The Ordovician, named after theWelsh tribe of theOrdovices, was defined byCharles Lapworth in 1879 to resolve a dispute between followers ofAdam Sedgwick andRoderick Murchison, who were placing the samerock beds inNorth Wales in the Cambrian and Silurian systems, respectively.[10] Lapworth recognized that thefossilfauna in the disputedstrata 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 theInternational 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 theOrdovician–Silurian extinction events. Invertebrates, namelymolluscs andarthropods, 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 plants are known from this period. TheGreat Ordovician Biodiversification Event considerably increased the diversity of life.Fish, the world's first truevertebrates, continued to evolve, andthose 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 theOrdovician meteor event.[11] It has been theorized that this increase in impacts may originate froma ring system that formed around Earth at the time.[12]
In 2008, theICS erected a formal international system of subdivisions for the Ordovician Period and System.[13] Pre-existing Baltoscandic, British, Siberian, North American, Australian, Chinese, Mediterranean and North-Gondwanan regional stratigraphic schemes are also used locally.[14]
The Ordovician Period in Britain was traditionally broken into Early (Tremadocian andArenig), Middle (Llanvirn (subdivided into Abereiddian and Llandeilian) andLlandeilo) and Late (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 theHirnantian.
The Ashgill Epoch, the last epoch of the British Ordovician, is made of four ages: the Hirnantian Age, the Rawtheyan Age, theCautleyan 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'sGeologic Time Scale.
Paleogeographic map of the Earth in the early Ordovician, 480 million years ago[citation needed]Paleogeographic map of the Earth in the middle Ordovician, 470 million years ago[citation needed]Paleogeographic map of the Earth in the late Ordovician, 450 million years ago[citation needed]
During the Ordovician, the southern continents were assembled intoGondwana, which reached from north of theequator to theSouth Pole. The Panthalassic Ocean, centered in the northern hemisphere, covered over half the globe.[16] 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.[17] The small continent ofAvalonia had just rifted from Gondwana and began to move north towards Baltica and Laurentia, opening theRheic Ocean between Gondwana and Avalonia.[18][19][20] Avalonia collided with Baltica towards the end of Ordovician.[21][22]
Other geographic features of the Ordovician world included theTornquist Sea, which separated Avalonia from Baltica;[17] the Aegir Ocean, which separated Baltica from Siberia;[23] and an oceanic area between Siberia, Baltica, and Gondwana which expanded to become the Paleoasian Ocean in Carboniferous time. TheMongol-Okhotsk Ocean formed 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.[24]
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 inback-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.[25][17]
TheTaconic orogeny, a major mountain-building episode, was well under way in Cambrian times.[26] This continued into the Ordovician, when at least twovolcanic island arcs collided 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.[27]
Theash fall of theMillburg/Big Bentonite bed, at about 454 Ma, was the largest in the last 590 million years. This had adense rock equivalent volume of as much as 1,140 cubic kilometres (270 cu mi). Remarkably, this appears to have had little impact on life.[28]
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.[17] The accretion of these arcs to the eastern margin of Gondwana was responsible for the Benambran Orogeny of eastern Australia.[29][30] Subduction also took place along what is now Argentina (Famatinian Orogeny) at 450 Ma.[31] This involved significant back arc rifting.[17] The interior of Gondwana was tectonically quiet until theTriassic.[17]
Towards the end of the Ordovician, Gondwana began to drift across the South Pole; this contributed to theHirnantian glaciation and the associated extinction event.[32]
TheOrdovician 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 theL chondrite parent body.[33] It is not associated with any major extinction event.[34][35][36] 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 system for a period of about 40 million years, with frequent falling debris causing these craters.[12]
External mold of Ordovicianbivalve showing that the originalaragonite shell dissolved on the sea floor, leaving a cemented mold for biological encrustation (Waynesville Formation of Franklin County, Indiana).
The Ordovician was a time ofcalcite sea geochemistry in which low-magnesiumcalcite was the primary inorganic marine precipitate ofcalcium carbonate.[37]Carbonate hardgrounds were 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.[38][39]
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.[40]
The Early Ordovician climate was very hot,[41] with intensegreenhouse conditions andsea surface temperatures comparable to those during the Early Eocene Climatic Optimum.[42]Carbon dioxide levels were very high at the Ordovician period's beginning.[43] By the late Early Ordovician, the Earth cooled,[44] giving way to a more temperate climate in the Middle Ordovician,[45] with the Earth likely entering theEarly Palaeozoic Ice Age during the Sandbian,[46][47] and possibly as early as the Darriwilian[48] or even the Floian.[44] The Dapingian and Sandbian saw major humidification events evidenced by trace metal concentrations in Baltoscandia from this time.[49] Evidence suggests that global temperatures rose briefly in the early Katian (Boda Event), depositing bioherms and radiating fauna across Europe.[50] The early Katian also witnessed yet another humidification event.[49] Further cooling during the Hirnantian, at the end of the Ordovician, led to theLate Ordovician glaciation.[51]
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.[40] The sea level rose more or less continuously throughout the Early Ordovician, leveling off somewhat during the middle of the period.[40] 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.[40] In particular, some researches interpret the fluctuations in sea level as pre-Hibernian glaciation,[52] but sedimentary evidence of glaciation is lacking until the end of the period.[22] There is evidence ofglaciers during the Hirnantian on theland we now know as Africa and South America, which were near theSouth Pole at the time, facilitating the formation of theice caps of the Hirnantian glaciation.
