Editing Ordovician

{{Short description|Second period of the Paleozoic Era}}

{{Infobox geologic timespan
| name                         = Ordovician
| color                        = Ordovician
| top_bar                      = 
| time_start                   = 486.85
| time_start_uncertainty       = 1.5
| time_end                     = 443.1
| time_end_uncertainty         = 0.9
| image_map                    = Mollweide Paleographic Map of Earth, 465 Ma (Darriwilian Age).png
| caption_map                  = A map of Earth as it appeared 465 million years ago during the Middle Ordovician Epoch
| timeline                     = Ordovician
<!--Etymology-->
| name_formality               = Formal
| name_accept_date             = 1960
| alternate_spellings          =
| synonym1                     =
| synonym1_coined              =
| synonym2                     =
| synonym2_coined              =
| synonym3                     =
| synonym3_coined              =
| nicknames                    =
| former_names                 =
| proposed_names               =
<!--Usage Information--> 
| celestial_body               = earth
| usage                        = Global ([[International Commission on Stratigraphy|ICS]])
| timescales_used              = ICS Time Scale
<!--Definition-->
| chrono_unit                  = Period
| strat_unit                   = System
| proposed_by                  = [[Charles Lapworth]], 1879
| timespan_formality           = Formal
| lower_boundary_def           = [[First appearance datum|FAD]] of the [[Conodont]] ''[[Iapetognathus fluctivagus]]''
| lower_gssp_location          = Greenpoint section, [[Green Point, Newfoundland|Green Point]], [[Newfoundland]], [[Canada]]
| lower_gssp_coords            = {{Coord|49.6829|N|57.9653|W|display=inline}}
| lower_gssp_accept_date       = 2000<ref>{{cite journal |last1=Cooper |first1=Roger |last2=Nowlan |first2=Godfrey |last3=Williams |first3=S. H. |title=Global Stratotype Section and Point for base of the Ordovician System |journal=Episodes |date=March 2001 |volume=24 |issue=1 |pages=19–28 |doi=10.18814/epiiugs/2001/v24i1/005 |doi-access=free |url=https://stratigraphy.org/gssps/files/tremadocian.pdf |access-date=6 December 2020 |archive-date=11 January 2021 |archive-url=https://web.archive.org/web/20210111103733/https://stratigraphy.org/gssps/files/tremadocian.pdf |url-status=live }}</ref>
| upper_boundary_def           = FAD of the [[Graptolite]] ''[[Akidograptus ascensus]]''
| upper_gssp_location          = [[Dob's Linn]], [[Moffat]], [[United Kingdom|U.K.]]
| upper_gssp_coords            = {{Coord|55.4400|N|3.2700|W|display=inline}}
| upper_gssp_accept_date       = 1984<ref>{{cite journal |last1=Lucas |first1=Sepncer |title=The GSSP Method of Chronostratigraphy: A Critical Review |journal=Frontiers in Earth Science |date=6 November 2018 |volume=6 |page=191 |doi=10.3389/feart.2018.00191 |bibcode=2018FrEaS...6..191L |doi-access=free }}</ref><ref>{{cite journal |last1=Holland |first1=C. |title=Series and Stages of the Silurian System |journal=Episodes |date=June 1985 |volume=8 |issue=2 |pages=101–103 |doi=10.18814/epiiugs/1985/v8i2/005 |url=https://timescalefoundation.org/references/Silurian1.pdf |access-date=11 December 2020 |doi-access=free |archive-date=19 January 2022 |archive-url=https://web.archive.org/web/20220119093253/https://timescalefoundation.org/references/Silurian1.pdf |url-status=live }}</ref>
<!--Atmospheric and Climatic Data-->
| sea_level                    = 180 m; rising to 220 m in Caradoc and falling sharply to 140 m in end-Ordovician glaciations<ref>{{cite journal | author = Haq, B. U.| year = 2008| doi = 10.1126/science.1161648 | title = A Chronology of Paleozoic Sea-Level Changes | journal = Science | volume = 322 | pages = 64–68 | pmid = 18832639 | last2 = Schutter | first2 = SR | issue = 5898 |bibcode = 2008Sci...322...64H | s2cid = 206514545}}</ref>
}}
The '''Ordovician''' ({{IPAc-en|ɔːr|d|ə|ˈ|v|ɪ|ʃ|i|.|ə|n|,_|-|d|oʊ|-|,_|-|ˈ|v|ɪ|ʃ|ən}} {{respell|or|də|VISH|ee|ən|,_|-|doh|-|,_|-|VISH|ən}})<ref>{{dictionary.com|Ordovician}}</ref> is a [[geologic period]] and [[System (geology)|system]], the second of six periods of the [[Paleozoic]] [[Era (geology)|Era]], and the second of twelve periods of the [[Phanerozoic]] [[Eon (geology)|Eon]]. The Ordovician spans 41.6 million years from the end of the [[Cambrian]] Period {{Period end|Cambrian}} [[Megaannum|Ma]] (million years ago) to the start of the [[Silurian]] Period {{Period start|Silurian}} Ma.<ref name="ICS2015">{{cite web | url=http://www.stratigraphy.org/ICSchart/ChronostratChart2015-01.pdf | title=International Chronostratigraphic Chart v.2015/01 | publisher=[[International Commission on Stratigraphy]] | date=January 2015 | access-date=2015-05-30 | archive-date=2015-04-02 | archive-url=https://web.archive.org/web/20150402142607/http://stratigraphy.org/ICSchart/ChronostratChart2015-01.pdf | url-status=live }}</ref>

The Ordovician, named after the [[Celtic Britons|Welsh]] 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 (geology)|rock]] beds in [[North Wales]] in the Cambrian and Silurian systems, respectively.<ref>Charles Lapworth (1879) [https://archive.org/details/lapworth-1879-geologicalmagazi-2618wood/  "On the Tripartite Classification of the Lower Palaeozoic Rocks"], ''Geological Magazine'', new series, '''6''' : 1-15.  From pp. 13–14:  "North Wales itself — at all events the whole of the great Bala district where Sedgwick first worked out the physical succession among the rocks of the intermediate or so-called ''Upper Cambrian'' or ''Lower Silurian'' system; and in all probability, much of the Shelve and the Caradoc area, whence Murchison first published its distinctive fossils — lay within the territory of the Ordovices; … Here, then, have we the hint for the appropriate title for the central system of the Lower Paleozoic.  It should be called the Ordovician System, after this old British tribe."</ref> Lapworth recognized that the [[fossil]] [[fauna]] in the disputed [[Stratum|strata]] were 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 the [[International Union of Geological Sciences|International Geological Congress]].

Life continued to flourish during the Ordovician as it had in the earlier Cambrian Period, although the end of the period was marked by the [[Ordovician–Silurian extinction events]]. Invertebrates, namely [[Mollusca|molluscs]] and [[arthropod]]s, dominated the oceans, with members of the latter group probably starting their establishment on land during this time, becoming fully established by the [[Devonian]]. The first [[land plants]] are known from this period. The [[Great Ordovician Biodiversification Event]] considerably increased the diversity of life. [[Fish]], the world's first true [[vertebrate]]s, continued to evolve, and [[Gnathostomata|those with jaws]] may 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 the [[Ordovician meteor event]].<ref>{{cite web | url=https://www.sciencedaily.com/releases/2016/06/160615135230.htm | title=New type of meteorite linked to ancient asteroid collision | website=Science Daily | date=15 June 2016 | access-date=20 June 2016 | archive-date=3 April 2019 | archive-url=https://web.archive.org/web/20190403222034/https://www.sciencedaily.com/releases/2016/06/160615135230.htm | url-status=live }}</ref> It has been theorized that this increase in impacts may originate from [[Rings of Earth|a ring system]] that formed around Earth at the time.<ref name=":0">{{Cite journal |last1=Tomkins |first1=Andrew G. |last2=Martin |first2=Erin L. |last3=Cawood |first3=Peter A. |date=2024-11-15 |title=Evidence suggesting that earth had a ring in the Ordovician |journal=Earth and Planetary Science Letters |volume=646 |pages=118991 |doi=10.1016/j.epsl.2024.118991 |issn=0012-821X|doi-access=free }}</ref>

==Subdivisions{{anchor|Subdivisions}}==
{{anchor|Tremadocian}}
In 2008, the [[International Classification for Standards|ICS]] erected a formal international system of subdivisions for the Ordovician Period and System.<ref>Details on the Dapingian are available at {{Cite journal| first1 = X.| first2 = S.| first3 = X.| first4 = Z.| first5 = C.| title = Dapingian Stage: standard name for the lowermost global stage of the Middle Ordovician Series| journal = [[Lethaia]]| volume = 42| issue = 3| pages = 377–380| last1 = Wang| year = 2009| doi = 10.1111/j.1502-3931.2009.00169.x| last2 = Stouge| last3 = Chen| last4 = Li| last5 = Wang}}</ref> Pre-existing Baltoscandic, British, Siberian, North American, Australian, Chinese, Mediterranean and North-[[Gondwana]]n regional stratigraphic schemes are also used locally.<ref>{{cite web |title=The Ordovician Period |url=https://ordovician.stratigraphy.org/period |website=Subcommission on Ordovician Stratigraphy |publisher=International Commission on Stratigraphy |access-date=7 June 2021 |date=2020 |archive-date=11 May 2022 |archive-url=https://web.archive.org/web/20220511072916/https://ordovician.stratigraphy.org/period |url-status=live }}</ref>

