during the Middle Ordovician, Darriwilian age[citation needed]
Chronology
Etymology
Name formalityFormal
Name ratified1960
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitPeriod
Stratigraphic unitSystem
First proposed byCharles Lapworth,1879
Time span formalityFormal
Lower boundary definitionFADof theConodontIapetognathus fluctivagus
Lower boundary GSSPGreenpoint section,Green Point,Newfoundland,Canada
49°40′58″N57°57′55″W/ 49.6829°N 57.9653°W/49.6829; -57.9653
Lower GSSP ratified2000[5]
Upper boundary definitionFAD of theGraptoliteAkidograptus ascensus
Upper boundary GSSPDob's Linn,Moffat,U.K.
55°26′24″N3°16′12″W/ 55.4400°N 3.2700°W/55.4400; -3.2700
Upper GSSP ratified1984[6][7]
Atmospheric and climatic data
Sea level above present day180 m; rising to 220 m in Caradoc and falling sharply to 140 m in end-Ordovician glaciations[8]

TheOrdovician(/ɔːrdəˈvɪʃi.ən,-doʊ-,-ˈvɪʃən/or-də-VISH-ee-ən, -⁠doh-, -⁠VISH-ən)[9]is ageologic periodandsystem,the second of six periods of thePaleozoicEra.The Ordovician spans 41.6 million years from the end of theCambrianPeriod 485.4Ma(million years ago) to the start of theSilurianPeriod 443.8 Ma.[10]

The Ordovician, named after theWelshtribe of theOrdovices,was defined byCharles Lapworthin 1879 to resolve a dispute between followers ofAdam SedgwickandRoderick Murchison,who were placing the samerockbeds inNorth Walesin the Cambrian and Silurian systems, respectively.[11]Lapworth recognized that thefossilfaunain the disputedstratawere different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own. The Ordovician received international approval in 1960 (forty years after Lapworth's death), when it was adopted as an official period of the Paleozoic Era by theInternational Geological Congress.

Life continued to flourish during the Ordovician as it did in the earlier Cambrian Period, although the end of the period was marked by theOrdovician–Silurian extinction events.Invertebrates, namelymolluscsandarthropods,dominated the oceans, with members of the latter group probably starting their establishment on land during this time, becoming fully established by theDevonian.The firstland plantsare known from this period. TheGreat Ordovician Biodiversification Eventconsiderably increased the diversity of life.Fish,the world's first truevertebrates,continued to evolve, andthose with jawsmay have first appeared late in the period. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today in a period known as theOrdovician meteor event.[12]It has been theorized that this increase in impacts may originate froma ring systemthat formed around Earth at the time.[13]

Subdivisions

[edit]

In 2008, theICSerected a formal international system of subdivisions for the Ordovician Period and System.[14]Pre-existing Baltoscandic, British, Siberian, North American, Australian, Chinese, Mediterranean and North-Gondwananregional stratigraphic schemes are also used locally.[15]

ICS (global) subdivisions

[edit]
The Ordovician subdivisions with "golden spikes"according toIUGS:[16]
System Series Stage/age Lower boundary (Ma)
Silurian Llandovery Rhuddanian 443.8±1.5
Ordovician Upper Ordovician Hirnantian 445.2±1.4
Katian 453.0±0.7
Sandbian 458.4±0.9
Middle Ordovician Darriwilian 467.3±1.1
Dapingian 470.0±1.4
Lower Ordovician Floian 477.7±1.4
Tremadocian 485.4±1.9
Cambrian Furongian Stage 10 older

Global/regional correlation

[edit]
Approximate correlation of Ordovician regional series and stages[17]
ICS series ICS stage British series British stage North American series North American stage Australian stage Chinese stage
Upper Ordovician Hirnantian Ashgill Hirnantian Cincinnatian Gamachian Bolindian Hirnantian
Katian Rawtheyan Richmondian Chientangkiangian
Cautleyan
Pusgillian Maysvillian Eastonian Neichianshanian
Caradoc Streffordian Edenian
Cheneyan Mohawkian Chatfieldian
Sandbian Burrellian Gisbornian
Turinian
Aurelucian
Whiterockian
Middle Ordovician Darriwilian Llanvirn Llandeilo Darriwilian Darriwilian
Abereiddian
Arenig Fennian
Dapingian Yapeenian Dapingian
Whitlandian Rangerian Castlemainian
Lower Ordovician Floian Ibexian Blackhillsian Chewtonian Yiyangian
Bendigonian
Moridunian
Tulean Lancefieldian
Tremadocian Tremadoc Migneintian Xinchangian
Stairsian
Cressagian
Skullrockian
Warendan

British stages and ages

[edit]

The Ordovician Period in Britain was traditionally broken into Early (Tremadocian andArenig), Middle (Llanvirn(subdivided into Abereiddian and Llandeilian) andLlandeilo) and Late (Caradocand Ashgill) epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column.

