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The Ordovician ( ) is a
geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth sciences, Earth scientists to describe t ...
and
system A system is a group of interacting Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way causal e ...
, the second of six periods of the
Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its popula ...
Era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth. Compar ...
. The Ordovician spans 41.6 million years from the end of the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These perio ...
Period million years ago (Mya) to the start of the
Silurian The Silurian ( ) is a spanning 24.6 million years from the end of the Period, at million years ago (), to the beginning of the Period, Mya. The Silurian is the shortest period of the . As with other periods, the beds that define the per ...
Period Mya. The Ordovician, named after the
Welsh Welsh may refer to: Related to Wales * Welsh, referring or related to Wales * Welsh language, a Brittonic Celtic language of the Indo-European language family, indigenous to the British Isles, spoken in Wales ** Patagonian Welsh, a dialect of Wels ...
tribe of the
Ordovices The Ordovices were one of the Celtic tribes living in Great Britain before the Roman invasion. Their tribal lands were located in present-day North Wales Wales ( cy, Cymru ) is a country that is Countries of the United Kingdom, part of the U ...
, was defined by
Charles Lapworth Charles Lapworth FRS FGS (20 September 1842 – 13 March 1920) was an English geologist who pioneered faunal analysis using index fossils and identified the Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the ...

Charles Lapworth
in 1879 to resolve a dispute between followers of
Adam Sedgwick Adam Sedgwick (; 22 March 1785 – 27 January 1873) was a British geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, as well as the processes that ...

Adam Sedgwick
and
Roderick Murchison Sir Roderick Impey Murchison, 1st Baronet, (22 February 1792 – 22 October 1871) was a British geologist who first described and investigated the Silurian system. Early life and work Murchison was born at Tarradale Castle, Tarradale House, M ...

Roderick Murchison
, who were placing the same
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
beds in
North Wales North Wales ( cy, Gogledd Cymru), also known as the North of Wales (or simply the North, or in Welsh language, Welsh '' in Wales), is a regions of Wales, geographic region of Wales, encompassing its northernmost areas. It borders Mid Wales (or Sout ...

North Wales
in the Cambrian and Silurian systems, respectively. Lapworth recognized that the
fossil A fossil (from Classical Latin Classical Latin is the form of Latin, Latin language recognized as a Literary language, literary standard language, standard by writers of the late Roman Republic and early Roman Empire. It was used from 75 ...

fossil
fauna Fauna is all of the animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular r ...

fauna
in the disputed
strata (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half of South America. Sharing the bulk of the Southern Cone with Chile to the west, the country is a ...
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 The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology. About The IUGS was founded in 1961 and is a Scientific Union member of the Inter ...
. 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 The Ordovician–Silurian extinction events, also known as the Late Ordovician mass extinction, are collectively the second-largest of the five major extinction events in Earth's history in terms of percentage of Genus, genera that became extinct. ...
. Invertebrates, namely
molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil species is es ...

molluscs
and
arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart fr ...
s, dominated the oceans, with members of the latter group probably starting their establishment on land during this time, becoming fully established by the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
. The first
land plants The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophyta is a clade within the Phragmoplastophyta, a larger clade that also includes several groups of green algae including the ...
are known from this period. The
Great Ordovician Biodiversification Event The Ordovician radiation, or the Great Ordovician Biodiversification Event (GOBE), was an evolutionary radiation of animal life throughout the Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six perio ...
considerably increased the diversity of life.
Fish Fish are , , -bearing animals that lack with . Included in this definition are the living , s, and and as well as various extinct related groups. Around 99% of living fish species are ray-finned fish, belonging to the class , with over 95 ...

Fish
, the world's first true
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s, continued to evolve, and
those with jaws
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.


Subdivisions

A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the
ICS ICS may refer to: Computing * Image Cytometry Standard, a digital multidimensional image file format used in life sciences microscopy * Industrial control system, computer systems and networks used to control industrial plants and infrastructure ...
erected a formal international system of subdivisions. There exist Baltoscandic, British, Siberian, North American, Australian, Chinese Mediterranean and North-
Gondwana Gondwana () or Gondwanaland was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (ge ...