=== Global/regional correlation ===
{| class="wikitable"
|+ Approximate correlation of Ordovician regional series and stages<ref>{{Citation |last1=Goldman |first1=D. |title=The Ordovician Period |date=2020 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780128243602000206 |work=Geologic Time Scale 2020 |pages=631–694 |access-date=2023-06-08 |publisher=Elsevier |language=en |doi=10.1016/b978-0-12-824360-2.00020-6 |isbn=978-0-12-824360-2 |last2=Sadler |first2=P.M. |last3=Leslie |first3=S.A. |last4=Melchin |first4=M.J. |last5=Agterberg |first5=F.P. |last6=Gradstein |first6=F.M. |archive-date=2023-01-06 |archive-url=https://web.archive.org/web/20230106135352/https://linkinghub.elsevier.com/retrieve/pii/B9780128243602000206 |url-status=live }}</ref>
|-
! [[Series (stratigraphy)|ICS series]] !! [[Stage (stratigraphy)|ICS stage]] !! British series!! British stage !! North American series !! North American stage !! Australian stage !! Chinese stage 
|-
| rowspan="10" |[[Upper Ordovician]] || [[Hirnantian]] || rowspan="4"| [[Ashgill series|Ashgill]]|| [[Hirnantian]] || rowspan="5" | Cincinnatian|| Gamachian|| rowspan="3" | Bolindian|| [[Hirnantian]]
|-
| rowspan="5"|[[Katian]] || Rawtheyan|| rowspan="2" | Richmondian|| rowspan="2" | Chientangkiangian
|-
| Cautleyan
|-
| Pusgillian|| Maysvillian
| rowspan="3" |Eastonian
| rowspan="7" |Neichianshanian
|-
| rowspan="6" |[[Caradoc series|Caradoc]] || Streffordian|| Edenian
|-
| Cheneyan
| rowspan="4" |Mohawkian
| rowspan="2" |Chatfieldian
|-
| rowspan="4" |[[Sandbian]] || rowspan="2" | Burrellian|| rowspan="4" |Gisbornian
|-
| rowspan="2" |Turinian
|-
| rowspan="2" | Aurelucian
|-
| rowspan="6" |[[Whiterockian]]
| rowspan="5" |
|-
| rowspan="5" |[[Middle Ordovician]]|| rowspan="3" |[[Darriwilian]]|| rowspan="2" |Llanvirn|| Llandeilo|| rowspan="3" | [[Darriwilian]]
| rowspan="3" |[[Darriwilian]]
|-
| Abereiddian
|-
| rowspan="7" |[[Arenig]]|| rowspan="2" | Fennian
|-
| rowspan="2" |[[Dapingian]]
|Yapeenian
| rowspan="2" |[[Dapingian]]
|-
| rowspan="3" |Whitlandian
|Rangerian
|Castlemainian
|-
| rowspan="9" |[[Lower Ordovician]]
| rowspan="4" |[[Floian]]
| rowspan="9" |Ibexian
| rowspan="3" |Blackhillsian||Chewtonian
| rowspan="4" |Yiyangian
|-
| rowspan="2" |Bendigonian
|-
| rowspan="2" | Moridunian
|-
| rowspan="2" |Tulean
| rowspan="5" |Lancefieldian
|-
| rowspan="5" | [[Tremadocian]]|| rowspan="5" |[[Tremadocian|Tremadoc]] || rowspan="2" | Migneintian|| rowspan="5" | Xinchangian
|-
| rowspan="2" |Stairsian
|-
| rowspan="3" |Cressagian
|-
| rowspan="2" | Skullrockian
|-
|Warendan
|}

=== British stages and ages ===
The Ordovician Period in Britain was traditionally broken into Early (Tremadocian and [[Arenig]]), Middle ([[Llanvirn Series|Llanvirn]] (subdivided into Abereiddian and Llandeilian) and [[Llandeilo Group|Llandeilo]]) and Late ([[Caradoc Series|Caradoc]] and 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 the [[Hirnantian]].

==== Ashgill ====
The Ashgill Epoch, the last epoch of the British Ordovician, is made of four ages: the Hirnantian Age, the Rawtheyan Age, the '''Cautleyan''' Age, 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's [[Geologic Time Scale]].

==Paleogeography and tectonics==
[[File:Earth Paleogeography 480 Ma (Early Ordovician, Tremadocian).png|thumb|Paleogeographic map of the Earth in the early Ordovician, 480 million years ago{{citation needed|reason=the image on Wikimedia Commons is unsourced|date=February 2025}}]]
[[File:Earth Paleogeography 470 Ma (Early-Middle Ordovician, Dapingian).png|thumb|Paleogeographic map of the Earth in the middle Ordovician, 470 million years ago{{citation needed|reason=the image on Wikimedia Commons is unsourced|date=February 2025}}]]
[[File:Earth Paleogeography 450 Ma (Late Ordovician, Katian).png|thumb|Paleogeographic map of the Earth in the late Ordovician, 450 million years ago{{citation needed|reason=the image on Wikimedia Commons is unsourced|date=February 2025}}]]
During the Ordovician, the southern continents were assembled into [[Gondwana]], which reached from north of the [[equator]] to the [[South Pole]]. The Panthalassic Ocean, centered in the northern hemisphere, covered over half the globe.<ref>{{cite book |last1=Torsvik |first1=Trond H. |last2=Cocks |first2=L. Robin M. |title=Earth history and palaeogeography |date=2017 |publisher=Cambridge University Press |location=Cambridge, United Kingdom |isbn=9781107105324 |page=102}}</ref> At the start of the period, the continents of [[Laurentia]] (in present-day [[North America]]), [[Siberia (continent)|Siberia]], and [[Baltica]] (present-day northern Europe) were separated from Gondwana by over {{convert|5000|km||}} of ocean. These smaller continents were also sufficiently widely separated from each other to develop distinct communities of benthic organisms.{{sfn|Torsvik|Cocks|2017|p=102}} The small continent of [[Avalonia]] had just rifted from Gondwana and began to move north towards Baltica and Laurentia, opening the [[Rheic Ocean]] between Gondwana and Avalonia.<ref>{{cite journal |last1=Pollock |first1=Jeffrey C. |last2=Hibbard |first2=James P. |last3=Sylvester |first3=Paul J. |title=Early Ordovician rifting of Avalonia and birth of the Rheic Ocean: U–Pb detrital zircon constraints from Newfoundland |journal=[[Journal of the Geological Society]] |date=May 2009 |volume=166 |issue=3 |pages=501–515 |doi=10.1144/0016-76492008-088|bibcode=2009JGSoc.166..501P |s2cid=129091590 }}</ref><ref>{{cite journal |last1=Nance |first1=R. Damian |last2=Gutiérrez-Alonso |first2=Gabriel |last3=Keppie |first3=J. Duncan |last4=Linnemann |first4=Ulf |last5=Murphy |first5=J. Brendan |last6=Quesada |first6=Cecilio |last7=Strachan |first7=Rob A. |last8=Woodcock |first8=Nigel H. |title=A brief history of the Rheic Ocean |journal=Geoscience Frontiers |date=March 2012 |volume=3 |issue=2 |pages=125–135 |doi=10.1016/j.gsf.2011.11.008|doi-access=free }}</ref>{{sfn|Torsvik|Cocks|2017|p=103}} Avalonia collided with Baltica towards the end of Ordovician.<ref>{{cite journal |last1=Trela |first1=Wieslaw |date=15 July 2005 |title=Condensation and phosphatization of the Middle and Upper Ordovician limestones on the Malopolska Block (Poland): Response to paleoceanographic conditions |url=https://www.sciencedirect.com/science/article/abs/pii/S0037073805001910 |journal=Sedimentary Geology |volume=117 |issue=3–4 |pages=219–236 |doi=10.1016/j.sedgeo.2005.05.005 |access-date=21 May 2023 |archive-date=22 May 2023 |archive-url=https://web.archive.org/web/20230522055504/https://www.sciencedirect.com/science/article/abs/pii/S0037073805001910 |url-status=live }}</ref>{{sfn|Torsvik|Cocks|2017|p=112}}

Other geographic features of the Ordovician world included the [[Tornquist Sea]], which separated Avalonia from Baltica;{{sfn|Torsvik|Cocks|2017|p=102}} the Aegir Ocean, which separated Baltica from Siberia;<ref>{{cite journal |last1=Torsvik |first1=Trond H. |last2=Rehnström |first2=Emma F. |title=Cambrian palaeomagnetic data from Baltica: implications for true polar wander and Cambrian palaeogeography |journal=[[Journal of the Geological Society]] |date=March 2001 |volume=158 |issue=2 |pages=321–329 |doi=10.1144/jgs.158.2.321|bibcode=2001JGSoc.158..321T |s2cid=54656066 }}</ref> and an oceanic area between Siberia, Baltica, and Gondwana which expanded to become the Paleoasian Ocean in Carboniferous time. The [[Mongol-Okhotsk Ocean]] formed a deep embayment between Siberia and the Central Mongolian [[terrane]]s. Most of the terranes of central Asia were part of an equatorial archipelago whose geometry is poorly constrained by the available evidence.{{sfn|Torsvik|Cocks|2017|pp=102, 106}}

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.<ref>{{cite journal |last1=van Staal |first1=C.R. |last2= Hatcher | first2= R.D. Jr. |year=2010 |title=Global setting of Ordovician orogenesis |journal=Geol Soc Am Spec Pap |volume=466 |pages=1–11 |doi=10.1130/2010.2466(01)|isbn=9780813724669 }}</ref>{{sfn|Torsvik|Cocks|2017|p=102}}
 
The [[Taconic orogeny]], a major mountain-building episode, was well under way in Cambrian times.{{sfn|Torsvik|Cocks|2017|pp=93-94}} This continued into the Ordovician, when at least two [[Volcanic arc|volcanic island arcs]] collided with Laurentia to form the [[Appalachian 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.{{sfn|Torsvik|Cocks|2017|pp=106-109}}

The [[ash fall]] of the Millburg/Big Bentonite bed, at about 454 Ma, was the largest in the last 590 million years. This had a [[dense rock equivalent]] volume of as much as {{convert|1140|km3||}}. Remarkably, this appears to have had little impact on life.<ref>{{cite journal |last1=Huff |first1=Warren D. |last2=Bergström |first2=Stig M. |last3=Kolata |first3=Dennis R. |title=Gigantic Ordovician volcanic ash fall in North America and Europe: Biological, tectonomagmatic, and event-stratigraphic significance |journal=[[Geology (journal)|Geology]] |date=1992-10-01 |volume=20 |issue=10 |pages=875–878 |doi=10.1130/0091-7613(1992)020<0875:GOVAFI>2.3.CO;2|bibcode=1992Geo....20..875H }}</ref>