For most of the Late Ordovician life continued to flourish, but at and near the end of the period there weremass-extinction events that seriously affectedconodonts andplanktonic forms likegraptolites. ThetrilobitesAgnostida andPtychopariida completely died out, and theAsaphida were much reduced.Brachiopods,bryozoans andechinoderms were also heavily affected, and theendoceridcephalopods 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 thefirst terrestrial plants,[53] as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history.
Endoceras, one of the largest predators of the OrdovicianFossiliferous limestone slab from the Liberty Formation (Upper Ordovician) of Caesar Creek State Park near Waynesville, Ohio.The trilobiteIsotelus fromWisconsin
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.[40] Though less famous than theCambrian explosion, theOrdovician radiation (also known as the Great Ordovician Biodiversification Event)[17] was no less remarkable; marine faunalgenera increased fourfold, resulting in 12% of all knownPhanerozoic marine fauna.[54] 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-feeding organisms.[55] The trilobite, inarticulate brachiopod,archaeocyathid, andeocrinoid faunas 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 inshelf communities. Their success epitomizes the greatly increased diversity ofcarbonate shell-secreting organisms in the Ordovician compared to the Cambrian.[56]
Ordovician geography had its effect on the diversity of fauna; Ordovician invertebrates displayed a very high degree of provincialism.[57] 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,[58] and Gondwana developed distinct fauna in its tropical and temperature zones.[59] The Tien Shan terrane maintained a biogeographic affinity with Gondwana,[60] and the Alborz margin of Gondwana was linked biogeographically to South China.[61] Southeast Asia's fauna also maintained strong affinities to Gondwana's.[62] North China was biogeographically connected to Laurentia and the Argentinian margin of Gondwana.[63] A Celtic biogeographic province also existed, separate from the Laurentian and Baltican ones.[64] 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.[65] Faunas become less provincial later in the Ordovician, partly due to the narrowing of the Iapetus Ocean,[66] though they were still distinguishable into the late Ordovician.[67]
Trilobites in particular were rich and diverse, and experienced rapid diversification in many regions.[68] 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 primitiveeurypterids and nautiloids while other trilobites such asAeglina 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.[69] Some trilobites such asAsaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.[70] Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician.[71] Although solitarycorals date back to at least theCambrian,reef-forming corals appeared in the early Ordovician, including the earliest knownoctocorals,[72][73] corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals.[40] Brachiopods surged in diversity, adapting to almost every type of marine environment.[74][75][76] Even after GOBE, there is evidence suggesting that Ordovician brachiopods maintained elevated rates of speciation.[77]Molluscs, which appeared during the Cambrian or even theEdiacaran, became common and varied, especiallybivalves,gastropods, andnautiloid cephalopods.[78][79] Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments.[80] Graptolites, which evolved in the preceding Cambrian period, thrived in the oceans.[81] This includes the distinctiveNemagraptus gracilis graptolite fauna, which was distributed widely during peak sea levels in the Sandbian.[82][22] Some new cystoids and crinoids appeared. It was long thought that the first truevertebrates (fish —Ostracoderms) appeared in the Ordovician, but recent discoveries inChina reveal that they probably originated in the EarlyCambrian.[83] The firstgnathostome (jawed fish) may have appeared in theLate Ordovician epoch.[84] Chitinozoans, which first appeared late in the Wuliuan, exploded in diversity during the Tremadocian, quickly becoming globally widespread.[85][86] Several groups of endobiotic symbionts appeared in the Ordovician.[87][88]
In the Early Ordovician, trilobites were joined by many new types of organisms, includingtabulate corals,strophomenid,rhynchonellid, and many neworthid brachiopods, 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,tentaculitids and echinoderms all flourished, tabulate corals diversified and the firstrugose corals appeared. 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.[89] 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.[90] It is marked by a sudden abundance of hard substrate trace fossils such asTrypanites,Palaeosabella,Petroxestes andOsprioneides.Bioerosion became an important process, particularly in the thick calcitic skeletons of corals, bryozoans and brachiopods, and on the extensivecarbonate hardgrounds that appear in abundance at this time.
