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67-521: The Moesian Platform is made up of four terranes that accreted together during the Ordovician and Devonian as the small continent Avalonia collided with Baltica and Laurentia to form Laurussia . These terrains are sometimes referred to as Caledonian terrains, drawing parallels with the Caledonian orogeny . The North Dobrogea terrane is a fifth, closely related terrane that lies closest to

134-600: 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. 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

201-603: A biogeographic affinity with Gondwana, and the Alborz margin of Gondwana was linked biogeographically to South China. Southeast Asia's fauna also maintained strong affinities to Gondwana's. North China was biogeographically connected to Laurentia and the Argentinian margin of Gondwana. A Celtic biogeographic province also existed, separate from the Laurentian and Baltican ones. However, tropical articulate brachiopods had

268-628: 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

335-468: A burst of volcanic activity that deposited new silicate rocks, which draw CO 2 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 CO 2 levels. The drop in CO 2 selectively affected the shallow seas where most organisms lived. It has also been suggested that shielding of

402-590: 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. 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

469-639: 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. 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

536-607: A distinct band around the Earth, and that the breakup of the parent body may have formed a ring system for a period of about 40 million years, with frequent falling debris causing these craters. 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

603-464: 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

670-693: 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. Faunas become less provincial later in the Ordovician, partly due to the narrowing of the Iapetus Ocean, though they were still distinguishable into the late Ordovician. Trilobites in particular were rich and diverse, and experienced rapid diversification in many regions. Trilobites in

737-670: A period known as the Ordovician meteor event . It has been theorized that this increase in impacts may originate from a ring system that formed around Earth at the time. In 2008, the ICS erected a formal international system of subdivisions for the Ordovician Period and System. Pre-existing Baltoscandic, British, Siberian, North American, Australian, Chinese, Mediterranean and North- Gondwanan regional stratigraphic schemes are also used locally. The Ordovician Period in Britain

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804-541: 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 . Towards the end of the period, Gondwana began to drift across

871-789: Is a geologic period and system , the second of six periods of the Paleozoic Era , and the second of twelve periods of the Phanerozoic Eon . The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 Ma (million years ago) to the start of the Silurian Period 443.8 Ma. The Ordovician, named after the Welsh tribe of the Ordovices , was defined by Charles Lapworth in 1879 to resolve

938-489: 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. Green algae were common in the Late Cambrian (perhaps earlier) and in

1005-801: Is subdivided into the South Dobrogea and Central Dobrogea terranes, split by the Capidava-Ovidiu Fault. The Pecenage-Camena Fault separates these terranes from the North Dobrogea terrane which abuts the Pre-Dobrogean Depression along the Sfantu-Gheorghe Fault in the north. Ordovician The Ordovician ( / ɔːr d ə ˈ v ɪ ʃ i . ə n , - d oʊ -, - ˈ v ɪ ʃ ən / or-də- VISH -ee-ən, -⁠doh-, -⁠ VISH -ən )

1072-406: Is that jaws are homologous to the gill arches . In jawless fishes a series of gills opened behind the mouth, and these gills became supported by cartilaginous elements. The first set of these elements surrounded the mouth to form the jaw. The upper portion of the second embryonic arch supporting the gill became the hyomandibular bone of jawed fish, which supports the skull and therefore links

1139-518: The Cambrian , reef -forming corals appeared in the early Ordovician, including the earliest known octocorals , corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals. Brachiopods surged in diversity, adapting to almost every type of marine environment. Even after GOBE, there is evidence suggesting that Ordovician brachiopods maintained elevated rates of speciation. Molluscs , which appeared during

1206-455: The Devonian . The first land plants are known from this period. The Great Ordovician Biodiversification Event considerably increased the diversity of life. Fish , the world's first true vertebrates , continued to evolve, and those with jaws may have first appeared late in the period. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today in

1273-766: The Rheic Ocean 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

1340-471: 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 , as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history. On the whole,

1407-624: The jawed vertebrates . Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. Most gnathostomes have retained ancestral traits like true teeth , a stomach , and paired appendages (pectoral and pelvic fins, arms, legs, wings, etc.). Other traits are elastin , a horizontal semicircular canal of the inner ear, myelin sheaths of neurons , and an adaptive immune system which has discrete lymphoid organs ( spleen and thymus ), and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in

