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Ordovician Period
485.4–443.8 million years ago
Mean atmospheric O
content over period duration
ca. 13.5 vol %[1]
(68 % of modern level)
Mean atmospheric CO
content over period duration
ca. 4200 ppm[2]
(15 times pre-industrial level)
(2 °C above modern level)

The Ordovician /ɔrdəˈvɪʃən/ is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 485.4 and 443.8 million years ago.[5] It follows the Cambrian Period and is followed by the Silurian Period. 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 periods, respectively.[6] Lapworth, recognizing that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own. While recognition of the distinct Ordovician period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1960, 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 Cambrian, although the end of the period was marked by a significant mass extinction. Invertebrates, namely molluscs and arthropods, dominated the oceans. 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.

Dating: extinction events[edit]

The Ordovician Period started at a major extinction event called the Cambrian–Ordovician extinction events about 485.4 Mya (million years ago), and lasted for about 42 million years. It ended with the Ordovician–Silurian extinction event, about 443.8 Mya (ICS, 2004) that wiped out 60% of marine genera.


The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is highly significant. Since 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.


Key events in the Ordovician
view • discuss • edit
-485 —
-480 —
-475 —
-470 —
-465 —
-460 —
-455 —
-450 —
-445 —
Key events of the Ordovician Period.
ICS approved stages.

Axis scale: millions of years ago.[8]

A number of regional terms have been used to refer to subdivisions of the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions, illustrated to the right.[9]

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[edit]




During the Ordovician, the southern continents were collected into a single continent called Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents Laurentia (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 to shrink between them. The small continent Avalonia separated from Gondwana and began to head north towards Baltica and Laurentia. The Rheic Ocean between Gondwana and Avalonia was formed as a result.

A major mountain-building episode was the Taconic orogeny that was well under way in Cambrian times. In 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 into the atmosphere, turning the planet into a hothouse. These volcanic island arcs eventually collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician these volcanic emissions had stopped. Gondwana had by that time neared or approached the pole and was largely glaciated.

Ordovician meteor event[edit]

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.[11][12][13]





Climate and sea level[edit]

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.[16] Sea level rose more or less continuously throughout the Early Ordovician, levelling off somewhat during the middle of the period.[16] Locally, some regressions occurred, but sea level rise continued in the beginning of the Late Ordovician. A great change was in the cards, however, and sea levels fell steadily in accord with the cooling temperatures for ~30 million years leading up to the Hirnantian glaciation. Within this icy stage, sea level seems to have risen and dropped somewhat, but despite much study the details remain unresolved.[16]





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, graptolites, and some groups of trilobites (Agnostida and Ptychopariida, which completely died out, and the Asaphida, which 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,[18] as the end of the Late Ordovician was one of the coldest times in the last 600 million years of earth history.


Nautiloids like Orthoceras were among the largest predators in the Ordovician.

On the whole, the fauna that emerged in the Ordovician set the template for the remainder of the Palaeozoic.[16] The fauna was dominated by tiered communities of suspension feeders, mainly with short food chains; this said, the ecological system reached a new grade of complexity far beyond that of the Cambrian fauna,[16] which has persisted until the present day.[16]

Though less famous than the Cambrian explosion, the Ordovician featured an adaptive radiation, the Ordovician radiation, that was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna.[19] Another change in the fauna was the strong increase in filter feeding organisms.[20] 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.[21] Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian.[21]





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.[22] 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.[23][24]



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.[25] Some trilobites such as Asaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.[26] Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician.[27]



Colonization of land would have been limited to shorelines


End of the period[edit]



The most commonly accepted theory is that these events were triggered by the onset of most 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, study of oxygen isotopes in fossil brachiopods showing that its duration could have been only 0.5 to 1.5 million years.[15] Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian.


Glaciation locks up water from the world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise; the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations lacking many whole families of organisms, then withdrew again with the next pulse of glaciation, eliminating biological diversity at each change.[32] Species limited to a single epicontinental sea on a given landmass were severely affected.[15] Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.[15]



An alternate extinction hypothesis, from Melott et al. (2004), suggested that a ten-second gamma-ray burst could have destroyed the ozone layer and exposed terrestrial and marine surface-dwelling life to deadly radiation and initiated global cooling.[33]


  1. ^ Image:Sauerstoffgehalt-1000mj.svg
  2. ^ Image:Phanerozoic Carbon Dioxide.png
  3. ^ Image:All palaeotemps.png
  11. ^ Heck, Philipp R.; Schmitz, Birger; Baur, Heinrich; Halliday, Alex N.; Wieler, Rainer (2004). "Fast delivery of meteorites to Earth after a major asteroid collision". Nature 430 (6997): 323–5. Bibcode:2004Natur.430..323H. doi:10.1038/nature02736. PMID 15254530. 
  12. ^ Haack, Henning; Farinella, Paolo; Scott, Edward R. D.; Keil, Klaus (1996). "Meteoritic, Asteroidal, and Theoretical Constraints on the 500 MA Disruption of the L Chondrite Parent Body". Icarus 119: 182–91. Bibcode:1996Icar..119..182H. doi:10.1006/icar.1996.0010. 
  17. ^ Explosion in marine biodiversity explained by climate change
  18. ^ Humble moss helped to cool Earth and spurred on life
  20. ^ Palaeos Paleozoic : Ordovician : The Ordovician Period
  24. ^ Vinn, O., Wilson, M.A., Mõtus, M.-A. and Toom, U. (2014). "The earliest bryozoan parasite: Middle Ordovician (Darriwilian) of Osmussaar Island, Estonia". Palaeogeography Palaeoclimatology Palaeoecology 414: 129–132. doi:10.1016/j.palaeo.2014.08.021. Retrieved 2014-01-09. 
  26. ^ A Guide to the Orders of Trilobites
  33. ^ Melott, Adrian; et al. (2004). "Did a gamma-ray burst initiate the late Ordovician mass extinction?". International Journal of Astrobiology 3: 55–61. arXiv:astro-ph/0309415. Bibcode:2004IJAsB...3...55M. doi:10.1017/S1473550404001910. 

External links[edit]

Preceded by Proterozoic Eon Phanerozoic Eon
Paleozoic Era Mesozoic Era Cenozoic Era
Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene 4ry