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.{{sfn|Torsvik|Cocks|2017|p=102}} The accretion of these arcs to the eastern margin of Gondwana was responsible for the Benambran Orogeny of eastern Australia.<ref>{{cite journal |last1=Glen |first1=R. A. |last2=Meffre |first2=S. |last3=Scott |first3=R. J. |title=Benambran Orogeny in the Eastern Lachlan Orogen, Australia |journal=[[Australian Journal of Earth Sciences]] |date=March 2007 |volume=54 |issue=2–3 |pages=385–415 |doi=10.1080/08120090601147019|bibcode=2007AuJES..54..385G |s2cid=129843558 }}</ref>{{sfn|Torsvik|Cocks|2017|p=105}} Subduction also took place along what is now Argentina (Famatinian Orogeny) at 450 Ma.<ref>{{cite book |last1=Ramos |first1=Victor A. |chapter=The Famatinian Orogen Along the Protomargin of Western Gondwana: Evidence for a Nearly Continuous Ordovician Magmatic Arc Between Venezuela and Argentina |title=The Evolution of the Chilean-Argentinean Andes |series=Springer Earth System Sciences |date=2018 |pages=133–161 |doi=10.1007/978-3-319-67774-3_6|isbn=978-3-319-67773-6 }}</ref> This involved significant back arc rifting.{{sfn|Torsvik|Cocks|2017|p=102}} The interior of Gondwana was tectonically quiet until the [[Triassic]].{{sfn|Torsvik|Cocks|2017|p=102}}

Towards the end of the Ordovician, Gondwana began to drift across the South Pole; this contributed to the [[Hirnantian glaciation]] and the associated extinction event.{{sfn|Torsvik|Cocks|2017|pp=103–105}}

===Ordovician meteor event===
The [[Ordovician meteor event]] is 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 the [[L chondrite]] parent body.<ref name=Lindskog>{{Cite journal|last1=Lindskog|first1=A. |last2=Costa|first2=M. M. |last3=Rasmussen|first3=C.M.Ø. |last4=Connelly|first4=J. N. |last5=Eriksson|first5=M. E. |date=2017-01-24|title=Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification|journal=Nature Communications|language=En|volume=8|pages=14066 |doi=10.1038/ncomms14066| pmid=28117834|pmc=5286199 |bibcode=2017NatCo...814066L |issn=2041-1723 }}</ref> It is not associated with any major extinction event.<ref name=Nature1>{{cite journal |bibcode=2004Natur.430..323H |title=Fast delivery of meteorites to Earth after a major asteroid collision |last1=Heck |first1=Philipp R. |last2=Schmitz |first2=Birger |last3=Baur |first3=Heinrich |last4=Halliday |first4=Alex N. | author-link4 = Alex N. Halliday |last5=Wieler |first5=Rainer |volume=430 |year=2004 |pages=323–5 |journal=[[Nature (journal)|Nature]] |doi=10.1038/nature02736 |pmid=15254530 |issue=6997|s2cid=4393398 }}</ref><ref>{{cite journal |bibcode=1996Icar..119..182H |title=Meteoritic, Asteroidal, and Theoretical Constraints on the 500 MA Disruption of the L Chondrite Parent Body |last1=Haack |first1=Henning |last2=Farinella |first2=Paolo |last3=Scott |first3=Edward R. D. |last4=Keil |first4=Klaus |volume=119 |issue=1 |year=1996 |pages=182–91 |journal=Icarus |doi=10.1006/icar.1996.0010}}</ref><ref>{{cite journal |bibcode=2007M&PS...42..113K |title=L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar-39Ar dating |last1=Korochantseva |first1=Ekaterina V. |last2=Trieloff |first2=Mario |last3=Lorenz |first3=Cyrill A. |last4=Buykin |first4=Alexey I. |last5=Ivanova |first5=Marina A. |last6=Schwarz |first6=Winfried H. |last7=Hopp |first7=Jens |last8=Jessberger |first8=Elmar K. |volume=42 |issue=1 |year=2007 |pages=113–30 |journal=[[Meteoritics & Planetary Science]] |doi=10.1111/j.1945-5100.2007.tb00221.x|s2cid=54513002 |doi-access=free }}</ref> 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 a [[ring system]] for a period of about 40 million years, with frequent falling debris causing these craters.<ref name=":0" />

== Geochemistry ==
[[File:Anomalodonta gigantea Waynesville Franklin Co IN.JPG|thumb|External mold of Ordovician [[Bivalvia|bivalve]] showing that the original [[aragonite]] shell dissolved on the sea floor, leaving a cemented mold for biological encrustation ([[Waynesville Formation]] of Franklin County, Indiana).]]
The Ordovician was a time of [[calcite sea]] geochemistry in which low-magnesium [[calcite]] was the primary inorganic marine precipitate of [[calcium carbonate]].<ref name="JonesEtAl2019">{{cite journal |last1=Jones |first1=David S. |last2=Brothers |first2=R. William |last3=Ahm |first3=Anne-Sofie Crüger |last4=Slater |first4=Nicholas |last5=Higgins |first5=John A. |last6=Fike |first6=David A. |date=9 December 2019 |title=Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates |journal=Geology |volume=48 |issue=2 |pages=194–199 |doi=10.1130/G46861.1 |s2cid=213408515 |doi-access=free }}</ref> [[Carbonate hardgrounds]] were thus very common, along with calcitic [[ooid]]s, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons. Biogenic [[aragonite]], like that composing the shells of most [[Mollusca|molluscs]], dissolved rapidly on the sea floor after death.<ref name="Stanley1998">{{Cite journal | last1 = Stanley | first1 = S. | last2 = Hardie | first2 = L. | doi = 10.1016/S0031-0182(98)00109-6 | title = Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry | journal = [[Palaeogeography, Palaeoclimatology, Palaeoecology]] | volume = 144 | issue = 1–2 | pages = 3–19 | year = 1998 | bibcode = 1998PPP...144....3S | doi-access = free }}</ref><ref name="Stanley1999">{{cite journal |last=Stanley |first=S. M. |author2=Hardie, L. A. |year=1999 |title=Hypercalcification; paleontology links plate tectonics and geochemistry to sedimentology |journal=GSA Today |volume=9 |pages=1&ndash;7 }}</ref>

