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 Global Boundary Stratotype Section and Point (GSSP) section on Newfoundland.[1]

Epochs in the Ordovician

-485 —

–

-480 —

–

-475 —

–

-470 —

–

-465 —

–

-460 —

–

-455 —

–

-450 —

–

-445 —

–

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Cambrian

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Silurian

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Tremadocian

Floian

Dapingian

Darriwilian

Sandbian

Katian

Hirnantian

 

 

 

Epochs of the Ordovician Period.
Axis scale: millions of years ago.

Naming

The Tremadocian is named after the village Tremadoc in Wales. The name was proposed by Adam Sedgwick in 1846 (as "Tremadoc group").

GSSP

The GSSP for the beginning of the Tremadocian is the Greenpoint section (49.6829°N 57.9653°W / 49.6829; -57.9653) in Gros Morne National Park, in western Newfoundland. It is defined as the first appearance of the conodont species Iapetognathus fluctivagus. This horizon can be found 101.8 m above the Greenpoint section datum within bed number 23.[1] The boundary lies within the Broom Point Member, of the Green Point Formation which is part of the Cow Head Group.[2] The first planktonic graptolites appear 4.8 m above the first appearance of Iapetognathus fluctivagus at Greenpoint section.[2]

The Tremadocian ends with the beginning of the Floian which is defined as the first appearance of Tetragraptus approximatus at the GSSP in Diabasbrottet quarry, Västergötland, Sweden.[1]

Regional stages

In North America the first stage of the Ordovician is the Gasconadian Stage. In Baltoscandic region the stages corresponding to Tremadocian are Pakerort stage (older) and Varangu stage (younger).

Evolution

The Cambrian Stage 10-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 species. Overall the amount of biodiversity of the Cambrian was maintained. The evolutionary radiation that would eventually triple the amount of genera during the Ordovician (the Great Ordovician Biodiversification Event) slowly picks up during the Tremadocian.[3]

Planktonic graptolites, an important index fossil, appear during the Tremadocian.[2]

Palaeontology

Agnathans

Agnathans of the Tremadocian
Taxa	Presence	Location	Description	Images
Arandaspis	Tremadocian-Floian	Alice Springs, Australia

Molluscs

Cephalopods

Cephalopods of the Tremadocian
Taxa	Presence	Location	Description	Images
Proterovaginoceras	Tremadocian-Darriwilian

Ocean and climate

Further information: Ordovician § Climate_and_sea_level

The Early Ordovician in general was a time of transgression. The climate was slowly cooling throughout the Ordovician.[4]

References

1. "GSSP Table - Paleozoic Era". Geologic TimeScale Foundation. Retrieved 24 November 2012.
2. Cooper, R. A.; G. S. Nowlan; S. H. Williams (2001). "Global Stratotype Section and Point for base of the Ordovician System" (PDF). Episodes. 24 (1): 19–28. Retrieved 20 September 2012.
3. Sepkoski, J. J. (1995). "The Ordovician Radiations: Diversification and Extinction Shown by Global Genus-Level Taxonomic Data". Retrieved 24 November 2012. Cite journal requires |journal= (help)
4. Munnecke, Axel; Calner, Mikael; Harper, David A.T.; Servais, Thomas (1 October 2010). "Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis". Palaeogeography, Palaeoclimatology, Palaeoecology. 296 (3–4): 389–413. doi:10.1016/j.palaeo.2010.08.001.