The Tremadocian is the lowest stage of

Ordovician 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 ...

. Together with the later

Floian The Floian is the second stage of the Ordovician Period. It succeeds the Tremadocian with which it forms the Lower Ordovician epoch. It precedes the Dapingian Stage of the Middle Ordovician. The Floian extended from to million years ago. The ...

Stage it forms the

Lower Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. Th ...

Epoch. The Tremadocian lasted from to million years ago. The base of the Tremadocian is defined as the

first appearance In American comic books and other stories with a long history, first appearance refers to the first issue to feature a fictional character. These issues are often highly valued by collectors due to their rarity and iconic status. Reader intere ...

of the

conodont Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...

species ''

Iapetognathus fluctivagus ''Iapetognathus fluctivagus'' is a species of denticulate cordylodan conodonts belonging to the genus '' Iapetognathus''. It existed during the Tremadocian Age ( million years ago) of the Ordovician. It is an important index fossil in biostrat ...

'' at the

Global Boundary Stratotype Section and Point A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. The effort to define GSSPs is conducted b ...

(GSSP) section on

Newfoundland Newfoundland and Labrador (; french: Terre-Neuve-et-Labrador; frequently abbreviated as NL) is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region ...

Naming

The Tremadocian is named after the village Tremadoc in

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

. The name was proposed by

Adam Sedgwick Adam Sedgwick (; 22 March 1785 – 27 January 1873) was a British geologist and Anglican priest, one of the founders of modern geology. He proposed the Cambrian and Devonian period of the geological timescale. Based on work which he did on W ...

in 1846 (as "Tremadoc group").

GSSP

The GSSP for the beginning of the Tremadocian is the Greenpoint section () in

Gros Morne National Park Gros Morne National Park is a National Parks of Canada, Canadian national park and World Heritage Site located on the west coast of Newfoundland and Labrador, Newfoundland. At , it is the second largest national park in Atlantic Canada after To ...

, in western

. It is defined as the

of the

species ''

''. This horizon can be found 101.8 m above the Greenpoint section datum within bed number 23. The boundary lies within the Broom Point Member, of the

Green Point Formation The Green Point Formation is a geologic formation in Newfoundland and Labrador. It preserves fossils dating back to the Ordovician period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation ...

which is part of the

Cow Head Group The Cow Head Group is a geologic group in Newfoundland and Labrador. It preserves fossils dating back to the Ordovician period. See also * List of fossiliferous stratigraphic units in Newfoundland and Labrador This is a list of fossiliferous ...

. The first planktonic

graptolites Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through th ...

appear 4.8 m above the first appearance of ''Iapetognathus fluctivagus'' at Greenpoint section. The Tremadocian ends with the beginning of the

which is defined as the first appearance of ''

Tetragraptus approximatus ''Tetragraptus approximatus'' is a species of Dichograptidae, dichograptid graptolite belonging to the genus ''Tetragraptus''. It existed during the Floian Age (geology), Age ( million years ago) of the Ordovician. It is an important index fossil ...

'' at the GSSP in Diabasbrottet quarry,

Västergötland Västergötland (), also known as West Gothland or the Latinized version Westrogothia in older literature, is one of the 25 traditional non-administrative provinces of Sweden (''landskap'' in Swedish), situated in the southwest of Sweden. Väs ...

Sweden Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic country located on ...

Regional stages

In North America the first stage of the Ordovician is the

Gasconadian Stage {{Infobox geologic timespan , name = Gascondian , color = , manual_color = , top_bar = , time_start = 488 , time_start_prefix = , ...

. In Baltoscandic region the stages corresponding to Tremadocian are Pakerort stage (older) and

Varangu stage Varangu may refer to several places in Estonia: * Varangu, Haljala Parish, village in Lääne-Viru County, Estonia * Varangu, Väike-Maarja Parish, village in Lääne-Viru County, Estonia {{geodis ...

(younger).

Evolution

The

Cambrian Stage 10 Stage 10 of the Cambrian is the still unnamed third and final stage of the Furongian series. It follows the Jiangshanian and precedes the Ordovician Tremadocian Stage. The proposed lower boundary is the first appearance of the trilobite '' Lotagnos ...

-Tremadocian boundary is marked by the Cambrian-Ordovician extinction event. It led to the extinction of many brachipods, conodonts and severely reduced the number of

trilobite Trilobites (; meaning "three lobes") are extinct marine 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 At ...

species. Overall the amount of biodiversity of the

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

was maintained. The evolutionary radiation that would eventually triple the amount of

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

during the

(the

Great Ordovician Biodiversification Event The Great Ordovician Biodiversification Event (GOBE), was an evolutionary radiation of animal life throughout the Ordovician period, 40 million years after the Cambrian explosion, whereby the distinctive Cambrian fauna fizzled out to be replaced ...

) slowly picks up during the Tremadocian. Conodonts diversified during the Tremadocian, though not to the same extent as they would in the following Floian. Planktonic graptolites, an important index fossil, appear during the Tremadocian. The middle of the Tremadocian witnessed an

extinction event An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. I ...

known as the Mid-Tremadocian Extinction Event or the Base Stairsian Mass Extinction Event, which is particularly known to have affected Baltican conodonts. This extinction event may have been caused by anoxia.

Ocean and climate

The

Early Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

in general was a time of transgression. The climate was slowly cooling throughout the Ordovician.

References

{{Geological history, p, p, state=collapsed . Ordovician geochronology