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.<ref name=Munnecke2010/>

==Climate and sea level==

The Early Ordovician climate was very hot,<ref>{{Cite journal |last1=M. Marcilly |first1=Chloé |last2=Maffre |first2=Pierre |last3=Le Hir |first3=Guillaume |last4=Pohl |first4=Alexandre |last5=Fluteau |first5=Frédéric |last6=Goddéris |first6=Yves |last7=Donnadieu |first7=Yannick |last8=H. Heimdal |first8=Thea |last9=Torsvik |first9=Trond H. |date=15 September 2022 |title=Understanding the early Paleozoic carbon cycle balance and climate change from modelling |url=https://www.sciencedirect.com/science/article/pii/S0012821X22003533 |journal=[[Earth and Planetary Science Letters]] |volume=594 |pages=117717 |doi=10.1016/j.epsl.2022.117717 |issn=0012-821X |access-date=17 September 2023 |hdl=10852/94890 |hdl-access=free |archive-date=7 October 2023 |archive-url=https://web.archive.org/web/20231007025150/https://www.sciencedirect.com/science/article/pii/S0012821X22003533 |url-status=live }}</ref> with intense [[Greenhouse and icehouse Earth|greenhouse]] conditions and [[sea surface temperature]]s comparable to those during the Early Eocene Climatic Optimum.<ref>{{cite journal |last1=Bergmann |first1=Kristin D. |last2=Finnegan |first2=Seth |last3=Creel |first3=Roger |last4=Eiler |first4=John M. |last5=Hughes |first5=Nigel C. |last6=Popov |first6=Leonid E. |last7=Fischer |first7=Woodward W. |date=1 March 2018 |title=A paired apatite and calcite clumped isotope thermometry approach to estimating Cambro-Ordovician seawater temperatures and isotopic composition |journal=[[Geochimica et Cosmochimica Acta]] |volume=224 |pages=18–41 |doi=10.1016/j.gca.2017.11.015 |bibcode=2018GeCoA.224...18B |doi-access=free }}</ref> [[Carbon dioxide]] levels were very high at the Ordovician period's beginning.<ref>{{Cite journal |last1=Brandt |first1=Danita S. |last2=Elias |first2=Robert J. |date=1989 |title=Temporal variations in tempestite thickness may be a geologic record of atmospheric CO2 |url=https://pubs.geoscienceworld.org/geology/article/17/10/951-952/186520 |journal=[[Geology (journal)|Geology]] |language=en |volume=17 |issue=10 |pages=951 |doi=10.1130/0091-7613(1989)017<0951:TVITTM>2.3.CO;2 |issn=0091-7613 |access-date=30 September 2023}}</ref> By the late Early Ordovician, the Earth cooled,<ref name="MayaElrick">{{cite journal |last1=Elrick |first1=Maya |date=1 October 2022 |title=Orbital-scale climate changes detected in Lower and Middle Ordovician cyclic limestones using oxygen isotopes of conodont apatite |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=603 |page=111209 |doi=10.1016/j.palaeo.2022.111209 |bibcode=2022PPP...603k1209E |doi-access=free }}</ref> giving way to a more temperate climate in the Middle Ordovician,<ref>{{Cite journal|last1=Goldberg|first1=Samuel L.|last2=Present|first2=Theodore M.|last3=Finnegan|first3=Seth|last4=Bergmann|first4=Kristin D.|date=2021-02-09|title=A high-resolution record of early Paleozoic climate|journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|language=en|volume=118|issue=6|pages=e2013083118|doi=10.1073/pnas.2013083118|pmid=33526667|pmc=8017688|bibcode=2021PNAS..11813083G|issn=0027-8424|doi-access=free }}</ref> with the Earth likely entering the [[Late Ordovician glaciation|Early Palaeozoic Ice Age]] during the Sandbian,<ref>{{cite journal |last1=Vandenbroucke |first1=Thijs R. A. |last2=Armstrong |first2=Howard A. |last3=Williams |first3=Mark |last4=Paris |first4=Florentin |last5=Sabbe |first5=Koen |last6=Zalasiewicz |first6=Jan A. |last7=Nõlvak |first7=Jaak |last8=Verniers |first8=Jacques |date=15 August 2010 |title=Epipelagic chitinozoan biotopes map a steep latitudinal temperature gradient for earliest Late Ordovician seas: Implications for a cooling Late Ordovician climate |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018209005215 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=294 |issue=3–4 |pages=202–219 |doi=10.1016/j.palaeo.2009.11.026 |bibcode=2010PPP...294..202V |access-date=29 December 2022 |archive-date=29 December 2022 |archive-url=https://web.archive.org/web/20221229202739/https://www.sciencedirect.com/science/article/abs/pii/S0031018209005215 |url-status=live }}</ref><ref>{{cite journal |last1=Rosenau |first1=Nicholas A. |last2=Hermann |first2=Achim D. |last3=Leslie |first3=Stephen A. |date=15 January 2012 |title=Conodont apatite δ18O values from a platform margin setting, Oklahoma, USA: Implications for initiation of Late Ordovician icehouse conditions |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018211005839 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=315-316 |pages=172–180 |doi=10.1016/j.palaeo.2011.12.003 |bibcode=2012PPP...315..172R |access-date=29 December 2022 |archive-date=7 April 2019 |archive-url=https://web.archive.org/web/20190407015109/https://www.sciencedirect.com/science/article/pii/S0031018211005839 |url-status=live }}</ref> and possibly as early as the Darriwilian<ref name ="EPIA">{{cite journal |last1=Pohl |first1=Alexandre |last2=Donnadieu |first2=Yannick |last3=Le Hir |first3=Guillaume |last4=Ladant |first4=Jean-Baptiste |last5=Dumas |first5=Christophe |last6=Alvarez-Solas |first6=Jorge |last7=Vandenbroucke |first7=Thijs R. A. |date=28 May 2016 |title=Glacial onset predated Late Ordovician climate cooling |journal=[[Paleoceanography and Paleoclimatology]] |volume=31 |issue=6 |pages=800–821 |doi=10.1002/2016PA002928 |bibcode=2016PalOc..31..800P |s2cid=133243759 |doi-access=free |hdl=1854/LU-8057556 |hdl-access=free }}</ref> or even the Floian.<ref name="MayaElrick" /> The Dapingian and Sandbian saw major humidification events evidenced by trace metal concentrations in Baltoscandia from this time.<ref name="HumidClimaticEvents">{{Cite journal |last1=Kiipli |first1=Enli |last2=Kiipli |first2=Tarmo |last3=Kallaste |first3=Toivo |last4=Pajusaar |first4=Siim |date=December 2017 |title=Trace elements indicating humid climatic events in the Ordovician–early Silurian |url=https://linkinghub.elsevier.com/retrieve/pii/S0009281917300168 |journal=Geochemistry |language=en |volume=77 |issue=4 |pages=625–631 |doi=10.1016/j.chemer.2017.05.002 |access-date=23 July 2024 |via=Elsevier Science Direct}}</ref> Evidence suggests that global temperatures rose briefly in the early Katian (Boda Event), depositing bioherms and radiating fauna across Europe.<ref>{{cite journal |last1=Fortey |first1=Richard A. |last2=Cocks |first2=L. Robin M. |title=Late Ordovician global warming—The Boda event |journal=[[Geology (journal)|Geology]] |date=2005 |volume=33 |issue=5 |pages=405 |doi=10.1130/G21180.1|bibcode=2005Geo....33..405F }}</ref> The early Katian also witnessed yet another humidification event.<ref name="HumidClimaticEvents" /> Further cooling during the Hirnantian, at the end of the Ordovician, led to the [[Late Ordovician glaciation]].<ref>{{Cite journal|last1=Trotter|first1=J. A.|last2=Williams|first2=I. S.|last3=Barnes|first3=C. R.|last4=Lecuyer|first4=C.|last5=Nicoll|first5=R. S.|date=2008-07-25|title=Did Cooling Oceans Trigger Ordovician Biodiversification? Evidence from Conodont Thermometry|url=https://www.science.org/doi/10.1126/science.1155814|journal=[[Science (journal)|Science]]|language=en|volume=321|issue=5888|pages=550–554|doi=10.1126/science.1155814|pmid=18653889|bibcode=2008Sci...321..550T|s2cid=28224399|issn=0036-8075|access-date=2022-06-30|archive-date=2022-10-06|archive-url=https://web.archive.org/web/20221006184654/https://www.science.org/doi/10.1126/science.1155814|url-status=live}}</ref>

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.<ref name=Munnecke2010/> The sea level rose more or less continuously throughout the Early Ordovician, leveling off somewhat during the middle of the period.<ref name=Munnecke2010/> 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, the sea level has risen and dropped somewhat. Despite much study, the details remain unresolved.<ref name=Munnecke2010/> In particular, some researches interpret the fluctuations in sea level as pre-Hibernian glaciation,<ref>{{cite journal |last1=Rasmussen |first1=Christian M. Ø. |last2=Ullmann |first2=Clemens V. |last3=Jakobsen |first3=Kristian G. |last4=Lindskog |first4=Anders |last5=Hansen |first5=Jesper |last6=Hansen |first6=Thomas |last7=Eriksson |first7=Mats E. |last8=Dronov |first8=Andrei |last9=Frei |first9=Robert |last10=Korte |first10=Christoph |last11=Nielsen |first11=Arne T. |last12=Harper |first12=David A.T. |title=Onset of main Phanerozoic marine radiation sparked by emerging Mid Ordovician icehouse |journal=[[Scientific Reports]] |date=May 2016 |volume=6 |issue=1 |pages=18884 |doi=10.1038/srep18884|pmid=26733399 |pmc=4702064 |bibcode=2016NatSR...618884R }}</ref> but sedimentary evidence of glaciation is lacking until the end of the period.{{sfn|Torsvik|Cocks|2017|p=112}} There is evidence of [[glacier]]s during the Hirnantian on the [[Gondwana|land we now know]] as Africa and South America, which were near the [[South Pole]] at the time, facilitating the formation of the [[ice cap]]s of the Hirnantian glaciation.

As with [[North America]] and [[Europe]], [[Gondwana]] was 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. The [[Panthalassic Ocean]] covered much of the [[Northern Hemisphere]], and other minor oceans included [[Proto-Tethys Ocean|Proto-Tethys]], [[Paleo-Tethys Ocean|Paleo-Tethys]], [[Khanty Ocean]], which was closed off by the Late Ordovician, [[Iapetus Ocean]], and the new [[Rheic Ocean]].


==Life==
[[File:Nmnh fg09.jpg|thumb|A [[diorama]] depicting Ordovician flora and fauna]]
For most of the Late Ordovician life continued to flourish, but at and near the end of the period there were [[Ordovician–Silurian extinction events|mass-extinction events]] that seriously affected [[conodont]]s and [[plankton]]ic forms like [[Graptolithina|graptolites]]. The [[trilobite]]s [[Agnostida]] and [[Ptychopariida]] completely died out, and the [[Asaphida]] were much reduced. [[Brachiopod]]s, [[bryozoa]]ns and [[echinoderm]]s were also heavily affected, and the [[endocerid]] [[cephalopod]]s died 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 the [[Moss#Geological history|first terrestrial plants]],<ref>{{cite news| url = https://www.bbc.co.uk/news/science-environment-16814669| title = Humble moss helped to cool Earth and spurred on life| work = BBC News| date = 2 February 2012| access-date = 21 June 2018| archive-date = 5 November 2018| archive-url = https://web.archive.org/web/20181105223704/https://www.bbc.co.uk/news/science-environment-16814669| url-status = live}}</ref> as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history.

