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.[6]

Tremadocian
485.4 ± 1.9 – 477.7 ± 1.4 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Chronology
Epochs in the Ordovician -485 — – -480 — – -475 — – -470 — – -465 — – -460 — – -455 — – -450 — – -445 — – P a l e o z o i c Ꞓ O r d o v i c i a n S F E a r l y M i d d l e L a t e L Stage 10 Tremadocian Floian Dapingian Darriwilian Sandbian Katian Hirnantian Rhuddanian       ← First land plant spores[1] ← Ordovician meteor event[2][3] Epochs of the Ordovician Period. Axis scale: millions of years ago.
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 49.6829°N 57.9653°W
GSSP ratified	2000[4]
Upper boundary definition	FAD of the Graptolite Tetragraptus approximatus
Upper boundary GSSP	Diabasbrottet quarry, Västergötland, Sweden 58.3589°N 12.5024°E
GSSP ratified	2002[5]

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) 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.[6] The boundary lies within the Broom Point Member, of the Green Point Formation which is part of the Cow Head Group.[4] The first planktonic graptolites appear 4.8 m above the first appearance of Iapetognathus fluctivagus at Greenpoint section.[4]

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.[6]

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.[7]

Planktonic graptolites, an important index fossil, appear during the Tremadocian.[4]

Palaeontology

Agnathans

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

Cephalopods

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

Ocean and climate

The Early Ordovician in general was a time of transgression. The climate was slowly cooling throughout the Ordovician.[8]

References

1. Wellman, C.H.; Gray, J. (2000). "The microfossil record of early land plants". Phil. Trans. R. Soc. B. 355 (1398): 717–732. doi:10.1098/rstb.2000.0612. PMC 1692785. PMID 10905606.
2. Korochantseva, Ekaterina; Trieloff, Mario; Lorenz, Cyrill; Buykin, Alexey; Ivanova, Marina; Schwarz, Winfried; Hopp, Jens; Jessberger, Elmar (2007). "L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40 Ar- 39 Ar dating". Meteoritics & Planetary Science. 42 (1): 113–130. Bibcode:2007M&PS...42..113K. doi:10.1111/j.1945-5100.2007.tb00221.x.
3. Lindskog, A.; Costa, M. M.; Rasmussen, C.M.Ø.; Connelly, J. N.; Eriksson, M. E. (2017-01-24). "Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification". Nature Communications. 8: 14066. doi:10.1038/ncomms14066. ISSN 2041-1723. PMC 5286199. PMID 28117834. 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
4. Cooper, Roger; Nowlan, Godfrey; Williams, S. H. (March 2001). "Global Stratotype Section and Point for base of the Ordovician System" (PDF). Episodes. 24 (1): 19–28. doi:10.18814/epiiugs/2001/v24i1/005. Retrieved 6 December 2020.
5. Bergström, M.; Löfgren, Anita; Maletz, Jörg (December 2004). "The GSSP of the Second (Upper) Stage of the Lower Ordovician Series: Diabasbrottet at Hunneberg, Province of Västergötland, Southwestern Sweden". Episodes. 27 (4): 265–272. doi:10.18814/epiiugs/2004/v27i4/005. Retrieved 13 December 2020.
6. "GSSP Table - Paleozoic Era". Geologic TimeScale Foundation. Retrieved 24 November 2012.
7. 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)
8. 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.