Ordovician

The Ordovician (/ɔːrdəˈvɪʃi.ən, -doʊ-, -ˈvɪʃən/ or-də-VISH-ee-ən, -⁠doh-, -⁠VISH-ən)^[9] is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 Ma (million years ago) to the start of the Silurian Period 443.8 Ma.^[10]

Ordovician
485.4 ± 1.9 – 443.8 ± 1.5 Ma PreꞒ Ꞓ O S D C P T J K Pg N
A map of Earth as it appeared 465 million years ago during the Middle Ordovician Epoch
Chronology
−485 — – −480 — – −475 — – −470 — – −465 — – −460 — – −455 — – −450 — – −445 — – P a l e o z o i c Cambrian O r d o v i c i a n Silurian E a r l y M i d d l e L a t e Tremadocian Floian Dapingian Darriwilian Sandbian Katian Hirnantian       ← First land plant spores[1] ← Ordovician meteor event[2][3] Subdivision of the Ordovician according to the ICS, as of 2023.[4] Vertical axis scale: Millions of years ago
Etymology
Name formality	Formal
Name ratified	1960
Usage information
Celestial body	Earth
Regional usage	Global (ICS)
Time scale(s) used	ICS Time Scale
Definition
Chronological unit	Period
Stratigraphic unit	System
First proposed by	Charles Lapworth, 1879
Time span formality	Formal
Lower boundary definition	FAD of the Conodont Iapetognathus fluctivagus
Lower boundary GSSP	Greenpoint section, Green Point, Newfoundland, Canada 49.6829°N 57.9653°W / 49.6829; -57.9653
Lower GSSP ratified	2000[5]
Upper boundary definition	FAD of the Graptolite Akidograptus ascensus
Upper boundary GSSP	Dob's Linn, Moffat, U.K. 55.4400°N 3.2700°W / 55.4400; -3.2700
Upper GSSP ratified	1984[6][7]
Atmospheric and climatic data
Sea level above present day	180 m; rising to 220 m in Caradoc and falling sharply to 140 m in end-Ordovician glaciations[8]

The Ordovician, named after the 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 beds in North Wales in the Cambrian and Silurian systems, respectively.^[11] Lapworth recognized that the fossil fauna in the disputed 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 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 molluscs and arthropods, 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 vertebrates, continued to evolve, and 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.^[12] It has been theorized that this increase in impacts may originate from a ring system that formed around Earth at the time.^[13]

Subdivisions

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

Global/regional correlation

Approximate correlation of Ordovician regional series and stages^[16]

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

British stages and ages

The Ordovician Period in Britain was traditionally broken into Early (Tremadocian and Arenig), Middle (Llanvirn (subdivided into Abereiddian and Llandeilian) and Llandeilo) 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 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

Paleogeographic map of the Earth in the early Ordovician, 480 million years ago

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

Paleogeographic map of the Earth in the late Ordovician, 450 million years ago

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.^[17] At the start of the period, the continents of Laurentia (in present-day North America), Siberia, and Baltica (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.^[18] 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.^[19][20][21] Avalonia collided with Baltica towards the end of Ordovician.^[22][23]

Other geographic features of the Ordovician world included the Tornquist Sea, which separated Avalonia from Baltica;^[18] the Aegir Ocean, which separated Baltica from Siberia;^[24] 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 terranes. Most of the terranes of central Asia were part of an equatorial archipelago whose geometry is poorly constrained by the available evidence.^[25]

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.^[26][18]

The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times.^[27] This continued into the Ordovician, when at least two 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.^[28]

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 1,140 cubic kilometres (270 cu mi). Remarkably, this appears to have had little impact on life.^[29]

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.^[18] The accretion of these arcs to the eastern margin of Gondwana was responsible for the Benambran Orogeny of eastern Australia.^[30][31] Subduction also took place along what is now Argentina (Famatinian Orogeny) at 450 Ma.^[32] This involved significant back arc rifting.^[18] The interior of Gondwana was tectonically quiet until the Triassic.^[18]

Towards the end of the period, Gondwana began to drift across the South Pole. This contributed to the Hibernian glaciation and the associated extinction event.^[33]

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.^[34] It is not associated with any major extinction event.^[35][36][37] 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.^[13]

Geochemistry

External mold of Ordovician 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.^[38] Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons. Biogenic aragonite, like that composing the shells of most molluscs, dissolved rapidly on the sea floor after death.^[39][40]

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.^[41]