===Fauna===
[[File:Endoceras life restoration.png|left|thumb|''[[Endoceras]]'', one of the largest predators of the Ordovician|570x570px]]
[[File:LibertyFormationSlab092313.jpg|thumb|[[Fossiliferous limestone]] slab from the Liberty Formation (Upper Ordovician) of Caesar Creek State Park near Waynesville, Ohio.]]
[[File:Isotelus trilobite from Wisconsin.jpg|thumb|The trilobite ''[[Isotelus]]'' from [[Wisconsin]]]]
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.<ref name=Munnecke2010>{{Cite journal| last1 = Munnecke| first1 = Axel| last2 = Calner| first2 = M.| last3 = Harper| first3 = David A. T.| author-link3 = David Harper (palaeontologist)| last4 = Servais| first4 = Thomas| title = Ordovician and Silurian sea-water chemistry, sea level, and climate: A synopsis| journal = [[Palaeogeography, Palaeoclimatology, Palaeoecology]]| url = https://www.sciencedirect.com/science/article/abs/pii/S0031018210004785| volume = 296| issue = 3–4| pages = 389–413| year = 2010| doi = 10.1016/j.palaeo.2010.08.001| bibcode = 2010PPP...296..389M| access-date = 16 August 2023| archive-date = 17 August 2023| archive-url = https://web.archive.org/web/20230817055224/https://www.sciencedirect.com/science/article/abs/pii/S0031018210004785| url-status = live}}</ref> Though less famous than the [[Cambrian explosion]], the [[Ordovician radiation]] (also known as the Great Ordovician Biodiversification Event){{sfn|Torsvik|Cocks|2017|p=102}} was no less remarkable; marine faunal [[genus|genera]] increased fourfold, resulting in 12% of all known [[Phanerozoic]] marine fauna.<ref name="Dixon2001">{{cite book |title=Atlas of Life on Earth |last=Dixon |first=Dougal |year=2001 |publisher=Barnes & Noble Books |location=New York |isbn=978-0-7607-1957-2 |pages=87 |display-authors=etal}}</ref> Several animals also went through a miniaturization process, becoming much smaller than their Cambrian counterparts.{{Citation needed|date=September 2023}} Another change in the fauna was the strong increase in [[Filter feeder|filter-feeding]] organisms.<ref>[http://www.palaeos.com/Paleozoic/Ordovician/Ordovician.htm Palaeos Paleozoic : Ordovician : The Ordovician Period] {{webarchive|url=https://web.archive.org/web/20071221094614/http://www.palaeos.com/Paleozoic/Ordovician/Ordovician.htm |date=21 December 2007}}</ref> The trilobite, inarticulate brachiopod, [[Archaeocyatha|archaeocyathid]], and [[Eocrinoidea|eocrinoid]] faunas of the Cambrian were succeeded by those that dominated the rest of the Paleozoic, such as articulate brachiopods, [[cephalopod]]s, and [[crinoid]]s. Articulate brachiopods, in particular, largely replaced trilobites in [[continental shelf|shelf]] communities. Their success epitomizes the greatly increased diversity of [[calcium carbonate|carbonate]] shell-secreting organisms in the Ordovician compared to the Cambrian.<ref name="Cooper1986">{{cite book |title=A Trip Through Time: Principles of Historical Geology |last=Cooper |first=John D. |author2=Miller, Richard H. |author3=Patterson, Jacqueline |year=1986 |publisher=Merrill Publishing Company |location=Columbus |isbn=978-0-675-20140-7 |pages=[https://archive.org/details/tripthroughtimep0000coop/page/247 247, 255&ndash;259] |url=https://archive.org/details/tripthroughtimep0000coop/page/247 }}</ref>[[File:20191205 Aegirocassis benmoulai Aegirocassis benmoulae.png|left|thumb|''[[Aegirocassis]]'', a large filter-feeding [[Hurdiidae|hurdiid]] [[Radiodonta|radiodont]] from [[Morocco]] ]]
Ordovician geography had its effect on the diversity of fauna; Ordovician invertebrates displayed a very high degree of provincialism.<ref>{{cite journal |last1=Heim |first1=Noel A. |date=8 April 2016 |title=A null biogeographic model for quantifying the role of migration in shaping patterns of global taxonomic richness and differentiation diversity, with implications for Ordovician biogeography |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/null-biogeographic-model-for-quantifying-the-role-of-migration-in-shaping-patterns-of-global-taxonomic-richness-and-differentiation-diversity-with-implications-for-ordovician-biogeography/2FB3853AA935DAD151FC1FB35181A986 |journal=[[Paleobiology (journal)|Paleobiology]] |volume=34 |issue=2 |pages=195–209 |doi=10.1666/0094-8373(2008)034[0195:ANBMFQ]2.0.CO;2 |access-date=18 May 2023 |archive-date=19 May 2023 |archive-url=https://web.archive.org/web/20230519042939/https://www.cambridge.org/core/journals/paleobiology/article/abs/null-biogeographic-model-for-quantifying-the-role-of-migration-in-shaping-patterns-of-global-taxonomic-richness-and-differentiation-diversity-with-implications-for-ordovician-biogeography/2FB3853AA935DAD151FC1FB35181A986 |url-status=live }}</ref> 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,<ref>{{Cite journal |last1=Cocks |first1=L. Robin M. |last2=Torsvik |first2=Trond H. |date=December 2021 |title=Ordovician palaeogeography and climate change |journal=[[Gondwana Research]] |language=en |volume=100 |pages=53–72 |doi=10.1016/j.gr.2020.09.008 |doi-access=free |hdl=10852/83447 |hdl-access=free }}</ref> and Gondwana developed distinct fauna in its tropical and temperature zones.<ref>{{Cite journal |last1=Cocks |first1=L. R. M. |last2=Fortey |first2=R. A. |date=January 1990 |title=Biogeography of Ordovician and Silurian faunas |url=https://www.lyellcollection.org/doi/10.1144/GSL.MEM.1990.012.01.08 |journal=Geological Society, London, Memoirs |language=en |volume=12 |issue=1 |pages=97–104 |doi=10.1144/GSL.MEM.1990.012.01.08 |issn=0435-4052 |access-date=17 September 2023}}</ref> The Tien Shan terrane maintained a biogeographic affinity with Gondwana,<ref>{{Cite journal |last1=Fortey |first1=Richard A. |last2=Cocks |first2=L.Robin M. |date=June 2003 |title=Palaeontological evidence bearing on global Ordovician–Silurian continental reconstructions |url=https://www.sciencedirect.com/science/article/pii/S0012825202001150 |journal=[[Earth-Science Reviews]] |language=en |volume=61 |issue=3–4 |pages=245–307 |doi=10.1016/S0012-8252(02)00115-0 |access-date=17 September 2023}}</ref> and the Alborz margin of Gondwana was linked biogeographically to South China.<ref>{{Cite journal |last1=Ghobadi Pour |first1=M. |last2=Popov |first2=L. E. |last3=Álvaro |first3=J. J. |last4=Amini |first4=A. |last5=Hairapetian |first5=V. |last6=Jahangir |first6=H. |date=23 December 2022 |title=Ordovician of North Iran: New lithostratigraphy, palaeogeography and biogeographical links with South China and the Mediterranean peri-Gondwana margin |url=http://www.geology.cz/bulletin/contents/art1830 |journal=Bulletin of Geosciences |volume=97 |issue=4 |pages=465–538 |access-date=17 September 2023 |archive-date=14 October 2023 |archive-url=https://web.archive.org/web/20231014211020/http://www.geology.cz/bulletin/contents/art1830 |url-status=live }}</ref> Southeast Asia's fauna also maintained strong affinities to Gondwana's.<ref>{{Cite journal |last1=Burrett |first1=Clive |last2=Stait |first2=Bryan |date=October 1985 |title=South East Asia as a part of an Ordovician Gondwanaland—a palaeobiogeographic test of a tectonic hypothesis |url=https://linkinghub.elsevier.com/retrieve/pii/0012821X85901001 |journal=[[Earth and Planetary Science Letters]] |language=en |volume=75 |issue=2–3 |pages=184–190 |doi=10.1016/0012-821X(85)90100-1 |access-date=17 September 2023 |archive-date=22 April 2024 |archive-url=https://web.archive.org/web/20240422215157/https://linkinghub.elsevier.com/retrieve/pii/0012821X85901001 |url-status=live }}</ref> North China was biogeographically connected to Laurentia and the Argentinian margin of Gondwana.<ref>{{Cite journal |last1=Ebbestad |first1=Jan Ove R. |last2=Frýda |first2=Jiří |last3=Wagner |first3=Peter J. |last4=Horný |first4=Radvan J. |last5=Isakar |first5=Mare |last6=Stewart |first6=Sarah |last7=Percival |first7=Ian G. |last8=Bertero |first8=Verònica |last9=Rohr |first9=David M. |last10=Peel |first10=John S. |last11=Blodgett |first11=Robert B. |last12=Högström |first12=Anette E. S. |date=November 2013 |title=Biogeography of Ordovician and Silurian gastropods, monoplacophorans and mimospirids |url=https://www.lyellcollection.org/doi/10.1144/M38.15 |journal=Geological Society, London, Memoirs |language=en |volume=38 |issue=1 |pages=199–220 |doi=10.1144/M38.15 |issn=0435-4052 |access-date=17 September 2023}}</ref> A Celtic biogeographic province also existed, separate from the Laurentian and Baltican ones.<ref>{{Cite journal |last1=Harper |first1=D.A.T. |last2=Mac Niocaill |first2=C. |last3=Williams |first3=S.H. |date=May 1996 |title=The palaeogeography of early Ordovician Iapetus terranes: an integration of faunal and palaeomagnetic constraints |url=https://linkinghub.elsevier.com/retrieve/pii/0031018295000798 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=121 |issue=3–4 |pages=297–312 |doi=10.1016/0031-0182(95)00079-8 |access-date=17 September 2023 |archive-date=16 April 2024 |archive-url=https://web.archive.org/web/20240416004322/https://linkinghub.elsevier.com/retrieve/pii/0031018295000798 |url-status=live }}</ref> However, tropical articulate brachiopods had a more [[cosmopolitan 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.<ref>{{cite journal |last1=Penny |first1=Amelia |last2=Kröger |first2=Björn |date=18 November 2019 |title=Impacts of spatial and environmental differentiation on early Palaeozoic marine biodiversity |url=https://www.nature.com/articles/s41559-019-1035-7 |journal=[[Nature Ecology and Evolution]] |volume=3 |issue=1 |pages=1655–1660 |doi=10.1038/s41559-019-1035-7 |access-date=3 June 2023 |hdl=10138/325369 |hdl-access=free |archive-date=3 June 2023 |archive-url=https://web.archive.org/web/20230603234416/https://www.nature.com/articles/s41559-019-1035-7 |url-status=live }}</ref> Faunas become less provincial later in the Ordovician, partly due to the narrowing of the Iapetus Ocean,<ref>{{Cite journal |last1=Pedersen |first1=R.B. |last2=Bruton |first2=D.L. |last3=Furnes |first3=H. |date=March 1992 |title=Ordovician faunas, island arcs and ophiolites in the Scandinavian Caledonides |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1365-3121.1992.tb00475.x |journal=[[Terra Nova (journal)|Terra Nova]] |language=en |volume=4 |issue=2 |pages=217–222 |doi=10.1111/j.1365-3121.1992.tb00475.x |issn=0954-4879 |access-date=17 September 2023 |archive-date=14 October 2023 |archive-url=https://web.archive.org/web/20231014211020/https://onlinelibrary.wiley.com/doi/10.1111/j.1365-3121.1992.tb00475.x |url-status=live }}</ref> though they were still distinguishable into the late Ordovician.{{sfn|Torsvik|Cocks|2017|p=112–113}} 
[[File:Eurypterids Pentecopterus Vertical.jpg|thumb|''[[Pentecopterus]]'', the earliest known eurypterid, and found in [[Iowa]]]]
[[Trilobite]]s in particular were rich and diverse, and experienced rapid diversification in many regions.<ref>{{Cite journal |last1=Zhiyi |first1=Zhou |last2=Wenwei |first2=Yuan |last3=Zhiqiang |first3=Zhou |date=19 March 2007 |title=Patterns, processes and likely causes of the Ordovician trilobite radiation in South China |url=https://onlinelibrary.wiley.com/doi/10.1002/gj.1076 |journal=[[Geological Journal]] |language=en |volume=42 |issue=3–4 |pages=297–313 |doi=10.1002/gj.1076 |issn=0072-1050 |access-date=12 September 2024 |via=Wiley Online Library}}</ref> 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 primitive [[eurypterid]]s and nautiloids while other trilobites such as ''Aeglina prisca'' evolved 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.<ref name="Palaeos.com">{{cite web |date=April 11, 2002 |title=Palaeos Paleozoic : Ordovician : The Ordovician Period |url=http://www.palaeos.com/Paleozoic/Ordovician/Ordovician.htm#Life |url-status=dead |archive-url=https://web.archive.org/web/20071221094614/http://www.palaeos.com/Paleozoic/Ordovician/Ordovician.htm#Life |archive-date=December 21, 2007}}</ref> Some trilobites such as ''Asaphus kowalewski'' evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.<ref>{{cite web |title=A Guide to the Orders of Trilobites<!-- Bot generated title --> |url=http://www.trilobites.info/ |access-date=2007-12-13 |archive-date=2019-02-18 |archive-url=https://web.archive.org/web/20190218193216/http://www.trilobites.info/ |url-status=live }}</ref> Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician.<ref>{{cite journal |last1=Garwood |first1=Russell J. |last2=Sharma |first2=Prashant P. |last3=Dunlop |first3=Jason A. |last4=Giribet |first4=Gonzalo |date=5 May 2014 |title=A Paleozoic Stem Group to Mite Harvestmen Revealed through Integration of Phylogenetics and Development |journal=[[Current Biology]] |volume=24 |issue=9 |pages=1017–1023 |doi=10.1016/j.cub.2014.03.039 |pmid=24726154 |doi-access=free }}</ref> Although solitary [[coral]]s date back to at least the [[Cambrian]], [[coral reef|reef]]-forming corals appeared in the early Ordovician, including the earliest known [[Octocorallia|octocorals]],<ref name="Taylor2013Pywackia">{{Cite journal|doi=10.1666/13-029|title=Reinterpretation of the Cambrian 'bryozoan' ''Pywackia'' as an octocoral|year=2013|last1=Taylor|first1=P.D.|last2=Berning|first2=B.|last3=Wilson|first3=M.A.|journal=Journal of Paleontology|volume=87|issue=6|pages=984–990|bibcode=2013JPal...87..984T|s2cid=129113026|url=https://zenodo.org/record/907861|access-date=2022-11-23|archive-date=2019-06-07|archive-url=https://web.archive.org/web/20190607161619/https://zenodo.org/record/907861|url-status=live}}</ref><ref>{{cite journal |last1=Bergström |first1=Stig M. |last2=Bergström |first2=Jan |last3=Kumpulainen |first3=Risto |last4=Ormö |first4=Jens |last5=Sturkell |first5=Erik |date=2007 |title=Maurits Lindström – A renaissance geoscientist |journal=[[GFF (journal)|GFF]] |volume=129 |issue=2 |pages=65–70 |doi=10.1080/11035890701292065 |s2cid=140593975}}</ref> corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals.<ref name=Munnecke2010/> Brachiopods surged in diversity, adapting to almost every type of marine environment.<ref>{{cite journal |last1=Song |first1=Zhenyu |last2=Xiao |first2=Yunpeng |last3=Xiao |first3=Chuantao |date=19 February 2020 |title=Early–Middle Ordovician brachiopod diversification in the middle Yangtze region of South China |url=https://cdnsciencepub.com/doi/full/10.1139/cjes-2019-0141 |journal=[[Canadian Journal of Earth Sciences]] |volume=57 |issue=8 |pages=999–1009 |doi=10.1139/cjes-2019-0141 |bibcode=2020CaJES..57..999S |s2cid=213757467 |access-date=22 November 2022}}</ref><ref>{{cite journal |last1=Harper |first1=David A. T. |last2=Zhan |first2=Ren-Bin |last3=Jin |first3=Jisuo |date=March–June 2015 |title=The Great Ordovician Biodiversification Event: Reviewing two decades of research on diversity's big bang illustrated by mainly brachiopod data |url=https://www.sciencedirect.com/science/article/abs/pii/S1871174X15000153 |journal=[[Palaeoworld]] |volume=24 |issue=1–2 |pages=75–85 |doi=10.1016/j.palwor.2015.03.003 |access-date=12 November 2022 |archive-date=13 November 2022 |archive-url=https://web.archive.org/web/20221113060658/https://www.sciencedirect.com/science/article/abs/pii/S1871174X15000153 |url-status=live }}</ref><ref>{{cite journal |last1=Zhan |first1=Renbin |last2=Rong |first2=Jiayu |last3=Cheng |first3=Jinghui |last4=Chen |first4=Pengfei |date=May 2005 |title=Early-Mid Ordovician brachiopod diversification in South China |url=https://link.springer.com/article/10.1360/03yd0586 |journal=[[Science China Earth Sciences]] |volume=48 |issue=5 |pages=662–675 |doi=10.1360/03yd0586 |s2cid=130038222 |access-date=12 November 2022 |archive-date=13 November 2022 |archive-url=https://web.archive.org/web/20221113060655/https://link.springer.com/article/10.1360/03yd0586 |url-status=live }}</ref> Even after GOBE, there is evidence suggesting that Ordovician brachiopods maintained elevated rates of speciation.<ref>{{cite journal |last1=Patzkowsky |first1=Mark E. |last2=Holland |first2=Steven M. |date=Fall 1997 |title=Patterns of turnover in Middle and Upper Ordovician brachiopods of the eastern United States: a test of coordinated stasis |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/patterns-of-turnover-in-middle-and-upper-ordovician-brachiopods-of-the-eastern-united-states-a-test-of-coordinated-stasis/1A029B92CA2AEE70CE305DF94F7C3623 |journal=[[Paleobiology (journal)|Paleobiology]] |volume=23 |issue=4 |pages=420–443 |doi=10.1017/S0094837300019825 |access-date=23 July 2023 |archive-date=23 July 2023 |archive-url=https://web.archive.org/web/20230723235042/https://www.cambridge.org/core/journals/paleobiology/article/abs/patterns-of-turnover-in-middle-and-upper-ordovician-brachiopods-of-the-eastern-united-states-a-test-of-coordinated-stasis/1A029B92CA2AEE70CE305DF94F7C3623 |url-status=live }}</ref> [[Mollusca|Molluscs]], which appeared during the Cambrian or even the [[Ediacaran]], became common and varied, especially [[Bivalvia|bivalves]], [[Gastropoda|gastropods]], and [[nautiloid]] cephalopods.<ref name="Novack-GottshallMiller2003">{{cite journal |last1=Novack-Gottshall |first1=Philip M. |last2=Miller |first2=Arnold I. |date=Fall 2003 |title=Comparative geographic and environmental diversity dynamics of gastropods and bivalves during the Ordovician Radiation |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/comparative-geographic-and-environmental-diversity-dynamics-of-gastropods-and-bivalves-during-the-ordovician-radiation/1199817CD0DB79492093621E11DA7A56 |journal=[[Paleobiology (journal)|Paleobiology]] |volume=29 |issue=4 |pages=576–604 |doi=10.1666/0094-8373(2003)029<0576:CGAEDD>2.0.CO;2 |s2cid=85724505 |access-date=8 December 2022 |archive-date=13 August 2023 |archive-url=https://web.archive.org/web/20230813015225/https://www.cambridge.org/core/journals/paleobiology/article/abs/comparative-geographic-and-environmental-diversity-dynamics-of-gastropods-and-bivalves-during-the-ordovician-radiation/1199817CD0DB79492093621E11DA7A56 |url-status=live }}</ref><ref name="RexCrick">{{cite journal |last1=Crick |first1=Rex M. |date=Spring 1981 |title=Diversity and evolutionary rates of Cambro-Ordovician nautiloids |url=https://www.cambridge.org/core/journals/paleobiology/article/abs/diversity-and-evolutionary-rates-of-cambroordovician-nautiloids/55BCD1C16ECA4FC01DFA2DA9092FE354 |journal=Paleobiology |volume=7 |issue=2 |pages=216–229 |doi=10.1017/S0094837300003997 |s2cid=83933056 |access-date=8 December 2022 |archive-date=13 August 2023 |archive-url=https://web.archive.org/web/20230813015225/https://www.cambridge.org/core/journals/paleobiology/article/abs/diversity-and-evolutionary-rates-of-cambroordovician-nautiloids/55BCD1C16ECA4FC01DFA2DA9092FE354 |url-status=live }}</ref> Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments.<ref name="KrögerAndYun-Bai2009">{{cite journal |last1=Kröger |first1=Björn |last2=Yun-Bai |first2=Zhang |title=Pulsed cephalopod diversification during the Ordovician |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |date=March 2009 |volume=273 |issue=1–2 |pages=174–183 |doi=10.1016/j.palaeo.2008.12.015|bibcode=2009PPP...273..174K }}</ref> Graptolites, which evolved in the preceding Cambrian period, thrived in the oceans.<ref>{{cite journal |last1=Heward |first1=A. P. |last2=Fortey |first2=R. A. |last3=Miller |first3=C. G. |last4=Booth |first4=G. A. |date=June 2023 |title=New Middle Ordovician (Darriwilian) faunas from the Sultanate of Oman |url=https://www.sciencedirect.com/science/article/abs/pii/S0016787823000202 |journal=Proceedings of the Geologists' Association |volume=134 |issue=3 |pages=251–268 |doi=10.1016/j.pgeola.2023.02.004 |access-date=16 August 2023 |archive-date=17 August 2023 |archive-url=https://web.archive.org/web/20230817055224/https://www.sciencedirect.com/science/article/abs/pii/S0016787823000202 |url-status=live }}</ref> This includes the distinctive ''Nemagraptus gracilis'' graptolite fauna, which was distributed widely during peak sea levels in the Sandbian.<ref name="FinneyAndBergström1986">{{cite journal |last1=Finney |first1=Stanley C. |last2=Bergström |first2=Stig M. |title=Biostratigraphy of the Ordovician Nemagraptus gracilis Zone |journal=Geological Society, London, Special Publications |date=1986 |volume=20 |issue=1 |pages=47–59 |doi=10.1144/GSL.SP.1986.020.01.06|bibcode=1986GSLSP..20...47F |s2cid=129733589 }}</ref>{{sfn|Torsvik|Cocks|2017|p=112}} Some new cystoids and crinoids appeared. It was long thought that the first true [[vertebrata|vertebrates]] (fish — [[Ostracoderm]]s) appeared in the Ordovician, but recent discoveries in [[China]] reveal that they probably originated in the Early [[Cambrian]].<ref>{{Cite web |date=2016-10-05 |title=12.7: Vertebrate Evolution |url=https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.07%3A_Vertebrate_Evolution |access-date=2022-06-07 |website=Biology LibreTexts |language=en |archive-date=2022-07-03 |archive-url=https://web.archive.org/web/20220703173215/https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.07%3A_Vertebrate_Evolution |url-status=live }}</ref> The first [[Gnathostomata|gnathostome]] (jawed fish) may have appeared in the [[Late Ordovician glaciation|Late Ordovician]] epoch.<ref name="Brazeau-Friedman-2015">{{cite journal |last1=Brazeau |first1=M. D. |last2=Friedman |first2=M. |date=2015 |title=The origin and early phylogenetic history of jawed vertebrates |journal=[[Nature (journal)|Nature]] |volume=520 |issue=7548 |pages=490–497 |bibcode=2015Natur.520..490B |doi=10.1038/nature14438 |pmc=4648279 |pmid=25903631}}</ref> Chitinozoans, which first appeared late in the Wuliuan, exploded in diversity during the Tremadocian, quickly becoming globally widespread.<ref name="NõlvakLiangHints2019PPP">{{cite journal |last1=Nõlvak |first1=Jaak |last2=Liang |first2=Yan |last3=Hints |first3=Olle |date=1 July 2019 |title=Early diversification of Ordovician chitinozoans on Baltica: New data from the Jägala waterfall section, northern Estonia |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018218309520 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=525 |pages=14–24 |doi=10.1016/j.palaeo.2019.04.002 |bibcode=2019PPP...525...14N |s2cid=135138918 |access-date=12 November 2022 |archive-date=13 November 2022 |archive-url=https://web.archive.org/web/20221113060656/https://www.sciencedirect.com/science/article/abs/pii/S0031018218309520 |url-status=live }}</ref><ref name="LiangEtAl2017PPP">{{cite journal |last1=Liang |first1=Yan |last2=Servais |first2=Thomas |last3=Tang |first3=Peng |last4=Lu |first4=Jianbo |last5=Wang |first5=Wenhui |date=December 2017 |title=Tremadocian (Early Ordovician) chitinozoan biostratigraphy of South China: An update |url=https://www.sciencedirect.com/science/article/abs/pii/S0034666716302676 |journal=[[Review of Palaeobotany and Palynology]] |volume=247 |pages=149–163 |doi=10.1016/j.revpalbo.2017.08.008 |access-date=12 November 2022 |archive-date=13 November 2022 |archive-url=https://web.archive.org/web/20221113060655/https://www.sciencedirect.com/science/article/abs/pii/S0034666716302676 |url-status=live }}</ref> Several groups of endobiotic symbionts appeared in the Ordovician.<ref name="VinnMotus2012">{{cite journal |author=Vinn, O. |author2=Mõtus, M.-A. |year=2012 |title=Diverse early endobiotic coral symbiont assemblage from the Katian (Late Ordovician) of Baltica |url=https://www.researchgate.net/publication/255569552 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=321–322 |pages=137–141 |bibcode=2012PPP...321..137V |doi=10.1016/j.palaeo.2012.01.028 |access-date=2014-06-11 |archive-date=2018-08-18 |archive-url=https://web.archive.org/web/20180818185257/https://www.researchgate.net/publication/255569552 |url-status=live }}</ref><ref name="VinnWilsonMotusToom2014">{{cite journal |last1=Vinn |first1=O. |last2=Wilson |first2=M.A. |last3=Mõtus |first3=M.-A. |last4=Toom |first4=U. |year=2014 |title=The earliest bryozoan parasite: Middle Ordovician (Darriwilian) of Osmussaar Island, Estonia |url=https://www.researchgate.net/publication/265853181 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=414 |pages=129–132 |bibcode=2014PPP...414..129V |doi=10.1016/j.palaeo.2014.08.021 |access-date=2014-01-09 |archive-date=2018-08-18 |archive-url=https://web.archive.org/web/20180818185255/https://www.researchgate.net/publication/265853181 |url-status=live }}</ref>

