Ordovician

The Ordovician ( /ɔːrdəˈvɪʃən/) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.[8]

The Ordovician, named after the Celtic 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 northern Wales into the Cambrian and Silurian systems, respectively.[9] 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 Ordovician–Silurian extinction events. Invertebrates, namely molluscs and arthropods, dominated the oceans. 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. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today.[10]

Ordovician Period
485.4–443.8 million years ago
O
Mean atmospheric O
2
content over period duration
c. 13.5 vol %[1][2]
(68 % of modern level)
Mean atmospheric CO
2
content over period duration
c. 4200 ppm[3]
(15 times pre-industrial level)
Mean surface temperature over period duration c. 16 °C[4]
(2 °C above modern level)
Sea level (above present day) 180 m; rising to 220 m in Caradoc and falling sharply to 140 m in end-Ordovician glaciations[5]
Key events in the Ordovician
-485 —
-480 —
-475 —
-470 —
-465 —
-460 —
-455 —
-450 —
-445 —
Key events of the Ordovician Period.
ICS-approved stages.

Axis scale: millions of years ago.[7]

Dating: extinction events

The Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya (million years ago). It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya (ICS, 2004) which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary slightly from those found in other sources. This second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs.

The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is highly significant. It correlates well with the occurrence of widespread graptolite, conodont, and trilobite species. The base (start) of the Tremadocian allows scientists to relate these species not only to each other, but also to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period.

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.[11] There exist Baltoscandic, British, Siberian, North American, Australian, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes.[12]

Ordovician regional stages
ICS Epoch ICS stage British epoch British stage North American epoch North American stage Australian epoch Australian stage Chinese epoch Chinese stage
Late Ordovician Hirnantian stage Ashgill stage Hirnant stage Cincinnati series Gamach stage Late Ordovician Bolinda stage Late Ordovician Hirnant stage
Katian stage Rawthey stage Richmond stage Chientangkiang stage
Cautley stage Maysville stage Easton stage Neichiashan stage
Pusgill stage Eden stage
Caradoc series Strefford stage Mohawk stage Chatfield stage
Cheney stage
Sandbian stage Burrell stage Turin stage Gisborne stage
Aureluc stage Whiterock stage Chazy stage
Middle Ordovician Darriwilian stage Llanvirn series Llandeil stage Middle Ordovician Darriwil stage Middle Ordovician Darriwil stage
Abereiddy stage Not defined
Dapingian stage Arenig series Fenn stage Early Ordovician Yapeen stage Daping stage
Whitland stage Ranger stage Castlemaine stage
Ibex series Black Hills stage Chewton stage
Bendigo stage
Early Ordovician Floian stage Moridun stage Tule stage Lancefield stage Early Ordovician Flo stage
Tremadocian stage Tremadoc series Migneint stage Stairs stage Tremadoc stage
Cressage stage Skullrock stage

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 faunal stages (subdivisions of epochs) from youngest to oldest are:

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)

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.[13]

Paleogeography

During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole.

Early in the Ordovician, the continents of Laurentia (in present-day North America), Siberia, and Baltica (present-day northern Europe) were still independent continents (since the break-up of the supercontinent Pannotia earlier), but Baltica began to move towards Laurentia later in the period, causing the Iapetus Ocean between them to shrink. The small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia.

The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.

Initially, sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but later glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved.

These volcanic island arcs eventually collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was largely glaciated.

Ordovician meteor event

The Ordovician meteor event is a proposed shower of meteors that occurred during the Middle Ordovician period, roughly 470 million years ago. It is not associated with any major extinction event.[14][15][16]

Geochemistry

Anomalodonta gigantea Waynesville Franklin Co IN
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. 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.[17][18]

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.[19]

Climate and sea level

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.[19] The sea level rose more or less continuously throughout the Early Ordovician, leveling off somewhat during the middle of the period.[19] Locally, some regressions occurred, but sea level rise continued in the beginning of the Late Ordovician. Sea levels fell steadily in accord with the cooling temperatures for ~30 million years leading up to the Hirnantian glaciation. During this icy stage, sea level seems to have risen and dropped somewhat, but despite much study the details remain unresolved.[19]

At the beginning of the period, around 485.4 million years ago, the climate was very hot due to high concentration of CO
2
(4200 ppm) in the atmosphere, which gave a strong greenhouse effect. By contrast, today the concentration is just above 400 ppm. Marine water temperatures are assumed to have averaged 45 °C (113 °F), which restricted the diversification of complex multi-cellular organisms. But over time, the climate became cooler, and around 460 million years ago, the ocean temperatures became comparable to those of present-day equatorial waters.[20]

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.