The Tremadoc corresponds to the ICS's Tremadocian. The Arenig corresponds to the Floian, all of the Dapingian and the early Darriwilian. The Llanvirn corresponds to the late Darriwilian. The Caradoc covers the Sandbian and the first half of the Katian. The Ashgill represents the second half of the Katian, plus theHirnantian.

Ashgill

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The Ashgill Epoch, the last epoch of the British Ordovician, is made of four ages: the Hirnantian Age, the Rawtheyan Age, theCautleyanAge, and the Pusgillian Age. These ages make up the time period from c. 450 Ma to c. 443 Ma.

The Rawtheyan, the second last of the Ashgill ages, was from c. 449 Ma to c. 445 Ma. It is in the Katian Age of the ICS'sGeologic Time Scale.

Paleogeography and tectonics

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Paleogeographic map of the Earth in the middle Ordovician, 470 million years ago

During the Ordovician, the southern continents were assembled intoGondwana,which reached from north of theequatorto theSouth Pole.The Panthalassic Ocean, centered in the northern hemisphere, covered over half the globe.[18]At the start of the period, the continents ofLaurentia(in present-dayNorth America),Siberia,andBaltica(present-day northern Europe) were separated from Gondwana by over 5,000 kilometres (3,100 mi) of ocean. These smaller continents were also sufficiently widely separated from each other to develop distinct communities of benthic organisms.[19]The small continent ofAvaloniahad just rifted from Gondwana and began to move north towards Baltica and Laurentia, opening theRheic Oceanbetween Gondwana and Avalonia.[20][21][22]Avalonia collided with Baltica towards the end of Ordovician.[23][24]

Other geographic features of the Ordovician world included theTornquist Sea,which separated Avalonia from Baltica;[19]the Aegir Ocean, which separated Baltica from Siberia;[25]and an oceanic area between Siberia, Baltica, and Gondwana which expanded to become the Paleoasian Ocean in Carboniferous time. TheMongol-Okhotsk Oceanformed a deep embayment between Siberia and the Central Mongolianterranes.Most of the terranes of central Asia were part of an equatorial archipelago whose geometry is poorly constrained by the available evidence.[26]

The period was one of extensive, widespread tectonism and volcanism. However,orogenesis(mountain-building) was not primarily due to continent-continent collisions. Instead, mountains arose along active continental margins during accretion of arc terranes or ribbon microcontinents. Accretion of new crust was limited to the Iapetus margin of Laurentia; elsewhere, the pattern was of rifting in back-arc basins followed by remerger. This reflected episodic switching from extension to compression. The initiation of new subduction reflected a global reorganization of tectonic plates centered on the amalgamation of Gondwana.[27][19]

TheTaconic orogeny,a major mountain-building episode, was well under way in Cambrian times.[28]This continued into the Ordovician, when at least twovolcanic island arcscollided with Laurentia to form theAppalachian Mountains.Laurentia was otherwise tectonically stable. An island arc accreted to South China during the period, while subduction along north China (Sulinheer) resulted in the emplacement of ophiolites.[29]

Theash fallof the Millburg/Big Bentonite bed, at about 454 Ma, was the largest in the last 590 million years. This had adense rock equivalentvolume of as much as 1,140 cubic kilometres (270 cu mi). Remarkably, this appears to have had little impact on life.[30]

There was vigorous tectonic activity along northwest margin of Gondwana during the Floian, 478 Ma, recorded in the Central Iberian Zone of Spain. The activity reached as far as Turkey by the end of Ordovician. The opposite margin of Gondwana, in Australia, faced a set of island arcs.[19]The accretion of these arcs to the eastern margin of Gondwana was responsible for the Benambran Orogeny of eastern Australia.[31][32]Subduction also took place along what is now Argentina (Famatinian Orogeny) at 450 Ma.[33]This involved significant back arc rifting.[19]The interior of Gondwana was tectonically quiet until theTriassic.[19]

