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
fossilfauna 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
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.[12]
Subdivisions
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]
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
ages (subdivisions of epochs) from youngest to oldest are:
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]
Approximate correlation of Ordovician regional series and stages[16]
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]
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,[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] Evidence suggests that global temperatures rose briefly in the early Katian (Boda Event), depositing bioherms and radiating fauna across Europe.[50] Further cooling during the Hirnantian, at the end of the Ordovician, led to the
Late 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.[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.
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
trilobitesAgnostida and
Ptychopariida completely died out, and the
Asaphida were much reduced.
Brachiopods,
bryozoans and
echinoderms were also heavily affected, and the
endoceridcephalopods 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,[53] as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history.
Fauna
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.[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]
Trilobites in particular were rich and diverse. 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.[68] Some trilobites such as Asaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.[69] Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician.[70] Although solitary
corals date back to at least the
Cambrian,
reef-forming corals appeared in the early Ordovician, including the earliest known
octocorals,[71][72] 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.[73][74][75] Even after GOBE, there is evidence suggesting that Ordovician brachiopods maintained elevated rates of speciation.[76]Molluscs, which appeared during the Cambrian or even the
Ediacaran, became common and varied, especially
bivalves,
gastropods, and
nautiloid cephalopods.[77][78] Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments.[79] Graptolites, which evolved in the preceding Cambrian period, thrived in the oceans.[80] This includes the distinctive Nemagraptus gracilis graptolite fauna, which was distributed widely during peak sea levels in the Sandbian.[81][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.[82] The first
gnathostome (jawed fish) may have appeared in the
Late Ordovician epoch.[83] Chitinozoans, which first appeared late in the Wuliuan, exploded in diversity during the Tremadocian, quickly becoming globally widespread.[84][85] Several groups of endobiotic symbionts appeared in the Ordovician.[86][87]
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.[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
edrioasteroidCystaster stellatus on a cobble from the Kope Formation in northern Kentucky with the cyclostome
bryozoanCorynotrypa 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
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.[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]
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.[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]
^Charles Lapworth (1879)
"On the Tripartite Classification of the Lower Palaeozoic Rocks"[permanent dead link], 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."
^Details on the Dapingian are available at Wang, 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.
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10.1111/j.1502-3931.2009.00169.x.
^"The Ordovician Period". Subcommission on Ordovician Stratigraphy. International Commission on Stratigraphy. 2020. Retrieved 7 June 2021.
^Ogg; Ogg; Gradstein, eds. (2008). The Concise Geological Timescale.
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"A brief history of the Rheic Ocean". Geoscience Frontiers. 3 (2): 125–135.
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^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: 133–161.
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^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.
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