Gondwana
n regional stratigraphic schemes. The Ordovician Period in Britain was traditionally broken into Early (Tremadocian and
Arenig In geology, the Arenigian (or 'Arenig') refers both to a time interval during the Lower Ordovician period and also to the suite of rocks which were deposited during this interval. History The term was first used by Adam Sedgwick in 1847 with ref ...
), Middle (
Llanvirn The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start o ...
(subdivided into Abereiddian and Llandeilian) and
Llandeilo Llandeilo () is a town and community in Carmarthenshire, Wales Wales ( cy, Cymru ) is a country that is Countries of the United Kingdom, part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish ...
) and Late (
Caradoc Caradoc Vreichvras (; Modern cy, Caradog Freichfras, ) was a semi-legendary ancestor to the Kingdom of Gwent, kings of Gwent. He may have lived during the 5th or 6th century. He is remembered in the Matter of Britain as a Knight of the Round Tab ...
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 faunal
stages Stage or stages may refer to: Acting * Stage (theatre) In theatre and performing arts, the stage (sometimes referred to as the deck in stagecraft) is a designated space for the performance of theatrical production, productions. The stage s ...
(subdivisions of epochs) from youngest to oldest are: Late Ordovician * Hirnantian stage/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)


British stages

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.


Paleogeography and tectonics

During the Ordovician, the southern continents were assembled into
Gondwana Gondwana () or Gondwanaland was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (ge ...

Gondwana
, which reached from north of the
equator The Equator is a , about in circumference, that divides into the and hemispheres. It is an located at 0 degrees , halfway between the and poles. In , as applied in , the equator of a rotating (such as a ) is the parallel (circle of l ...

equator
to the
South Pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the where intersects its surface. It is the southernmost point on Earth and lies on the of Earth from the . Situated on the ...
. The Panthalassic Ocean, centered in the northern hemisphere, covered over half the globe. At the start of the period, the continents of
Laurentia Image:North america craton nps.gif, upright=1.4, Laurentia, also called the North American craton Laurentia or the North American Craton is a large continental craton that forms the Geology of North America, ancient geological core of North Ameri ...
(in present-day
North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continen ...

North America
),
Siberia Siberia (; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region spanning much of Northern Asia. Siberia has been Russian conquest of Siberia, part of modern Russia since the latter half of th ...
, and
Baltica Baltica is a paleocontinent A paleocontinent or palaeocontinent is a distinct area of continental crust 350px, The thickness of Crust (geology)#Earth's crust, Earth's crust (km) Continental crust is the layer of Igneous rock, igneous, Sedim ...
(present-day northern Europe) were separated from Gondwana by over of ocean. These smaller continents were also sufficiently widely separated from each other to develop distinct communities of benthic organisms. The small continent of
Avalonia Avalonia was a microcontinent in the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek ''palaiós'' (), "old" and ''zōḗ'' (), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon. It ...

Avalonia
had just rifted from Gondwana and began to move north towards Baltica and Laurentia, opening the
Rheic Ocean The Rheic Ocean was an ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.world map, originally prepared by Joan Blaeu for his ''Atlas Maior'', published ...
between Gondwana and Avalonia. Avalonia collided with Baltica towards the end of Ordovician. Other geographic features of the Ordovician world included the Tornquist Sea, which separated Avalonia from Baltica; the Aegir Ocean, which separated Baltica from Siberia; and an oceanic area between Siberia, Baltica, and Gondwana which expanded to become the Paleoasian Ocean in Carboniferous time. The Mongol-Okhotsk Ocean formed a deep embayment between Siberia and the Central Mongolian
terrane In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes ...
s. Most of the terranes of central Asia were part of an equatorial archipelago whose geometry is poorly constrained by the available evidence. The period was one of extensive, widespread tectonism and volcanism. However,
orogenesis Orogeny is the primary mechanism by which mountains are formed on continents. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. This leads to both structural deformation Deformation ...
(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. The
Taconic orogeny 300px, Illustration of the Taconic orogeny The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern-day New England New England is a region comprising six states in the Northeastern Unit ...