Towards the end of the period, Gondwana began to drift across the South Pole. This contributed to the Hibernian glaciation and the associated extinction event.[34]

Ordovician meteor event

[edit]

TheOrdovician meteor eventis a proposed shower of meteors that occurred during the Middle Ordovician Epoch, about 467.5 ± 0.28 million years ago, due to the break-up of theL chondriteparent body.[35]It is not associated with any major extinction event.[36][37][38]A 2024 study found that craters from this event cluster in a distinct band around the Earth, and that the breakup of the parent body may have formed aring systemfor a period of about 40 million years, with frequent falling debris causing these craters.[13]

Geochemistry

[edit]
External mold of Ordovicianbivalveshowing that the originalaragoniteshell dissolved on the sea floor, leaving a cemented mold for biological encrustation (Waynesville Formationof Franklin County, Indiana).

The Ordovician was a time ofcalcite seageochemistry in which low-magnesiumcalcitewas the primary inorganic marine precipitate ofcalcium carbonate.[39]Carbonate hardgroundswere thus very common, along with calciticooids,calcitic cements, and invertebrate faunas with dominantly calcitic skeletons. Biogenicaragonite,like that composing the shells of mostmolluscs,dissolved rapidly on the sea floor after death.[40][41]

Unlike Cambrian times, when calcite production was dominated by microbial and non-biological processes, animals (and macroalgae) became a dominant source of calcareous material in Ordovician deposits.[42]

Climate and sea level

[edit]

The Early Ordovician climate was very hot,[43]with intensegreenhouseconditions andsea surface temperaturescomparable to those during the Early Eocene Climatic Optimum.[44]Carbon dioxidelevels were very high at the Ordovician period's beginning.[45]By the late Early Ordovician, the Earth cooled,[46]giving way to a more temperate climate in the Middle Ordovician,[47]with the Earth likely entering theEarly Palaeozoic Ice Ageduring the Sandbian,[48][49]and possibly as early as the Darriwilian[50]or even the Floian.[46]The Dapingian and Sandbian saw major humidification events evidenced by trace metal concentrations in Baltoscandia from this time.[51]Evidence suggests that global temperatures rose briefly in the early Katian (Boda Event), depositing bioherms and radiating fauna across Europe.[52]The early Katian also witnessed yet another humidification event.[51]Further cooling during the Hirnantian, at the end of the Ordovician, led to theLate Ordovician glaciation.[53]

The Ordovician saw the highest sea levels of the Paleozoic, and the low relief of the continents led to many shelf deposits being formed under hundreds of metres of water.[42]The sea level rose more or less continuously throughout the Early Ordovician, leveling off somewhat during the middle of the period.[42]Locally, some regressions occurred, but the sea level rise continued in the beginning of the Late Ordovician. Sea levels fell steadily due to the cooling temperatures for about 3 million years leading up to the Hirnantian glaciation. During this icy stage, sea level seems to have risen and dropped somewhat. Despite much study, the details remain unresolved.[42]In particular, some researches interpret the fluctuations in sea level as pre-Hibernian glaciation,[54]but sedimentary evidence of glaciation is lacking until the end of the period.[24]There is evidence ofglaciersduring the Hirnantian on theland we now knowas Africa and South America, which were near theSouth Poleat the time, facilitating the formation of theice capsof the Hirnantian glaciation.

As withNorth AmericaandEurope,Gondwanawas largely covered with shallow seas during the Ordovician. Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts. ThePanthalassic Oceancovered much of theNorthern Hemisphere,and other minor oceans includedProto-Tethys,Paleo-Tethys,Khanty Ocean,which was closed off by the Late Ordovician,Iapetus Ocean,and the newRheic Ocean.

Life

[edit]
Adioramadepicting Ordovician flora and fauna

For most of the Late Ordovician life continued to flourish, but at and near the end of the period there weremass-extinction eventsthat seriously affectedconodontsandplanktonicforms likegraptolites.ThetrilobitesAgnostidaandPtychopariidacompletely died out, and theAsaphidawere much reduced.Brachiopods,bryozoansandechinodermswere also heavily affected, and theendoceridcephalopodsdied out completely, except for possible rare Silurian forms. The Ordovician–Silurian extinction events may have been caused by an ice age that occurred at the end of the Ordovician Period, due to the expansion of thefirst terrestrial plants,[55]as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history.