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1474-408: The sister taxon of Gnathostomata. Jaw development in vertebrates is likely a product of the supporting gill arches. This development would help push water into the mouth by the movement of the jaw, so that it would pass over the gills for gas exchange. The repetitive use of the newly formed jaw bones would eventually lead to the ability to bite in some gnathostomes. Newer research suggests that

1541-601: 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 . Until recently these ancestors, known as antiarchs , were thought to have lacked pectoral or pelvic fins. In addition to this, some placoderms (extinct fish with bony plates) 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 (bony armored jawless fish) are generally considered

1608-460: The Cambrian or even the Ediacaran , became common and varied, especially bivalves , gastropods , and nautiloid cephalopods. Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments. Graptolites, which evolved in the preceding Cambrian period, thrived in the oceans. This includes the distinctive Nemagraptus gracilis graptolite fauna, which

1675-472: 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. Ordovician geography had its effect on

1742-967: The Dapingian and the early Darriwilian. The Llanvirn corresponds to the late Darriwilian. The Caradoc covers the Sandbian and the first half of the Katian. The Ashgill represents the second half of the Katian, plus the Hirnantian . The Ashgill Epoch, the last epoch of the British Ordovician, is made of four ages: the Hirnantian Age, the Rawtheyan Age, the Cautleyan Age, and the Pusgillian Age. These ages make up

1809-848: The Early Eocene Climatic Optimum. Carbon dioxide levels were very high at the Ordovician period's beginning. By the late Early Ordovician, the Earth cooled, giving way to a more temperate climate in the Middle Ordovician, with the Earth likely entering the Early Palaeozoic Ice Age during the Sandbian, and possibly as early as the Darriwilian or even the Floian. The Dapingian and Sandbian saw major humidification events evidenced by trace metal concentrations in Baltoscandia from this time. Evidence suggests that global temperatures rose briefly in

1876-532: 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 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,

1943-737: The East European craton, although it accreted last in the Carboniferous . Its current position is inferred to be the result of strike-slip faulting in the Mesozoic with the opening of Mediterranean-style oceanic basins. Romanian Geological Survey borehole data was used to map the Moesian Platform, with results published in 2007 indicating a large southern section, the West Moesian Platform underlying much of

2010-559: The Great Ordovician Biodiversification Event) was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna. Several animals also went through a miniaturization process, becoming much smaller than their Cambrian counterparts. Another change in the fauna was the strong increase in filter-feeding organisms. The trilobite, inarticulate brachiopod, archaeocyathid , and eocrinoid faunas of

2077-507: The Hirnantian glaciation. As with North America and Europe , Gondwana was largely covered with shallow seas during the Ordovician. Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts. The Panthalassic Ocean covered much of the Northern Hemisphere , and other minor oceans included Proto-Tethys , Paleo-Tethys , Khanty Ocean , which

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2144-424: 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. Gnathostomata Gnathostomata ( / ˌ n æ θ oʊ ˈ s t ɒ m ə t ə / ; from Ancient Greek : γνάθος ( gnathos ) 'jaw' + στόμα ( stoma ) 'mouth') are

2211-469: 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 . Though stromatolites had declined from their peak in the Proterozoic, they continued to exist in localised settings. The Ordovician came to a close in a series of extinction events that, taken together, comprise

2278-421: 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

2345-582: 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. The ash fall of the Millburg/Big Bentonite bed, at about 454 Ma, was the largest in the last 590 million years. This had

2412-428: 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). The dip may have been caused by

2479-557: The Ordovician. Terrestrial plants probably evolved from green algae, first appearing as tiny non- vascular forms resembling 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 mycorrhizal symbiosis , which makes mineral nutrients available to plant cells; such fossilized fungal hyphae and spores from

2546-831: The Romanian section of the Danube River and the coastal city of Varna . This structural unit is separated from the Balkan Terrane to the south by Paleogene , Pliocene and Quaternary uplift in the Balkan Thrust Front and to the north from other structural units by the Neogene Carpathian Front. The East Moesian Platform extends up the coast, separated from the West Moesian Platform by the Intra-Moesian Fault. It in-turn

2613-613: The South Pole. This contributed to the Hibernian glaciation and the associated extinction 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. It is not associated with any major extinction event. A 2024 study found that craters from this event cluster in

2680-564: The Wuliuan, exploded in diversity during the Tremadocian, quickly becoming globally widespread. Several groups of endobiotic symbionts appeared in the Ordovician. 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

