|485.4 ± 1.9 – 477.7 ± 1.4 Ma|
|Regional usage||Global (ICS)|
|Time scale(s) used||ICS Time Scale|
|Time span formality||Formal|
|Lower boundary definition||FAD of the Conodont Iapetognathus fluctivagus.|
|Lower boundary GSSP||Greenpoint section, Green Point, Newfoundland, Canada|
|Upper boundary definition||FAD of the Graptolite Tetragraptus approximatus|
|Upper boundary GSSP||Diabasbrottet quarry, Västergötland, Sweden|
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.
The GSSP for the beginning of the Tremadocian is the Greenpoint section (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.) in
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.
Planktonic graptolites, an important index fossil, appear during the Tremadocian.
|Agnathans of the Tremadocian|
|Tremadocian-Floian||Alice Springs, Australia|
|Cephalopods of the Tremadocian|
Ocean and climate
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It has been suggested that the Middle Ordovician meteorite bombardment played a crucial role in the Great Ordovician Biodiversification Event, but this study shows that the two phenomena were unrelated
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