Fauna

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Endoceras,one of the largest predators of the Ordovician
Fossiliferous limestoneslab from the Liberty Formation (Upper Ordovician) of Caesar Creek State Park near Waynesville, Ohio.
The trilobiteIsotelusfromWisconsin

On the whole, the fauna that emerged in the Ordovician were the template for the remainder of the Palaeozoic. The fauna was dominated by tiered communities of suspension feeders, mainly with short food chains. The ecological system reached a new grade of complexity far beyond that of the Cambrian fauna, which has persisted until the present day.[42]Though less famous than theCambrian explosion,theOrdovician radiation(also known as the Great Ordovician Biodiversification Event)[19]was no less remarkable; marine faunalgeneraincreased fourfold, resulting in 12% of all knownPhanerozoicmarine fauna.[56]Several animals also went through a miniaturization process, becoming much smaller than their Cambrian counterparts.[citation needed]Another change in the fauna was the strong increase infilter-feedingorganisms.[57]The trilobite, inarticulate brachiopod,archaeocyathid,andeocrinoidfaunas of the Cambrian were succeeded by those that dominated the rest of the Paleozoic, such as articulate brachiopods,cephalopods,andcrinoids.Articulate brachiopods, in particular, largely replaced trilobites inshelfcommunities. Their success epitomizes the greatly increased diversity ofcarbonateshell-secreting organisms in the Ordovician compared to the Cambrian.[58]

Aegirocassis,a large filter-feedinghurdiidradiodontfromMorocco

Ordovician geography had its effect on the diversity of fauna; Ordovician invertebrates displayed a very high degree of provincialism.[59]The widely separated continents of Laurentia and Baltica, then positioned close to the tropics and boasting many shallow seas rich in life, developed distinct trilobite faunas from the trilobite fauna of Gondwana,[60]and Gondwana developed distinct fauna in its tropical and temperature zones.[61]The Tien Shan terrane maintained a biogeographic affinity with Gondwana,[62]and the Alborz margin of Gondwana was linked biogeographically to South China.[63]Southeast Asia's fauna also maintained strong affinities to Gondwana's.[64]North China was biogeographically connected to Laurentia and the Argentinian margin of Gondwana.[65]A Celtic biogeographic province also existed, separate from the Laurentian and Baltican ones.[66]However, tropical articulate brachiopods had a morecosmopolitan distribution,with less diversity on different continents. During the Middle Ordovician, beta diversity began a significant decline as marine taxa began to disperse widely across space.[67]Faunas become less provincial later in the Ordovician, partly due to the narrowing of the Iapetus Ocean,[68]though they were still distinguishable into the late Ordovician.[69]

Pentecopterus,the earliest known eurypterid, and found inIowa

Trilobitesin particular were rich and diverse, and experienced rapid diversification in many regions.[70]Trilobites in the Ordovician were very different from their predecessors in the Cambrian. Many trilobites developed bizarre spines and nodules to defend against predators such as primitiveeurypteridsand nautiloids while other trilobites such asAeglina priscaevolved to become swimming forms. Some trilobites even developed shovel-like snouts for ploughing through muddy sea bottoms. Another unusual clade of trilobites known as the trinucleids developed a broad pitted margin around their head shields.[71]Some trilobites such asAsaphus kowalewskievolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.[72]Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician.[73]Although solitarycoralsdate back to at least theCambrian,reef-forming corals appeared in the early Ordovician, including the earliest knownoctocorals,[74][75]corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals.[42]Brachiopods surged in diversity, adapting to almost every type of marine environment.[76][77][78]Even after GOBE, there is evidence suggesting that Ordovician brachiopods maintained elevated rates of speciation.[79]Molluscs,which appeared during the Cambrian or even theEdiacaran,became common and varied, especiallybivalves,gastropods,andnautiloidcephalopods.[80][81]Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments.[82]Graptolites, which evolved in the preceding Cambrian period, thrived in the oceans.[83]This includes the distinctiveNemagraptus gracilisgraptolite fauna, which was distributed widely during peak sea levels in the Sandbian.[84][24]Some new cystoids and crinoids appeared. It was long thought that the first truevertebrates(fish —Ostracoderms) appeared in the Ordovician, but recent discoveries inChinareveal that they probably originated in the EarlyCambrian.[85]The firstgnathostome(jawed fish) may have appeared in theLate Ordovicianepoch.[86]Chitinozoans, which first appeared late in the Wuliuan, exploded in diversity during the Tremadocian, quickly becoming globally widespread.[87][88]Several groups of endobiotic symbionts appeared in the Ordovician.[89][90]

In the Early Ordovician, trilobites were joined by many new types of organisms, includingtabulatecorals,strophomenid,rhynchonellid,and many neworthidbrachiopods, bryozoans, planktonic graptolites and conodonts, and many types of molluscs and echinoderms, including the ophiuroids ( "brittle stars" ) and the firstsea stars.Nevertheless, the arthropods remained abundant; all the Late Cambrian orders continued, and were joined by the new groupPhacopida.The first evidence of land plants also appeared (seeevolutionary history of life).