Taconic orogeny
, a major mountain-building episode, was well under way in Cambrian times. This continued into the Ordovician, when at least two volcanic island arcs collided with Laurentia to form the
Appalachian Mountains The Appalachian Mountains, often called the Appalachians, are a system of mountains in eastern North America North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can als ...

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. The
ash fall , Chile Chile (, ; ), officially the Republic of Chile (), is a country in western South America. It occupies a long, narrow strip of land between the Andes to the east and the Pacific Ocean to the west. Chile covers an area of and has a po ...
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 . Remarkably, this appears to have had little impact on life. 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. The accretion of these arcs to the eastern margin of Gondwana was responsible for the Benambran Orogeny of eastern Australia. Subduction also took place along what is now Argentina (Famatinian Orogeny) at 450 Ma. This involved significant back arc rifting. The interior of Gondwana was tectonically quiet until the
Triassic The Triassic ( ) is a geologic period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions int ...

Triassic
. 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.


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 L, or l, is the twelfth Letter (alphabet), letter of the modern English alphabet and the ISO basic Latin alphabet. Its name in English is English alphabet#Letter names, ''el'' (pronounced ), plural ''els''. History Lamedh may have come from a ...
parent body. It is not associated with any major extinction event.


Geochemistry

The Ordovician was a time of calcite sea geochemistry in which low-magnesium
calcite Calcite is a and the most stable of (CaCO3). The , based on , defines value 3 as "calcite". Other polymorphs of calcium carbonate are the minerals and . Aragonite will change to calcite over timescales of days or less at temperatures exceed ...

calcite
was the primary inorganic marine precipitate of
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

calcium carbonate
. Carbonate hardgrounds were thus very common, along with calcitic
ooid , The Bahamas. (Middle Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous ...
s, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons. Biogenic
aragonite Aragonite is a , one of the three most common naturally occurring of , (the other forms being the s and ). It is formed by biological and physical processes, including precipitation from marine and freshwater environments. The of aragonite d ...

aragonite
, like that composing the shells of most
molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil species is es ...

molluscs
, dissolved rapidly on the sea floor after death. 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.


Climate and sea level

The early Ordovician climate was very hot, with intense
greenhouse A greenhouse (also called a glasshouse, or, if with sufficient heating, a hothouse) is a structure with walls and roof made chiefly of transparent material, such as glass, in which plant Plants are predominantly photosynthetic Photo ...
conditions giving way to a more temperate climate in the Middle Ordovician. Further cooling led to the
Late Ordovician glaciationThe Late Ordovician Glaciation is the first part of the Andean-Saharan glaciation. It was centered on the Sahara region in late Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleoz ...
. 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. The sea level rose more or less continuously throughout the Early Ordovician, leveling off somewhat during the middle of the period. 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 30 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. In particular, some researches interpret the fluctuations in sea level as pre-Hibernian glaciation, but sedimentary evidence of glaciation is lacking until the end of the period. There is also evidence that global temperatures rose briefly in the early Katian (Boda Event), depositing bioherms and radiating fauna across Europe. As with
North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continen ...

North America
and
Europe Europe is a which is also recognised as part of , located entirely in the and mostly in the . It comprises the westernmost peninsulas of the of Eurasia, it shares the continental landmass of with both and , and is bordered by the to the ...

Europe
,
Gondwana Gondwana () or Gondwanaland was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (ge ...

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 The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remain ...

