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 ± 1.9 to 477.7 ± 1.4 million years ago. The base of the Tremadocian is defined as the first appearance of the conodont species Iapetognathus fluctivagus at the GSSP section in Newfoundland.
The GSSP for the beginning of the Tremadocian is the Greenpoint section ( ) 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. The boundary lies within the Broom Point Member, of the Green Point Formation which is part of the Cow Head Group. The first planktonic graptolites appear 4.8 m above the first appearance of Iapetognathus fluctivagus at Greenpoint section.
Regional Stages 
In North America the first stage of the Ordovician is the Gasconadian Stage.
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.
Ocean and Climate 
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- Gradstein, F. M., ed. (2012). The Geologic Time Scale 2012. Elsevier Science Ltd. p. 504. ISBN 978-0444594259.
- "GSSP Table - Paleozoic Era". Geologic TimeScale Foundation. Retrieved 24 November 2012.
- Cooper, R. A.; G. S. Nowlan, S. H. Williams (2001). "Global Stratotype Section and Point for base of the Ordovician System". Episodes 24 (1): 19–28. Retrieved 20 September 2012.
- Sepkoski, J. J. (1995). The Ordovician Radiations: Diversification and Extinction Shown by Global Genus-Level Taxonomic Data. Retrieved 24 November 2012.
- 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.
|Preceded by Proterozoic Eon||542 Ma - Phanerozoic Eon- Present|
|542 Ma - Paleozoic Era -251 Ma||251 Ma - Mesozoic Era - 65 Ma||65 Ma - Cenozoic Era - Present|