In the Middle Ordovician, the trilobite-dominated Early Ordovician communities were replaced by generally more mixed ecosystems, in which brachiopods, bryozoans, molluscs,cornulitids,tentaculitidsand echinoderms all flourished, tabulate corals diversified and the firstrugose coralsappeared. The planktonic graptolites remained diverse, with the Diplograptina making their appearance. One of the earliest known armouredagnathan( "ostracoderm") vertebrates,Arandaspis,dates from the Middle Ordovician.[91]During the Middle Ordovician there was a large increase in the intensity and diversity of bioeroding organisms. This is known as the Ordovician Bioerosion Revolution.[92]It is marked by a sudden abundance of hard substrate trace fossils such asTrypanites,Palaeosabella,PetroxestesandOsprioneides.Bioerosionbecame an important process, particularly in the thick calcitic skeletons of corals, bryozoans and brachiopods, and on the extensivecarbonate hardgroundsthat appear in abundance at this time.

  • Upper Ordovician edrioasteroid Cystaster stellatus on a cobble from the Kope Formation in northern Kentucky with the cyclostome bryozoan Corynotrypa in the background
    Upper OrdovicianedrioasteroidCystaster stellatuson a cobble from the Kope Formation in northern Kentucky with the cyclostomebryozoanCorynotrypain the background
  • Middle Ordovician fossiliferous shales and limestones at Fossil Mountain, west-central Utah
    Middle Ordovician fossiliferous shales and limestones at Fossil Mountain, west-central Utah
  • Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana
    Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana
  • Outcrop of Upper Ordovician limestone and minor shale, central Tennessee
    Outcrop of Upper Ordovician limestone and minor shale, central Tennessee
  • Trypanites borings in an Ordovician hardground, southeastern Indiana[93]
    Trypanitesborings in an Ordovicianhardground,southeastern Indiana[93]
  • Petroxestes borings in an Ordovician hardground, southern Ohio[92]
    Petroxestesborings in an Ordovicianhardground,southern Ohio[92]
  • Outcrop of Ordovician kukersite oil shale, northern Estonia
    Outcrop of Ordovician kukersiteoil shale,northernEstonia
  • Bryozoan fossils in Ordovician kukersite oil shale, northern Estonia
    Bryozoan fossils in Ordoviciankukersiteoil shale, northernEstonia
  • Brachiopods and bryozoans in an Ordovician limestone, southern Minnesota
    Brachiopodsandbryozoansin an Ordovician limestone, southern Minnesota
  • Vinlandostrophia ponderosa, Maysvillian (Upper Ordovician) near Madison, Indiana (scale bar is 5.0 mm)
    Vinlandostrophia ponderosa,Maysvillian (Upper Ordovician) near Madison, Indiana (scale bar is 5.0 mm)
  • The Ordovician cystoid Echinosphaerites (an extinct echinoderm) from northeastern Estonia; approximately 5 cm in diameter
    The Ordovician cystoidEchinosphaerites(an extinctechinoderm) from northeastern Estonia; approximately 5 cm in diameter
  • Prasopora, a trepostome bryozoan from the Ordovician of Iowa
    Prasopora,a trepostomebryozoanfrom the Ordovician of Iowa
  • An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan
    An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan
  • The heliolitid coral Protaraea richmondensis encrusting a gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana
    The heliolitid coralProtaraea richmondensisencrusting a gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana
  • Zygospira modesta, atrypid brachiopods, preserved in their original positions on a trepostome bryozoan from the Cincinnatian (Upper Ordovician) of southeastern Indiana
    Zygospira modesta,atrypid brachiopods, preserved in their original positions on a trepostome bryozoan from the Cincinnatian (Upper Ordovician) of southeastern Indiana
  • Graptolites (Amplexograptus) from the Ordovician near Caney Springs, Tennessee
    Graptolites (Amplexograptus) from the Ordovician near Caney Springs, Tennessee