Upper OrdovicianedrioasteroidCystaster stellatus on a cobble from the Kope Formation in northern Kentucky with the cyclostomebryozoanCorynotrypa in the background
Vinlandostrophia ponderosa, Maysvillian (Upper Ordovician) near Madison, Indiana (scale bar is 5.0 mm)
The Ordovician cystoidEchinosphaerites (an extinctechinoderm) from northeastern Estonia; approximately 5 cm in diameter
Prasopora, a trepostomebryozoan from the Ordovician of Iowa
An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan
The heliolitid coralProtaraea richmondensis encrusting 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
Graptolites (Amplexograptus) from the Ordovician near Caney Springs, Tennessee
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 forms resemblingliverworts, in the middle to late Ordovician.[92] Fossil spores found in Ordovician sedimentary rock are typical of bryophytes.[93]
Colonization of land would have been limited to shorelines
Among the first landfungi may have beenarbuscular mycorrhiza fungi (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 fungalhyphae andspores 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-vascularbryophytes.[94]
TheAnji Biota (Wenchang Formation,Zhejiang Province,China) preserves abundant and diverseglass sponges and graptolites as well as rare examples of other marine animals (such as the eurypteridArchopterus) living at a depth of several hundred metres. It is dated to just after theHirnantian mass extinction at the end of the Ordovician period.[96]
The extinctions occurred approximately 447–444 million years ago and mark the boundary between the Ordovician and the followingSilurian Period. At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever;brachiopods andbryozoans were greatly reduced, along with manytrilobite,conodont andgraptolite families.
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, stablegreenhouse conditions typical of the Ordovician.
The ice age was possibly not long-lasting. Oxygenisotopes in fossil brachiopods show its duration may have been only 0.5 to 1.5 million years.[97] Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian.
Thelate Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm).[98][99] The dip may have been caused by a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode.[99] Another possibility is thatbryophytes and lichens, which colonized land in the middle to late Ordovician, may have increased weathering enough to draw down CO2 levels.[92] 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 theOrdovician meteor event, may have also led to the glaciation.[12] As the southern supercontinentGondwana drifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata ofNorth 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.[100] Species limited to a single epicontinental sea on a given landmass were severely affected.[39] Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.[39]
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.[101]
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 burst could have destroyed theozone layer and exposed terrestrial and marine surface-dwelling life to deadly ultravioletradiation and initiated global cooling.[103]
Recent work considering thesequence 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.[104]
^Charles Lapworth (1879)"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-calledUpper Cambrian orLower 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."
^Details on the Dapingian are available atWang, X.; Stouge, S.; Chen, X.; Li, Z.; Wang, C. (2009). "Dapingian Stage: standard name for the lowermost global stage of the Middle Ordovician Series".Lethaia.42 (3):377–380.Bibcode:2009Letha..42..377W.doi:10.1111/j.1502-3931.2009.00169.x.
^"The Ordovician Period".Subcommission on Ordovician Stratigraphy. International Commission on Stratigraphy. 2020.Archived from the original on 11 May 2022. Retrieved7 June 2021.
^Torsvik, Trond H.; Cocks, L. Robin M. (2017).Earth history and palaeogeography. Cambridge, United Kingdom: Cambridge University Press. p. 102.ISBN978-1-107-10532-4.
^Ramos, Victor A. (2018). "The Famatinian Orogen Along the Protomargin of Western Gondwana: Evidence for a Nearly Continuous Ordovician Magmatic Arc Between Venezuela and Argentina".The Evolution of the Chilean-Argentinean Andes. Springer Earth System Sciences. pp. 133–161.doi:10.1007/978-3-319-67774-3_6.ISBN978-3-319-67773-6.
^Haack, Henning; Farinella, Paolo; Scott, Edward R. D.; Keil, Klaus (1996). "Meteoritic, Asteroidal, and Theoretical Constraints on the 500 MA Disruption of the L Chondrite Parent Body".Icarus.119 (1):182–91.Bibcode:1996Icar..119..182H.doi:10.1006/icar.1996.0010.
Mehrtens, Charlotte."Chazy Reef at Isle La Motte".Archived from the original on 6 March 2016. Retrieved27 December 2006. An Ordovician reef in Vermont.
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![Trypanites borings in an Ordovician hardground, southeastern Indiana[91]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aLibertyBorings.jpg)
![Petroxestes borings in an Ordovician hardground, southern Ohio[90]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPetroxestes_borings_Ordovician.jpg)
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