In the Early Ordovician, trilobites were joined by many new types of organisms, including [[Tabulata|tabulate]] corals, [[Strophomenida|strophomenid]], [[Rhynchonellida|rhynchonellid]], and many new [[Orthida|orthid]] brachiopods, bryozoans, planktonic graptolites and conodonts, and many types of molluscs and echinoderms, including the ophiuroids ("brittle stars") and the first [[Starfish|sea stars]]. Nevertheless, the arthropods remained abundant; all the Late Cambrian orders continued, and were joined by the new group [[Phacopida]]. The first evidence of land plants also appeared (see [[evolutionary 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, [[Cornulitida|cornulitids]], [[tentaculites|tentaculitids]] and echinoderms all flourished, tabulate corals diversified and the first [[Rugosa|rugose corals]] appeared. The planktonic graptolites remained diverse, with the Diplograptina making their appearance. One of the earliest known armoured [[agnatha]]n ("[[ostracoderm]]") vertebrates, ''[[Arandaspis]]'', dates from the Middle Ordovician.<ref name="RitchieGilbert-Tomlinson1976">{{cite journal |last1=Ritchie |first1=Alexander |last2=Gilbert-Tomlinson |first2=Joyce |date=24 November 1976 |title=First Ordovician vertebrates from the Southern Hemisphere |url=https://www.tandfonline.com/doi/abs/10.1080/03115517708527770 |journal=[[Alcheringa (journal)|Alcheringa]] |volume=1 |issue=4 |pages=351–368 |doi=10.1080/03115517708527770 |access-date=12 November 2022}}</ref> 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.<ref name="WilsonPalmer2006">{{cite journal |last=Wilson |first=M. A. |author2=Palmer, T. J. |year=2006 |title=Patterns and processes in the Ordovician Bioerosion Revolution |url=http://www3.wooster.edu/geology/WilsonPalmer06.pdf |url-status=dead |journal=Ichnos |volume=13 |issue=3 |pages=109&ndash;112 |doi=10.1080/10420940600850505 |archive-url=https://web.archive.org/web/20081216220233/http://www.wooster.edu/geology/WilsonPalmer06.pdf |archive-date=2008-12-16 |s2cid=128831144}}</ref> It is marked by a sudden abundance of hard substrate trace fossils such as ''[[Trypanites]]'', ''Palaeosabella'', ''[[Petroxestes]]'' and ''[[Osprioneides]]''. [[Bioerosion]] became an important process, particularly in the thick calcitic skeletons of corals, bryozoans and brachiopods, and on the extensive [[carbonate hardgrounds]] that appear in abundance at this time. 
<!--Bryozoa in Cambrian: {{doi|10.1130/G30870.1}}</ref>-->