As the Ordovician progressed, we see evidence of glaciers on the land we now know as Africa and South America, which were near the South Pole at the time, and covered by ice caps.

Life

Nmnh fg09
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 planktonic forms like conodonts and 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,[21] as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history.

Fauna

Orthoceras BW
Nautiloids like Orthoceras were among the largest predators in the Ordovician.
LibertyFormationSlab092313
Fossiliferous limestone slab from the Liberty Formation (Upper Ordovician) of Caesar Creek State Park near Waynesville, Ohio.

On the whole, the fauna that emerged in the Ordovician were the template for the remainder of the Palaeozoic.[19] 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,[19] which has persisted until the present day.[19]

Though less famous than the Cambrian explosion, the Ordovician radiation was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna.[22] Another change in the fauna was the strong increase in filter-feeding organisms.[23] 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.[24] Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian.[24]

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 corals date back to at least the Cambrian, 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.[19]

Molluscs, which appeared during the Cambrian or even the Ediacaran, became common and varied, especially bivalves, gastropods, and nautiloid cephalopods.

Now-extinct marine animals called graptolites thrived in the oceans. 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. The very first gnathostome (jawed fish) appeared in the 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.[25] 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.[26][27]

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 trilobites 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. 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.[28] Some trilobites such as Asaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.[29] Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician.[30]

The earliest-known octocorals date from the Ordovician.[31]

OrdovicianEdrio

The Upper Ordovician edrioasteroid Cystaster stellatus on a cobble from the Kope Formation in northern Kentucky. In the background is the cyclostome bryozoan Corynotrypa.

FossilMtnUT

Fossil Mountain, west-central Utah; Middle Ordovician fossiliferous shales and limestones in the lower half.

Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana

Outcrop of Upper Ordovician rubbly limestone and shale, southern Indiana; College of Wooster students.

OrdOutcropTN

Outcrop of Upper Ordovician limestone and minor shale, central Tennessee; College of Wooster students.

LibertyBorings

Trypanites borings in an Ordovician hardground, southeastern Indiana.[32]

Petroxestes borings Ordovician

Petroxestes borings in an Ordovician hardground, southern Ohio.[25]

OilShaleEstonia

Outcrop of Ordovician kukersite oil shale, northern Estonia.

OilShaleFossilsEstonia

Bryozoan fossils in Ordovician kukersite oil shale, northern Estonia.

OrdFossilsMN

Brachiopods and bryozoans in an Ordovician limestone, southern Minnesota.

PlatystrophiaOrdovician

Vinlandostrophia ponderosa, Maysvillian (Upper Ordovician) near Madison, Indiana. Scale bar is 5.0 mm.

Echinosphaerites

The Ordovician cystoid Echinosphaerites (an extinct echinoderm) from northeastern Estonia; approximately 5 cm in diameter.

Prasopora

Prasopora, a trepostome bryozoan from the Ordovician of Iowa.

EncrustedStroph

An Ordovician strophomenid brachiopod with encrusting inarticulate brachiopods and a bryozoan.

Protaraea

The heliolitid coral Protaraea richmondensis encrusting a gastropod; Cincinnatian (Upper Ordovician) of southeastern Indiana.

ZygospiraAttached

Zygospira modesta, atrypid brachiopods, preserved in their original positions on a trepostome bryozoan; Cincinnatian (Upper Ordovician) of southeastern Indiana.

DiplograptusCaneySprings

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. Fossil spores from land plants have been identified in uppermost Ordovician sediments.

Ordovician Land Scene
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.[33]

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 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.[18] 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).[34][35] The dip was triggered by a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode.[35] This 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.[36] Species limited to a single epicontinental sea on a given landmass were severely affected.[18] Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.[18]

Those species able to adapt to the changing conditions survived to fill the ecological niches left by the extinctions.

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.

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.[37]

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.[38]

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External links

Andean-Saharan glaciation

The Andean-Saharan glaciation occurred during the Paleozoic from 450 Ma to 420 Ma, during the late Ordovician and the Silurian period. For the Ordovician/Saharan part, see the more extensive article on the Late Ordovician glaciation.

