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.[7]
Tremadocian | |||||||||||
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Chronology | |||||||||||
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Etymology | |||||||||||
Name formality | Formal | ||||||||||
Usage information | |||||||||||
Celestial body | Earth | ||||||||||
Regional usage | Global (ICS) | ||||||||||
Time scale(s) used | ICS Time Scale | ||||||||||
Definition | |||||||||||
Chronological unit | Age | ||||||||||
Stratigraphic unit | Stage | ||||||||||
Time span formality | Formal | ||||||||||
Lower boundary definition | FAD of the Conodont Iapetognathus fluctivagus. | ||||||||||
Lower boundary GSSP | Greenpoint section, Green Point, Newfoundland, Canada 49°40′58″N 57°57′55″W / 49.6829°N 57.9653°W | ||||||||||
Lower GSSP ratified | 2000[5] | ||||||||||
Upper boundary definition | FAD of the Graptolite Tetragraptus approximatus | ||||||||||
Upper boundary GSSP | Diabasbrottet quarry, Västergötland, Sweden 58°21′32″N 12°30′09″E / 58.3589°N 12.5024°E | ||||||||||
Upper GSSP ratified | 2002[6] |
The Tremadocian is named after the village Tremadoc in Wales. The name was proposed by Adam Sedgwick in 1846 (as "Tremadoc group").
The GSSP for the beginning of the Tremadocian is the Green Point section (49°40′58″N 57°57′55″W / 49.6829°N 57.9653°W)[7] 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.[8] The boundary lies within the Broom Point Member, of the Green Point Formation which is part of the Cow Head Group.[5] The first planktonic graptolites appear 4.8 m above the first appearance of Iapetognathus fluctivagus at Greenpoint section.[5]
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.[9]
In North America the first stage of the Ordovician is the Gasconadian Stage.[10] In Baltic region the stages corresponding to Tremadocian are Pakerort stage (older) and Varangu stage (younger).[11][12]
The Cambrian-Tremadocian boundary is marked by the Cambrian-Ordovician extinction event. Overall the amount of biodiversity of the Cambrian was maintained.[13] At the beginning of the Tremadocian, about 485.4 million years ago, biodiversity, which had been at a low level, began its long increase phase, known as the Great Ordovician Biodiversification Event.[14]
At the Furongian‒Tremadocian boundary, a mantle plume event occurred on the territory of the northwestern Gondwana, which is now the Iberian Peninsula. Ollo de Sapo magmatic event continued in this region further into Ordovician.[15]
The middle of the Tremadocian witnessed an extinction event known as the Mid-Tremadocian Extinction Event[16] or the Base Stairsian Mass Extinction Event,[17] which is particularly known to have affected Baltican conodonts.[16] This extinction event may have been caused by anoxia.[18][19]
Planktonic graptolites, an important index fossil, appear during the Tremadocian.[5] Tremadocian cephalopods were not very different from their Cambrian predecessors. Specimens of Ellesmeroceras and possibly Bassleroceras, found in Santa Rosita Formation, northwestern Argentina, show that cephalopods first migrated to the waters off western Gondwana already in the early Tremadocian. In the middle Tremadocian, cephalopods became more diverse and occupied new ecological niches.[20] During Tremadocian there was an exchange of fauna between Avalonia and Gondwana across the Rheic Ocean, as evidenced by the findings of morphologically similar trilobites of the genus Platypeltoides in Belgium, Wales (both were parts of Avalonia) and Morocco (Gondwana).[21]
The Early Ordovician in general was a time of transgression. The climate was slowly cooling throughout the Ordovician.[22]
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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