Climate and sea level

The Early Ordovician climate was very hot,^[42] with intense greenhouse conditions and sea surface temperatures comparable to those during the Early Eocene Climatic Optimum.^[43] Carbon dioxide levels were very high at the Ordovician period's beginning.^[44] By the late Early Ordovician, the Earth cooled,^[45] giving way to a more temperate climate in the Middle Ordovician,^[46] with the Earth likely entering the Early Palaeozoic Ice Age during the Sandbian,^[47][48] and possibly as early as the Darriwilian^[49] or even the Floian.^[45] The Dapingian and Sandbian saw major humidification events evidenced by trace metal concentrations in Baltoscandia from this time.^[50] Evidence suggests that global temperatures rose briefly in the early Katian (Boda Event), depositing bioherms and radiating fauna across Europe.^[51] The early Katian also witnessed yet another humidification event.^[50] Further cooling during the Hirnantian, at the end of the Ordovician, led to the Late Ordovician glaciation.^[52]

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.^[41] The sea level rose more or less continuously throughout the Early Ordovician, leveling off somewhat during the middle of the period.^[41] Locally, some regressions occurred, but the sea level rise continued in the beginning of the Late Ordovician. Sea levels fell steadily due to the cooling temperatures for about 3 million years leading up to the Hirnantian glaciation. During this icy stage, sea level seems to have risen and dropped somewhat. Despite much study, the details remain unresolved.^[41] In particular, some researches interpret the fluctuations in sea level as pre-Hibernian glaciation,^[53] but sedimentary evidence of glaciation is lacking until the end of the period.^[23] There is evidence of glaciers during the Hirnantian on the land we now know as Africa and South America, which were near the South Pole at the time, facilitating the formation of the ice caps 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, Paleo-Tethys, Khanty Ocean, which was closed off by the Late Ordovician, Iapetus Ocean, and the new Rheic Ocean.

Life

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 mass-extinction events that seriously affected conodonts and planktonic forms like graptolites. The trilobites Agnostida and Ptychopariida completely died out, and the Asaphida were much reduced. Brachiopods, bryozoans and echinoderms were also heavily affected, and the endocerid cephalopods 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 first terrestrial plants,^[54] as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history.

Fauna

Endoceras, one of the largest predators of the Ordovician

Fossiliferous limestone slab from the Liberty Formation (Upper Ordovician) of Caesar Creek State Park near Waynesville, Ohio.

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.^[41] Though less famous than the Cambrian explosion, the Ordovician radiation (also known as the Great Ordovician Biodiversification Event)^[18] was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna.^[55] 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 in filter-feeding organisms.^[56] The trilobite, inarticulate brachiopod, archaeocyathid, and eocrinoid faunas of the Cambrian were succeeded by those that dominated the rest of the Paleozoic, such as articulate brachiopods, cephalopods, and crinoids. Articulate brachiopods, in particular, largely replaced trilobites in shelf communities. Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian.^[57]

Aegirocassis, a large filter-feeding hurdiid radiodont from Morocco

Ordovician geography had its effect on the diversity of fauna; Ordovician invertebrates displayed a very high degree of provincialism.^[58] 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,^[59] and Gondwana developed distinct fauna in its tropical and temperature zones.^[60] The Tien Shan terrane maintained a biogeographic affinity with Gondwana,^[61] and the Alborz margin of Gondwana was linked biogeographically to South China.^[62] Southeast Asia's fauna also maintained strong affinities to Gondwana's.^[63] North China was biogeographically connected to Laurentia and the Argentinian margin of Gondwana.^[64] A Celtic biogeographic province also existed, separate from the Laurentian and Baltican ones.^[65] 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.^[66] Faunas become less provincial later in the Ordovician, partly due to the narrowing of the Iapetus Ocean,^[67] though they were still distinguishable into the late Ordovician.^[68]