Northern Hemisphere
, and other minor oceans included Proto-Tethys,
Paleo-Tethys The Paleo-Tethys or Palaeo-Tethys Ocean was an ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
,
Khanty Ocean Khanty or Khanty-Mansi Ocean was a small ocean located between Baltica and an island arc (the Kipchak Arc) near Siberia Siberia (; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region spa ...
, which was closed off by the Late Ordovician,
Iapetus Ocean upright=1.35, Reconstruction of how the Iapetus Ocean and surrounding continents might have been arranged during the late Ediacaran period (geology), period The Iapetus Ocean (pronounced ) was an ocean that existed in the late Neoproterozoic and e ...
, and the new
Rheic Ocean The Rheic Ocean was an ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.world map, originally prepared by Joan Blaeu for his ''Atlas Maior'', published ...
. As the Ordovician progressed, there is evidence of
glacier A glacier (; ) is a persistent body of dense that is constantly moving under its own weight. A glacier forms where the accumulation of exceeds its over many years, often . Glaciers slowly deform and flow under stresses induced by their wei ...

glacier
s on the as Africa and South America, which were near the
South Pole The South Pole, also known as the Geographic South Pole, Terrestrial South Pole or 90th Parallel South, is one of the where intersects its surface. It is the southernmost point on Earth and lies on the of Earth from the . Situated on the ...
at the time, resulting in the
ice cap upright=1.35, Vatnajökull, Iceland ">Iceland.html" ;"title="Vatnajökull, Iceland">Vatnajökull, Iceland In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses coveri ...
s of the
Late Ordovician glaciationThe Late Ordovician Glaciation is the first part of the Andean-Saharan glaciation. It was centered on the Sahara region in late Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleoz ...
.


Life

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
conodont Conodonts (Greek language, Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the Class (biology), class Conodonta. For many years, they were known only from their ...
s and
plankton Plankton are the diverse collection of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by tax ...

plankton
ic forms like graptolites. The
trilobite Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

trilobite
s
Agnostida Agnostida is an order (biology), order of arthropod which first developed near the end of the Cambrian, Early Cambrian period and thrived during the Middle Cambrian. They are present in the Lower Cambrian fossil record along with trilobites from t ...
and
Ptychopariida Ptychopariida is a large, heterogeneous order of trilobite Trilobites (; meaning "three lobes") are a group of extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient Greek, Greek: ὀργ ...
completely died out, and the
Asaphida Asaphida is a large, morphologically diverse order (biology), order of trilobites found in marine Stratum, strata dated from the Middle Cambrian until their extinction during the Silurian. Asaphida contains six Superfamily (zoology), superfamilies ...
were much reduced.
Brachiopod Brachiopods (), phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a ...
s,
bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the ...

bryozoa
ns and
echinoderm An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of ...
s were also heavily affected, and the
endocerid Endocerida is an extinct Nautiloidea, nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in its siphuncle. Endocerida comprises a diverse group of cephalopods that lived from the Early Ordovician possibly to ...
cephalopod A cephalopod is any member of the mollusca Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the gram ...
s died out completely, except for possible rare Silurian forms. The Ordovician–Silurian extinction events may have been caused by an ice age that occurred at the end of the Ordovician Period, due to the expansion of the first terrestrial plants, 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. Though less famous than the Cambrian explosion, the Ordovician radiation (also known as the Great Ordovician Biodiversification Event) was no less remarkable; marine faunal
genera Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
increased fourfold, resulting in 12% of all known
Phanerozoic The Phanerozoic Eon is the current in the , and the one during which abundant and has existed. It covers million years to the present, and it began with the Period when animals first developed hard shells preserved in the fossil record. The ...
marine fauna. Another change in the fauna was the strong increase in
filter-feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding ...
organisms. The trilobite, inarticulate brachiopod, , and eocrinoid faunas of the Cambrian were succeeded by those that dominated the rest of the Paleozoic, such as articulate brachiopods,
cephalopod A cephalopod is any member of the mollusca Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the gram ...
s, and
crinoid Crinoids are marine animals that make up the Crinoidea, one of the classes of the phylum ata, which also includes the , s, s and s. Those crinoids which, in their adult form, are attached to the sea bottom by a stalk are commonly called sea lil ...

crinoid
s. Articulate brachiopods, in particular, largely replaced trilobites in communities. Their success epitomizes the greatly increased diversity of
carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline mi ...