According to Eyles and Young, "A major glacial episode at c. 440 Ma, is recorded in Late Ordovician strata (predominantly Ashgillian) in West Africa (Tamadjert Formation of the Sahara), in Morocco (Tindouf Basin) and in west-central Saudi Arabia, all areas at polar latitudes at the time. From the Late Ordovician to the Early Silurian the centre of glaciation moved from northern Africa to southwestern South America."During this period glaciation is known from Arabia, Sahara, West Africa, the south Amazon, and the Andes. The center of glaciation migrated from Sahara in the Ordovician (450–440 Ma) to South America in the Silurian (440–420 Ma). The maximum extent of glaciation developed in Africa and eastern Brazil.A minor ice age, the Andean-Saharan was preceded by the Cryogenian ice ages (720–630 Ma, the Sturtian and Marinoan glaciations), often referred to as Snowball Earth, and followed by the Karoo Ice Age (350–260 Ma).

Avalonia

Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, southern Ireland, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States. Avalonia is named for the Avalon Peninsula in Newfoundland.

Avalonia developed as a volcanic arc on the northern margin of Gondwana. It eventually rifted off, becoming a drifting microcontinent. The Rheic Ocean formed behind it, and the Iapetus Ocean shrank in front. It collided with the continents Baltica, then Laurentia, and finally with Gondwana, ending up in the interior of Pangea. When Pangea broke up, Avalonia's remains were divided by the rift which became the Atlantic Ocean.

Caledonian orogeny

The Caledonian orogeny was a mountain-building era recorded in the northern parts of Ireland and Britain, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma). It was caused by the closure of the Iapetus Ocean when the continents and terranes of Laurentia, Baltica and Avalonia collided.

The Caledonian orogeny is named for Caledonia, the Latin name for Scotland. The name was first used in 1885 by Austrian geologist Eduard Suess for an episode of mountain building in northern Europe that predated the Devonian period. Geologists like Émile Haug and Hans Stille saw the Caledonian orogeny as one of several episodic phases of mountain building that had occurred during Earth's history. Current understanding has it that the Caledonian orogeny encompasses a number of tectonic phases that can laterally be diachronous. The name "Caledonian" can therefore not be used for an absolute period of geological time, it applies only to a series of tectonically related events.

Cambrian–Ordovician extinction event

The Cambrian–Ordovician extinction event occurred approximately 488 million years ago (m.y.a.). This early Phanerozoic Eon extinction event eliminated many brachiopods and conodonts, and severely reduced the number of trilobite species.

It was preceded by the less-documented (but probably worse) End-Botomian extinction event around 517 million years ago and the Dresbachian extinction event about 502 million years ago.

The Cambrian–Ordovician event ended the Cambrian Period, and led into the Ordovician Period in the Paleozoic Era.

Gnathostomata

Gnathostomata are the jawed vertebrates. The term derives from Greek: γνάθος (gnathos) "jaw" + στόμα (stoma) "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheathes of neurons. Another is an adaptive immune system that uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.It is now assumed that Gnathostomata evolved from ancestors that already possessed a pair of both pectoral and pelvic fins. These ancestors, known as antiarchs, were previously thought to not possess pectoral or pelvic fins until recently. In addition to this, some placoderms were shown to have a third pair of paired appendages, that had been modified to claspers in males and basal plates in females--a pattern not seen in any other vertebrate group.The Osteostraci are generally considered the sister taxon of Gnathostomata.It is believed that the jaws evolved from anterior gill support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively – the buccal pump mechanism. The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would, with time, become stronger and tougher, being transformed into real jaws.

Newer research suggests that a branch of Placoderms was most likely the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm named Entelognathus had a bony skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in Chondrichthyes is a derived trait. The fossil findings of primitive bony fishes such as Guiyu oneiros and Psarolepis, which lived contemporaneously with Entelognathus and had pelvic girdles more in common with placoderms than with other bony fish, show that it was a relative rather than a direct ancestor of the extant gnathostomes. It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes. Fossils findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making it impossible to replace or regrow as they broke or wore down as they grew older, proves the common ancestor of all gnathostomes had teeth and place the origin of teeth along with, or soon after, the evolution of jaws.Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians or "shark-like fishes", may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the early Silurian, and become abundant by the start of the Devonian.