<gallery>
File:OrdovicianEdrio.jpg|Upper Ordovician [[edrioasteroid]] ''Cystaster stellatus'' on a cobble from the Kope Formation in northern Kentucky with the cyclostome [[bryozoan]] ''Corynotrypa'' in the background
File:FossilMtnUT.jpg|Middle Ordovician fossiliferous shales and limestones at [[Fossil Mountain (Utah)|Fossil Mountain, west-central Utah]]
File:Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana.jpg|Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana
File:OrdOutcropTN.JPG|Outcrop of Upper Ordovician limestone and minor shale, central Tennessee
File:LibertyBorings.jpg|''[[Trypanites]]'' borings in an Ordovician [[hardground]], southeastern Indiana<ref name="WilsonPalmer2001">{{cite journal |last=Wilson |first=M. A. |author2=Palmer, T. J. |year=2001 |title=Domiciles, not predatory borings: a simpler explanation of the holes in Ordovician shells analyzed by Kaplan and Baumiller, 2000 |journal=PALAIOS |volume=16 |issue= 5|pages=524&ndash;525 |doi=10.1669/0883-1351(2001)016<0524:DNPBAS>2.0.CO;2|bibcode=2001Palai..16..524W |s2cid=130036115 }}</ref>
File:Petroxestes borings Ordovician.jpg|''[[Petroxestes]]'' borings in an Ordovician [[hardground]], southern Ohio<ref name="WilsonPalmer2006" />
File:Tootuskihind läbilõige.jpg|Outcrop of Ordovician kukersite [[oil shale]], northern [[Estonia]]
File:OilShaleFossilsEstonia.jpg|Bryozoan fossils in Ordovician [[kukersite]] oil shale, northern [[Estonia]]
File:OrdFossilsMN.JPG|[[Brachiopod]]s and [[bryozoa]]ns in an Ordovician limestone, southern Minnesota
File:PlatystrophiaOrdovician.jpg|''Vinlandostrophia ponderosa'', Maysvillian (Upper Ordovician) near Madison, Indiana (scale bar is 5.0&nbsp;mm)
File:Echinosphaerites.JPG|The Ordovician cystoid ''[[Echinosphaerites]]'' (an extinct [[echinoderm]]) from northeastern Estonia; approximately 5&nbsp;cm in diameter
File:Prasopora.JPG|''Prasopora'', a trepostome [[bryozoan]] from the Ordovician of Iowa
File:EncrustedStroph.JPG|An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan
File:Protaraea.jpg|The heliolitid coral ''Protaraea richmondensis'' encrusting a gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana
File:ZygospiraAttached.jpg|''Zygospira modesta'', atrypid brachiopods, preserved in their original positions on a trepostome bryozoan from the Cincinnatian (Upper Ordovician) of southeastern Indiana
File:DiplograptusCaneySprings.jpg|Graptolites (''Amplexograptus'') from the Ordovician near Caney Springs, Tennessee 
</gallery>