Flora

[edit]

Green algaewere common in the Late Cambrian (perhaps earlier) and in the Ordovician. Terrestrial plants probably evolved from green algae, first appearing as tiny non-vascularforms resemblingliverworts,in the middle to late Ordovician.[94]Fossil spores found in Ordovician sedimentary rock are typical of bryophytes.[95]

Colonization of land would have been limited to shorelines

Among the first landfungimay have beenarbuscular mycorrhizafungi (Glomerales), playing a crucial role in facilitating the colonization of land by plants throughmycorrhizal symbiosis,which makes mineral nutrients available to plant cells; such fossilized fungalhyphaeandsporesfrom the Ordovician of Wisconsin have been found with an age of about 460 million years ago, a time when the land flora most likely only consisted of plants similar to non-vascularbryophytes.[96]

Microbiota

[edit]

Though stromatolites had declined from their peak in the Proterozoic, they continued to exist in localised settings.[97]

End of the period

[edit]
Main article:Ordovician–Silurian extinction events

The Ordovician came to a close in a series ofextinction eventsthat, taken together, comprise the second largest of the five major extinction events inEarth's historyin terms of percentage ofgenerathat became extinct. The only larger one was thePermian–Triassic extinction event.

The extinctions occurred approximately 447–444 million years ago and mark the boundary between the Ordovician and the followingSilurianPeriod. At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever;brachiopodsandbryozoanswere greatly reduced, along with manytrilobite,conodontandgraptolitefamilies.

The most commonly accepted theory is that these events were triggered by the onset of cold conditions in the late Katian, followed by anice age,in the Hirnantian faunal stage, that ended the long, stablegreenhouseconditions typical of the Ordovician.

The ice age was possibly not long-lasting. Oxygenisotopesin fossil brachiopods show its duration may have been only 0.5 to 1.5 million years.[98]Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian.

Thelate Ordovician glaciationevent was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm).[99][100]The dip may have been caused by a burst of volcanic activity that deposited new silicate rocks, which draw CO2out of the air as they erode.[100]Another possibility is thatbryophytesand lichens, which colonized land in the middle to late Ordovician, may have increased weathering enough to draw down CO2levels.[94]The drop in CO2selectively affected the shallow seas where most organisms lived. It has also been suggested that shielding of the sun's rays from the proposed Ordovician ring system, which also caused theOrdovician meteor event,may have also led to the glaciation.[13]As the southern supercontinentGondwanadrifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata ofNorth Africaand then-adjacent northeastern South America, which were south-polar locations at the time.

As glaciers grew, the sea level dropped, and the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches. When they returned, they carried diminished founder populations that lacked many whole families of organisms. They then withdrew again with the next pulse of glaciation, eliminating biological diversity with each change.[101]Species limited to a single epicontinental sea on a given landmass were severely affected.[41]Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.[41]

Those species able to adapt to the changing conditions survived to fill the ecological niches left by the extinctions. For example, there is evidence the oceans became more deeply oxygenated during the glaciation, allowing unusual benthic organisms (Hirnantian fauna) to colonize the depths. These organisms were cosmopolitan in distribution and present at most latitudes.[69]

At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Orders. Recovery was characterized by an unusual number of "Lazarus taxa", disappearing during the extinction and reappearing well into the Silurian, which suggests that the taxa survived in small numbers inrefugia.[102]

An alternate extinction hypothesis suggested that a ten-secondgamma-ray burstcould have destroyed theozone layerand exposed terrestrial and marine surface-dwelling life to deadly ultravioletradiationand initiated global cooling.[103]

Recent work considering thesequence stratigraphyof the Late Ordovician argues that the mass extinction was a single protracted episode lasting several hundred thousand years, with abrupt changes in water depth and sedimentation rate producing two pulses of last occurrences of species.[104]

References

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  3. ^Lindskog, A.; Costa, M. M.; Rasmussen, C.M.Ø.; Connelly, J. N.; Eriksson, M. E. (2017-01-24)."Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification".Nature Communications.8:14066.doi:10.1038/ncomms14066.ISSN2041-1723.PMC5286199.PMID28117834.It has been suggested that the Middle Ordovician meteorite bombardment played a crucial role in the Great Ordovician Biodiversification Event, but this study shows that the two phenomena were unrelated
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