Kope Formation

The Kope Formation is one of the three component bedrock formations of the Maquoketa Group that primarily consists of shale (75%) with some limestone (25%) interbedded. In general, it has a bluish-gray color that weathers light gray to yellowish-gray and it occurs in northern Kentucky, southwest Ohio, and southeast Indiana, United States.

Marchantiophyta

The Marchantiophyta (listen) are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information.

It is estimated that there are about 9000 species of liverworts. Some of the more familiar species grow as a flattened leafless thallus, but most species are leafy with a form very much like a flattened moss. Leafy species can be distinguished from the apparently similar mosses on the basis of a number of features, including their single-celled rhizoids. Leafy liverworts also differ from most (but not all) mosses in that their leaves never have a costa (present in many mosses) and may bear marginal cilia (very rare in mosses). Other differences are not universal for all mosses and liverworts, but the occurrence of leaves arranged in three ranks, the presence of deep lobes or segmented leaves, or a lack of clearly differentiated stem and leaves all point to the plant being a liverwort.

Liverworts are typically small, usually from 2–20 mm wide with individual plants less than 10 cm long, and are therefore often overlooked. However, certain species may cover large patches of ground, rocks, trees or any other reasonably firm substrate on which they occur. They are distributed globally in almost every available habitat, most often in humid locations although there are desert and Arctic species as well. Some species can be a nuisance in shady greenhouses or a weed in gardens.

Ordovician meteor event

The Ordovician meteor event was a dramatic increase in the rate at which L chondrite meteorites fell to Earth during the Middle Ordovician period, about 467.5 million years ago. This is indicated by abundant fossil L chondrite meteorites in a quarry in Sweden and enhanced concentrations of ordinary chondritic chromite grains in sedimentary rocks from this time. This temporary increase in the impact rate was most likely caused by the destruction of the L-chondrite parent body 468 ± 0.3 million years ago having scattered fragments into Earth-crossing orbits, a chronology which is also supported by shock ages in numerous L-chondrite meteorites that fall to Earth today. It has been hypothesized that this influx contributed to, or possibly even instigated, the Great Ordovician Biodiversification Event, although this has been refuted.

Ordovician–Silurian extinction events

The Ordovician–Silurian extinction events, when combined, are the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. This event greatly affected marine communities, which caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, and graptolites. The Ordovician–Silurian extinction occurred during the Hirnantian stage of the Ordovician Period and the subsequent Rhuddanian stage of the Silurian Period. The last event is dated in the interval of 455–430 Ma ago, i.e., lasting from the Middle Ordovician to Early Silurian, thus including the extinction period. This event was the first of the big five Phanerozoic events and was the first to significantly affect animal-based communities.Almost all major taxonomic groups were affected during this extinction event. Extinction was global during this period, eliminating 49-60% of marine genera and nearly 85% of marine species.Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected. Before the late Ordovician cooling, temperatures were relatively warm and it is the suddenness of the climate changes and the elimination of habitats due to sea-level fall that are believed to have precipitated the extinctions. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially driven cooling were likely driving agents for the Ordovician mass extinction.

Paleozoic

The Paleozoic (or Palaeozoic) Era ( ; from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon. It is the longest of the Phanerozoic eras, lasting from 541 to 251.902 million years ago, and is subdivided into six geologic periods (from oldest to youngest): the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Paleozoic comes after the Neoproterozoic Era of the Proterozoic Eon and is followed by the Mesozoic Era.

The Paleozoic was a time of dramatic geological, climatic, and evolutionary change. The Cambrian witnessed the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion, in which most modern phyla first appeared. Arthropods, molluscs, fish, amphibians, synapsids and diapsids all evolved during the Paleozoic. Life began in the ocean but eventually transitioned onto land, and by the late Paleozoic, it was dominated by various forms of organisms. Great forests of primitive plants covered the continents, many of which formed the coal beds of Europe and eastern North America. Towards the end of the era, large, sophisticated diapsids and synapsids were dominant and the first modern plants (conifers) appeared.

The Paleozoic Era ended with the largest extinction event in the history of Earth, the Permian–Triassic extinction event. The effects of this catastrophe were so devastating that it took life on land 30 million years into the Mesozoic Era to recover. Recovery of life in the sea may have been much faster.