===Flora===

[[Green algae]] were common in the Late Cambrian (perhaps earlier) and in the Ordovician. Terrestrial plants probably evolved from green algae, first appearing as tiny non-[[vascular plants|vascular]] forms resembling [[Marchantiophyta|liverwort]]s, in the middle to late Ordovician.<ref name="Porado-etal-2016">{{cite journal |last1=Porada |first1=P. |last2=Lenton |first2=T. M. |last3=Pohl |first3=A. |last4=Weber |first4=B. |last5=Mander |first5=L. |last6=Donnadieu |first6=Y. |last7=Beer |first7=C. |last8=Pöschl |first8=U. |last9=Kleidon |first9=A. |title=High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician |journal=[[Nature Communications]] |date=November 2016 |volume=7 |issue=1 |pages=12113 |doi=10.1038/ncomms12113|pmid=27385026 |pmc=4941054 |bibcode=2016NatCo...712113P }}</ref> Fossil spores found in Ordovician sedimentary rock are typical of bryophytes.<ref>{{cite journal |last1=Steemans |first1=P. |last2=Herisse |first2=A. L. |last3=Melvin |first3=J. |last4=Miller |first4=M. A. |last5=Paris |first5=F. |last6=Verniers |first6=J. |last7=Wellman |first7=C. H. |title=Origin and Radiation of the Earliest Vascular Land Plants |journal=[[Science (journal)|Science]] |date=2009-04-17 |volume=324 |issue=5925 |pages=353 |doi=10.1126/science.1169659 |pmid=19372423 |bibcode=2009Sci...324..353S |hdl=1854/LU-697223 |s2cid=206518080 |url=https://biblio.ugent.be/publication/697223 |hdl-access=free |access-date=2022-06-08 |archive-date=2021-08-15 |archive-url=https://web.archive.org/web/20210815182721/https://biblio.ugent.be/publication/697223 |url-status=live }}</ref>

[[File:Ordovician Land Scene.jpg|thumb|Colonization of land would have been limited to shorelines]]
Among the first land [[fungi]] may have been [[arbuscular mycorrhiza]] fungi ([[Glomerales]]), playing a crucial role in facilitating the colonization of land by plants through [[Mycorrhiza|mycorrhizal symbiosis]], which makes mineral nutrients available to plant cells; such fossilized fungal [[hypha]]e and [[spore]]s from 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-vascular [[bryophyte]]s.<ref>{{cite journal |last=Redecker |first=D. |author2=Kodner, R.  |author3=Graham, L. E.  |year=2000 |title=Glomalean fungi from the Ordovician |journal=[[Science (journal)|Science]] |volume=289 |issue=5486 |pages=1920–1921 |doi=10.1126/science.289.5486.1920 | pmid=10988069 |bibcode = 2000Sci...289.1920R |s2cid=43553633 }}</ref>

=== Microbiota ===
Though stromatolites had declined from their peak in the Proterozoic, they continued to exist in localised settings.<ref>{{Cite journal |last1=Kershaw |first1=Stephen |last2=Chitnarin |first2=Anisong |last3=Noipow |first3=Nitipon |last4=Forel |first4=Marie-Béatrice |last5=Junrattanamanee |first5=Thitikan |last6=Charoenmit |first6=Jeerasak |date=10 June 2019 |title=Microbialites and associated facies of the Late Ordovician system in Thailand: paleoenvironments and paleogeographic implications |url=http://link.springer.com/10.1007/s10347-019-0579-y |journal=Facies |language=en |volume=65 |issue=3 |doi=10.1007/s10347-019-0579-y |issn=0172-9179 |access-date=13 September 2024 |via=Springer Link}}</ref>

==End of the period==
{{Main|Ordovician–Silurian extinction events}}

[[File:Anji Biota.jpg|thumb|The [[Anji Biota]] (Wenchang Formation, [[Zhejiang]] Province, [[China]]) preserves abundant and diverse [[Hexactinellid|glass sponges]] and graptolites as well as rare examples of other marine animals (such as the eurypterid [[Archopterus]]) living at a depth of several hundred metres. It is dated to just after the [[Late_Ordovician_mass_extinction|Hirnantian mass extinction]] at the end of the Ordovician period.<ref>{{Cite journal |last1=Wang |first1=Han |last2=Braddy |first2=Simon J. |last3=Botting |first3=Joseph |last4=Zhang |first4=Yuandong |date=2023 |title=The first documentation of an Ordovician eurypterid (Chelicerata) from China |url=https://www.cambridge.org/core/journals/journal-of-paleontology/article/first-documentation-of-an-ordovician-eurypterid-chelicerata-from-china/2F0762857D48061467555E2C91387957 |journal=Journal of Paleontology |language=en |volume=97 |issue=3 |pages=606–611 |doi=10.1017/jpa.2023.21 |issn=0022-3360}}</ref>]]

The Ordovician came to a close in a series of [[extinction event]]s that, taken together, comprise the second largest of the five major extinction events in [[History of Earth|Earth's history]] in terms of percentage of [[genus|genera]] that became extinct. The only larger one was the [[Permian–Triassic extinction event]].

The extinctions occurred approximately 447–444 million years ago and mark the boundary between the Ordovician and the following [[Silurian]] Period. At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever; [[brachiopods]] and [[bryozoans]] were greatly reduced, along with many [[trilobite]], [[conodont]] and [[Graptolithina|graptolite]] families.

The most commonly accepted theory is that these events were triggered by the onset of cold conditions in the late Katian, followed by an [[ice age]], in the Hirnantian faunal stage, that ended the long, stable [[greenhouse]] conditions typical of the Ordovician.

The ice age was possibly not long-lasting. Oxygen [[isotopes]] in fossil brachiopods show its duration may have been only 0.5 to 1.5 million years.<ref name="Stanley1999b">{{cite book |title=Earth System History |last=Stanley |first=Steven M. |year=1999 |publisher=W.H. Freeman and Company |location=New York |isbn=978-0-7167-2882-5 |pages=358, 360 }}</ref> Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian.

The [[late Ordovician glaciation]] event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm).<ref name="Young 2010">{{cite journal | last1 = Young | first1 = Seth A. | last2 = Saltzman | first2 = Matthew R. | last3 = Ausich | first3 = William I. | last4 = Desrochers | first4 = André | last5 = Kaljo | first5 = Dimitri | year = 2010 | title = Did changes in atmospheric CO2 coincide with latest Ordovician glacial–interglacial cycles? | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 296 | issue = 3–4| pages = 376–388 | doi=10.1016/j.palaeo.2010.02.033| bibcode = 2010PPP...296..376Y }}</ref><ref name="Hecht 2010">Jeff Hecht, [https://www.newscientist.com/article/dn18618-highcarbon-ice-age-mystery-solved.html High-carbon ice age mystery solved] {{Webarchive|url=https://web.archive.org/web/20150423082538/http://www.newscientist.com/article/dn18618-highcarbon-ice-age-mystery-solved.html |date=2015-04-23 }}, ''[[New Scientist]]'', 8 March 2010 (retrieved 30 June 2014)</ref> The dip may have been caused by a burst of volcanic activity that deposited new silicate rocks, which draw CO<sub>2</sub> out of the air as they erode.<ref name="Hecht 2010" /> Another possibility is that [[bryophyte]]s and lichens, which colonized land in the middle to late Ordovician, may have increased weathering enough to draw down {{CO2}} levels.<ref name="Porado-etal-2016"/> The drop in {{CO2}} selectively 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 the [[Ordovician meteor event]], may have also led to the glaciation.<ref name=":0" /> As the southern supercontinent [[Gondwana]] drifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata of [[North Africa]] and 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.<ref>Emiliani, Cesare. (1992). ''Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment'' (Cambridge University Press) p. 491</ref> Species limited to a single epicontinental sea on a given landmass were severely affected.<ref name="Stanley1999" /> Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.<ref name="Stanley1999" />

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.{{sfn|Torsvik|Cocks|2017|p=112-113}}

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 in [[Refugium (population biology)|refugia]].{{sfn|Torsvik|Cocks|2017|pp=122-123}}

An alternate extinction hypothesis suggested that a ten-second [[gamma-ray burst]] could have destroyed the [[ozone layer]] and exposed terrestrial and marine surface-dwelling life to deadly ultraviolet [[radiation]] and initiated global cooling.<ref name="Melott2006">{{cite journal |last=Melott |first=Adrian |year=2004 |title=Did a gamma-ray burst initiate the late Ordovician mass extinction? |journal=International Journal of Astrobiology |volume=3 |issue= 1|pages=55&ndash;61 |doi=10.1017/S1473550404001910 |bibcode=2004IJAsB...3...55M|arxiv = astro-ph/0309415 |display-authors=etal|hdl=1808/9204 |s2cid=13124815 }}</ref>

Recent work considering the [[sequence stratigraphy]] of 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.<ref>{{Cite journal|doi = 10.1111/pala.12188|title = The stratigraphy of mass extinction|journal = Palaeontology|volume = 58|issue = 5|pages = 903–924|year = 2015|last1 = Holland|first1 = Steven M|last2 = Patzkowsky|first2 = Mark E|s2cid = 129522636|doi-access = free}}</ref>

==References==
{{Reflist}}

== External links ==
{{Wikisource portal|Paleozoic#Ordovician}}
{{EB1911 poster|Ordovician System}}
{{Commons category|Ordovician}}
*{{cite web |url=http://www.stratigraphy.org/gssp.htm |title=Overview of Global Boundary Stratotype Sections and Points (GSSP's) |access-date=2006-04-30 |last=Ogg |first=Jim |date=June 2004 |archive-url = https://web.archive.org/web/20060423084018/http://www.stratigraphy.org/gssp.htm |archive-date = 2006-04-23}}
*{{cite web |url=http://www.anr.state.vt.us/dec/geo/chazytxt.htm |title=Chazy Reef at Isle La Motte |last=Mehrtens |first=Charlotte |access-date=2006-12-27 |archive-date=2016-03-06 |archive-url=https://web.archive.org/web/20160306233902/http://www.anr.state.vt.us/dec/geo/chazytxt.htm |url-status=live }} An Ordovician reef in Vermont.
*[http://members.wri.com/jeffb/Fossils Ordovician fossils of the famous Cincinnatian Group] {{Webarchive|url=https://web.archive.org/web/20090103233229/http://members.wri.com/jeffb/Fossils/ |date=2009-01-03 }}
*[https://ghkclass.com/ghkC.html?ordovician Ordovician (chronostratigraphy scale)] {{Webarchive|url=https://web.archive.org/web/20221006184657/https://ghkclass.com/ghkC.html?ordovician |date=2022-10-06 }}

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