Pentecopterus, the earliest known eurypterid, and found in Iowa

Trilobites in particular were rich and diverse, and experienced rapid diversification in many regions.^[69] 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 eurypterids 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.^[70] Some trilobites such as Asaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.^[71] Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician.^[72] Although solitary corals date back to at least the Cambrian, reef-forming corals appeared in the early Ordovician, including the earliest known octocorals,^[73][74] corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals.^[41] Brachiopods surged in diversity, adapting to almost every type of marine environment.^[75][76][77] Even after GOBE, there is evidence suggesting that Ordovician brachiopods maintained elevated rates of speciation.^[78] Molluscs, which appeared during the Cambrian or even the Ediacaran, became common and varied, especially bivalves, gastropods, and nautiloid cephalopods.^[79][80] Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments.^[81] Graptolites, which evolved in the preceding Cambrian period, thrived in the oceans.^[82] This includes the distinctive Nemagraptus gracilis graptolite fauna, which was distributed widely during peak sea levels in the Sandbian.^[83][23] Some new cystoids and crinoids appeared. It was long thought that the first true vertebrates (fish — Ostracoderms) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian.^[84] The first gnathostome (jawed fish) may have appeared in the Late Ordovician epoch.^[85] Chitinozoans, which first appeared late in the Wuliuan, exploded in diversity during the Tremadocian, quickly becoming globally widespread.^[86][87] Several groups of endobiotic symbionts appeared in the Ordovician.^[88][89]

In the Early Ordovician, trilobites were joined by many new types of organisms, including tabulate corals, strophomenid, rhynchonellid, and many new orthid brachiopods, bryozoans, planktonic graptolites and conodonts, and many types of molluscs and echinoderms, including the ophiuroids ("brittle stars") and the first 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, cornulitids, tentaculitids and echinoderms all flourished, tabulate corals diversified and the first rugose corals appeared. The planktonic graptolites remained diverse, with the Diplograptina making their appearance. One of the earliest known armoured agnathan ("ostracoderm") vertebrates, Arandaspis, dates from the Middle Ordovician.^[90] 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.^[91] 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.

• 
  Upper Ordovician edrioasteroid Cystaster stellatus on a cobble from the Kope Formation in northern Kentucky with the cyclostome bryozoan Corynotrypa in the background
• 
  Middle Ordovician fossiliferous shales and limestones at Fossil Mountain, west-central Utah
• 
  Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana
• 
  Outcrop of Upper Ordovician limestone and minor shale, central Tennessee
• 
  Trypanites borings in an Ordovician hardground, southeastern Indiana^[92]
• 
  Petroxestes borings in an Ordovician hardground, southern Ohio^[91]
• 
  Outcrop of Ordovician kukersite oil shale, northern Estonia
• 
  Bryozoan fossils in Ordovician kukersite oil shale, northern Estonia
• 
  Brachiopods and bryozoans in an Ordovician limestone, southern Minnesota
• 
  Vinlandostrophia ponderosa, Maysvillian (Upper Ordovician) near Madison, Indiana (scale bar is 5.0 mm)
• 
  The Ordovician cystoid Echinosphaerites (an extinct echinoderm) from northeastern Estonia; approximately 5 cm in diameter
• 
  Prasopora, a trepostome bryozoan from the Ordovician of Iowa
• 
  An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan
• 
  The heliolitid coral Protaraea richmondensis encrusting a gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana
• 
  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

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 forms resembling liverworts, in the middle to late Ordovician.^[93] Fossil spores found in Ordovician sedimentary rock are typical of bryophytes.^[94]

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 mycorrhizal symbiosis, which makes mineral nutrients available to plant cells; such fossilized fungal hyphae and spores 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 bryophytes.^[95]

Microbiota

Though stromatolites had declined from their peak in the Proterozoic, they continued to exist in localised settings.^[96]

End of the period

Main article: Ordovician–Silurian extinction events

The Anji Biota (Wenchang Formation, Zhejiang Province, China) preserves abundant and diverse 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 Hirnantian mass extinction at the end of the Ordovician period.^[97]

The Ordovician came to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of 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 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.^[98] 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).^[99][100] 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.^[100] Another possibility is that bryophytes and lichens, which colonized land in the middle to late Ordovician, may have increased weathering enough to draw down CO₂ levels.^[93] 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.^[13] 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.^[101] Species limited to a single epicontinental sea on a given landmass were severely affected.^[40] Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.^[40]

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.^[68]

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 refugia.^[102]

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.^[103]

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.^[104]

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External links

• Ogg, Jim (June 2004). "Overview of Global Boundary Stratotype Sections and Points (GSSP's)". Archived from the original on 2006-04-23. Retrieved 2006-04-30.
• Mehrtens, Charlotte. "Chazy Reef at Isle La Motte". Archived from the original on 2016-03-06. Retrieved 2006-12-27. An Ordovician reef in Vermont.
• Ordovician fossils of the famous Cincinnatian Group Archived 2009-01-03 at the Wayback Machine
• Ordovician (chronostratigraphy scale) Archived 2022-10-06 at the Wayback Machine