Sea urchin

Sea urchins or urchins () are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres (16,000 ft; 2,700 fathoms). Their tests (hard shells) are round and spiny, typically from 3 to 10 cm (1 to 4 in) across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, and triggerfish.

Like other echinoderms, urchins have fivefold symmetry as adults, but their pluteus larvae have bilateral (mirror) symmetry, indicating that they belong to the Bilateria, the large group of animal phyla that includes chordates, arthropods, annelids and molluscs. They are widely distributed across all the oceans, all climates from tropical to polar, and inhabit marine benthic (sea bed) habitats from rocky shores to hadal zone depths. Echinoids have a rich fossil record dating back to the Ordovician, some 450 million years ago. Their closest relatives among the echinoderms are the sea cucumbers (Holothuroidea); both are deuterostomes, a clade which includes the chordates.

The animals have been studied since the 19th century as model organisms in developmental biology, as their embryos were easy to observe; this has continued with studies of their genomes because of their unusual fivefold symmetry and relationship to chordates. Species such as the slate pencil urchin are popular in aquariums, where they are useful for controlling algae. Fossil urchins have been used as protective amulets.

Silurian

The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out.

A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life also began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes, streams, and coastlines, and terrestrial arthropods are also first found on land during the Silurian. However, terrestrial life would not greatly diversify and affect the landscape until the Devonian.

St. Peter Sandstone

The St. Peter Sandstone is an Ordovician formation in the Chazyan stage of the Champlainian series. This sandstone originated as a sheet of sand in clear, shallow water near the shore of a Paleozoic sea and consists of fine-to-medium-size, well-rounded quartz grains with frosted surfaces. The extent of the formation spans north-south from Minnesota to Arkansas and east-west from Illinois into Nebraska and South Dakota. The formation was named by Owen (1847) after the Minnesota River, then known as the St. Peter River. The type locality is at the confluence of the Mississippi and Minnesota Rivers near Fort Snelling, Minnesota. In eastern Missouri the stone consists of quartz sand that is 99.44% silica.

Teleostomi

Teleostomi is an obsolete clade of jawed vertebrates that supposedly includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives. The teleostomes include all jawed vertebrates except the chondrichthyans and the extinct class Placodermi.

Recent studies indicate that Osteichthyes evolved from placoderms like Entelognathus, while acanthodians are more closely related to modern chondrichthyes. Teleostomi, therefore, is not a valid, natural clade, but a polyphyletic group of species.The clade Teleostomi should not be confused with the similar-sounding fish clade Teleostei.

Tremadocian

The Tremadocian is the lowest stage of Ordovician. Together with the later Floian stage it forms the Lower Ordovician epoch. The Tremadocian lasted from 485.4 to 477.7 million years ago. The base of the Tremadocian is defined as the first appearance of the conodont species Iapetognathus fluctivagus at the GSSP section on Newfoundland.

Trenton Formation

The Trenton Formation is a geologic formation in Michigan. It preserves fossils dating back to the Ordovician period.

Trilobite

Trilobites ( ; meaning "three lobes") are a group of extinct marine arachnomorph arthropods that form the class 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 Atdabanian stage of the Early Cambrian period (521 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except the Proetids died out. Trilobites disappeared in the mass extinction at the end of the Permian about 252 million years ago. The trilobites were among the most successful of all early animals, existing in oceans for over 300 million years.By the time trilobites first appeared in the fossil record, they were already highly diversified and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton, they left an extensive fossil record, with some 50,000 known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology, and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata), although several alternative taxonomies are found in the literature.

Trilobites had many lifestyles; some moved over the sea bed as predators, scavengers, or filter feeders, and some swam, feeding on plankton. Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where scientific debate continues). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.

Utica Shale

The Utica Shale is a stratigraphical unit of Upper Ordovician age in the Appalachian Basin. It

underlies much of the northeastern United States and adjacent parts of Canada.

It takes the name from the city of Utica, New York, as it was first described as an outcrop along the Starch Factory Creek east of the city by Ebenezer Emmons in 1842.

Cenozoic era
(present–66.0 Mya)
Mesozoic era
(66.0–251.902 Mya)
Paleozoic era
(251.902–541.0 Mya)
Proterozoic eon
(541.0 Mya–2.5 Gya)
Archean eon (2.5–4 Gya)
Hadean eon (4–4.6 Gya)

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