Ordovician

The Ordovician (

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 did in the earlier Cambrian Period, although the end of the period was marked by the

A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions.^[13] There exist Baltoscandic, British, Siberian, North American, Australian, Chinese, Mediterranean and North-Gondwanan regional stratigraphic schemes.^[14]

ICS (global) subdivisions

• Upper Ordovician epoch (458.4 Ma – 443.8 Ma)
  • Hirnantian stage/age (445.2 Ma – 443.8 Ma)
  • Katian stage/age (453.0 Ma – 445.2 Ma)
  • Sandbian stage/age (458.4 Ma – 453.0 Ma)
• Middle Ordovician epoch (470.0 Ma – 458.4 Ma)
  • Darriwilian stage/age (467.3 Ma – 458.4 Ma)
  • Dapingian stage/age (470.0 Ma – 467.3 Ma)
• Lower Ordovician epoch (485.4 Ma – 470.0 Ma)
  • Floian stage/age (477.7 Ma – 470.0 Ma)
  • Tremadocian stage/age (485.4 Ma – 477.7 Ma)

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 (modern) Tremadocian. The Floian corresponds to the early Arenig; the Arenig continues until the early Darriwilian, subsuming the Dapingian. The Llanvirn occupies the rest of the Darriwilian, and terminates with it at the start of the Late Ordovician. The Sandbian represents the first half of the Caradoc; the Caradoc ends in the mid-Katian, and the Ashgill represents the last half of the Katian, plus the Hirnantian.

The British

"Late Ordovician"

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

"Middle Ordovician"

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

"Early Ordovician"

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

The Tremadoc corresponds to the (modern) Tremadocian. The Floian corresponds to the lower Arenig; the Arenig continues until the early Darriwilian, subsuming the Dapingian. The Llanvirn occupies the rest of the Darriwilian, and terminates with it at the base of the Late Ordovician. The Sandbian represents the first half of the Caradoc; the Caradoc ends in the mid-Katian, and the Ashgill represents the last half of the Katian, plus the Hirnantian.^[15]

Paleogeography and tectonics

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,

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

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]

Geochemistry

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,

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,^[52] 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

For most of the Late Ordovician life continued to flourish, but at and near the end of the period there were

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.

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.^[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 in filter-feeding organisms.^[55] 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.^[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 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.^[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]

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.^[88] 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.^[89] 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

  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
• 
  hardground, southeastern Indiana^[90]
• 
  hardground, southern Ohio^[89]
• 
  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

vascular forms resembling liverworts, in the middle to late Ordovician.^[91] Fossil spores found in Ordovician sedimentary rock are typical of bryophytes.^[92]

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.^[93]

End of the period

Main article:

Ordovician–Silurian extinction events

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

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.^[94]

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).^[95][96] 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.^[96] 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.^[91] The drop in CO₂ selectively affected the shallow seas where most organisms lived. 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.[97] 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.^[67]

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.^[98]

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.^[99]

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.^[100]

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External links

Wikisource has original works on the topic: Paleozoic#Ordovician

Wikisource has the text of the 1911 Encyclopædia Britannica article "Ordovician System".

Wikimedia Commons has media related to Ordovician.

• 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". An Ordovician reef in Vermont.
• Ordovician fossils of the famous Cincinnatian Group
• Ordovician (chronostratigraphy scale)