2747-486: The deep remodelling of the vertebrate skull that must have taken place as early jaws evolved. The ancestor of all jawed vertebrates have gone through two rounds of whole genome duplication. The first happened before the gnathostome and cyclostome split, and appears to have been an autopolyploidy event (happened within the same species). The second occurred after the split, and was an allopolyploidy event (the result of hybridization between two lineages). The customary view

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2814-456: 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

2881-473: 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 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

2948-422: The diversity of fauna; Ordovician invertebrates displayed a very high degree of provincialism. 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, and Gondwana developed distinct fauna in its tropical and temperature zones. The Tien Shan terrane maintained

3015-559: The early Katian (Boda Event), depositing bioherms and radiating fauna across Europe. The early Katian also witnessed yet another humidification event. Further cooling during the Hirnantian, at the end of the Ordovician, led to the Late Ordovician glaciation . 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

3082-481: The early vertebrate jaw has been described as "a crucial innovation" and "perhaps the most profound and radical evolutionary step in the vertebrate history". Fish without jaws had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period. However studies of the cyclostomes , the jawless hagfishes and lampreys that did survive, have yielded little insight into

3149-425: The extinction and reappearing well into the Silurian, which suggests that the taxa survived in small numbers in 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

3216-485: 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

3283-578: 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. 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

3350-502: The globe. 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. The small continent of Avalonia had just rifted from Gondwana and began to move north towards Baltica and Laurentia, opening

3417-419: The jawless craniates Agnatha .   † Placodermi    [REDACTED]    Acanthodians , incl. Chondrichthyes (cartilaginous fishes) [REDACTED]    Actinopterygii    [REDACTED]    Amphibia    [REDACTED]    Sauria    [REDACTED]    Mammalia    [REDACTED] The appearance of

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3484-548: 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

3551-405: 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

3618-400: 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

3685-426: 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

3752-512: 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

3819-425: The shells of most 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. The Early Ordovician climate was very hot, with intense greenhouse conditions and sea surface temperatures comparable to those during

3886-415: The start of the Devonian . Gnathostomata is traditionally a infraphylum , broken into three top-level groupings: Chondrichthyes , or the cartilaginous fish; Placodermi , an extinct grade of armored fish; and Teleostomi , which includes the familiar classes of bony fish , birds , mammals , reptiles , and amphibians . Some classification systems have used the term Amphirhina . It is a sister group of

3953-492: The sun's rays from the proposed Ordovician ring system, which also caused the Ordovician meteor event , may have also led to the glaciation. 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,

4020-577: The time period from c. 450 Ma to c. 443 Ma. The Rawtheyan, the second last of the Ashgill ages, was from c. 449 Ma to c. 445 Ma. It is in the Katian Age of the ICS's Geologic Time Scale . 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

4087-399: 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. Although solitary corals date back to at least

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4154-458: 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. Fossil findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making them 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

4221-693: Was adopted as an official period of the Paleozoic Era by the International Geological Congress . Life continued to flourish during the Ordovician as it had in the earlier Cambrian Period, although the end of the period was marked by the Ordovician–Silurian extinction events . Invertebrates, namely molluscs and arthropods , dominated the oceans, with members of the latter group probably starting their establishment on land during this time, becoming fully established by

4288-537: Was closed off by the Late Ordovician, Iapetus Ocean , and the new Rheic Ocean . For most of the Late Ordovician life continued to flourish, but at and near the end of the period there were mass-extinction events that seriously affected conodonts and planktonic forms like graptolites . The trilobites Agnostida and Ptychopariida completely died out, and the Asaphida were much reduced. Brachiopods , bryozoans and echinoderms were also heavily affected, and

4355-554: Was distributed widely during peak sea levels in the Sandbian. 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 first gnathostome (jawed fish) may have appeared in the Late Ordovician epoch. Chitinozoans, which first appeared late in

4422-495: 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 , a major mountain-building episode, was well under way in Cambrian times. This continued into

4489-412: Was traditionally broken into Early (Tremadocian and Arenig ), Middle ( Llanvirn (subdivided into Abereiddian and Llandeilian) and Llandeilo ) and Late ( Caradoc and Ashgill) epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column. The Tremadoc corresponds to the ICS's Tremadocian. The Arenig corresponds to the Floian, all of

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