carbonate
shell-secreting organisms in the Ordovician compared to the Cambrian. Ordovician geography had its effect on the diversity of fauna. The widely separated continents of Laurentia and Baltica developed distinct trilobite fauna from the trilobite fauna of Gondwana, and Gondwana developed distinct fauna in its tropical and temperature zones. However, tropical articulate brachiopods had a more
cosmopolitan distribution In biogeography Biogeography is the study of the distribution of species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A sp ...
, with less diversity on different continents. Faunas become less provincial later in the Ordovician, though they were still distinguishable into the late Ordovician. In North America and Europe, the Ordovician was a time of shallow continental seas rich in life. Trilobites and brachiopods in particular were rich and diverse. Although solitary
coral Corals are marine invertebrates Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a ver ...

coral
s date back to at least the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These perio ...
,
reef A reef is a ridge or shoal A tidal sandbar connecting the islands of Waya and Wayasewa of the Yasawa Islands, Fiji ">Fiji.html" ;"title="Yasawa Islands, Fiji">Yasawa Islands, Fiji In oceanography, geomorphology, and earth sciences, a s ...

reef
-forming corals appeared in the early Ordovician, corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals.
Molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil species is es ...

Molluscs
, which appeared during the Cambrian or even the
Ediacaran The Ediacaran Period ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of d ...
, became common and varied, especially
bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged p ...

bivalves
,
gastropods The gastropods (), commonly known as snails and slugs, belong to a large of s within the called Gastropoda (). This class comprises s and s from saltwater, from freshwater, and from the land. There are many thousands of species of s and , as ...

gastropods
, and nautiloid cephalopods. Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments. Now-extinct marine animals called graptolites thrived in the oceans. This includes the distinctive ''Nemagraptus gracilis'' graptolite fauna, which was distributed widely during peak sea levels in the Sandbian. Some new cystoids and crinoids appeared. It was long thought that the first true vertebrata, vertebrates (fish — Ostracoderms) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These perio ...
. The first Gnathostomata, gnathostome (jawed fish) appeared in the Late Ordovician glaciation, Late Ordovician Epoch. 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. It is marked by a sudden abundance of hard substrate trace fossils such as ''Trypanites'', ''Palaeosabella'', ''Petroxestes'' and ''Osprioneides''. Several groups of endobiotic symbionts appeared in the Ordovician. In the Early Ordovician, trilobites were joined by many new types of organisms, including Tabulata, tabulate corals, Strophomenida, strophomenid, Rhynchonellida, rhynchonellid, and many new Orthida, orthid brachiopods, bryozoans, planktonic graptolites and conodonts, and many types of molluscs and echinoderms, including the ophiuroids ("brittle stars") and the first Starfish, sea stars. Nevertheless, the arthropods remained abundant, all the Late Cambrian orders continued, and were joined by the new group Phacopida. The first evidence of land plants also appeared (see evolutionary history of life). In the Middle Ordovician, the trilobite-dominated Early Ordovician communities were replaced by generally more mixed ecosystems, in which brachiopods, bryozoans, molluscs, Cornulitida, cornulitids, tentaculites, tentaculitids and echinoderms all flourished, tabulate corals diversified and the first Rugosa, rugose corals appeared. The planktonic graptolites remained diverse, with the Diplograptina making their appearance. 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. One of the earliest known armoured agnathan ("ostracoderm") vertebrate, ''Arandaspis'', dates from the Middle Ordovician. 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. Some trilobites such as ''Asaphus kowalewski'' evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely. Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician. The earliest-known Octocorallia, octocorals date from the Ordovician. File:OrdovicianEdrio.jpg, Upper Ordovician edrioasteroid ''Cystaster stellatus'' on a cobble from the Kope Formation in northern Kentucky with the cyclostome bryozoan ''Corynotrypa'' in the background File:FossilMtnUT.jpg, Middle Ordovician fossiliferous shales and limestones at Fossil Mountain, west-central Utah File:Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana.jpg, Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana File:OrdOutcropTN.JPG, Outcrop of Upper Ordovician limestone and minor shale, central Tennessee File:LibertyBorings.jpg, ''Trypanites'' borings in an Ordovician hardground, southeastern Indiana File:Petroxestes borings Ordovician.jpg, ''Petroxestes'' borings in an Ordovician hardground, southern Ohio File:OilShaleEstonia.jpg, Outcrop of Ordovician kukersite oil shale, northern Estonia File:OilShaleFossilsEstonia.jpg, Bryozoan fossils in Ordovician kukersite oil shale, northern Estonia File:OrdFossilsMN.JPG,
Brachiopod Brachiopods (), phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a ...
s and
bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the ...

bryozoa
ns in an Ordovician limestone, southern Minnesota File:PlatystrophiaOrdovician.jpg, ''Vinlandostrophia ponderosa'', Maysvillian (Upper Ordovician) near Madison, Indiana (scale bar is 5.0 mm) File:Echinosphaerites.JPG, The Ordovician cystoid ''Echinosphaerites'' (an extinct
echinoderm An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of ...
) from northeastern Estonia; approximately 5 cm in diameter File:Prasopora.JPG, ''Prasopora'', a trepostome bryozoan from the Ordovician of Iowa File:EncrustedStroph.JPG, An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan File:Protaraea.jpg, The heliolitid coral ''Protaraea richmondensis'' encrusting a gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana File:ZygospiraAttached.jpg, ''Zygospira modesta'', atrypid brachiopods, preserved in their original positions on a trepostome bryozoan from the Cincinnatian (Upper Ordovician) of southeastern Indiana File:DiplograptusCaneySprings.jpg, Graptolites (''Amplexograptus'') from the Ordovician near Caney Springs, Tennessee


Flora

Green algae were common in the Late Cambrian (perhaps earlier) and in the Ordovician. Terrestrial plants probably evolved from green algae, first appearing as tiny non-vascular plants, vascular forms resembling Marchantiophyta, liverworts, in the middle to late Ordovician. Fossil spores found in Ordovician sedimentary rock are typical of bryophytes. Among the first land fungi may have been arbuscular mycorrhiza fungi (Glomerales), playing a crucial role in facilitating the colonization of land by plants through Mycorrhiza, mycorrhizal symbiosis, which makes mineral nutrients available to plant cells; such fossilized fungal 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.


End of the period

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 History of Earth, Earth's history in terms of percentage of
genera Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
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 The Silurian ( ) is a spanning 24.6 million years from the end of the Period, at million years ago (), to the beginning of the Period, Mya. The Silurian is the shortest period of the . As with other periods, the beds that define the per ...
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 Trilobites (; meaning "three lobes") are a group of marine s that form the Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the of the ( ...

trilobite
,
conodont Conodonts (Greek language, Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the Class (biology), class Conodonta. For many years, they were known only from their ...
and Graptolithina, graptolite families. The most commonly accepted theory is that these events were triggered by the onset of cold conditions in the late Katian, followed by an ice age, in the Hirnantian faunal stage, that ended the long, stable greenhouse conditions typical of the Ordovician. The ice age was possibly not long-lasting. Oxygen isotopes in fossil brachiopods show its duration may have been only 0.5 to 1.5 million years. 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).Jeff Hecht
High-carbon ice age mystery solved
''New Scientist'', 8 March 2010 (retrieved 30 June 2014)
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. Another possibility is that bryophytes and lichens, which colonized land in the middle to late Ordovician, may have increased weathering enough to draw down levels. The drop in selectively affected the shallow seas where most organisms lived. As the southern supercontinent
Gondwana Gondwana () or Gondwanaland was a supercontinent In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (ge ...

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. Species limited to a single epicontinental sea on a given landmass were severely affected. Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse. 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. At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Orders. Recovery was characterized by an unusual number of "Lazarus taxa", disappearing during the extinction and reappearing well into the Silurian, which suggests that the taxa survived in small numbers in Refugium (population biology), refugia. 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. 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.


References


External links

* * An Ordovician reef in Vermont.
Ordovician fossils of the famous Cincinnatian Group
{{Authority control Ordovician, Geological periods 1879 in paleontology