Arenig

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Arenig Fawr, the mountain which lends its name to the geological series Arenig Fawr w.JPG
Arenig Fawr, the mountain which lends its name to the geological series

In geology, the Arenigian (or 'Arenig') refers both to a time interval during the Lower Ordovician period and also to the suite of rocks which were deposited during this interval.

Contents

History

The term was first used by Adam Sedgwick in 1847 with reference to the "Arenig Ashes and Porphyries" in the neighbourhood of Arenig Fawr, in Merioneth, North Wales. The rock-succession in the Arenig district has been recognized by W. G. Fearnsides (“On the Geology of Arenig Fawr and Moel Llanfnant", Q.J.G.S. vol. lxi., 1905, pp. 608–640, with maps). The above succession is divisible into:

  1. A lower series of gritty and calcareous sediments, the "Arenig Series" as it is now understood;
  2. A middle series, mainly volcanic, with shale, the "Llandeilo Series"; and
  3. The shale and limestones of the Bala or Caradoc Stage.

It was to the middle series (2) that Sedgwick first applied the term "Arenig". In the typical region and in North Wales generally the Arenig series appears to be unconformable upon the Cambrian rocks; this is not the case in South Wales. [1]

The Arenig series is represented in North Wales by the Garth Grit and Ty Obry beds, by the Shelve series of the Corndon district, the Skiddaw Slates of the Lake District, the Ballantrae Group of Ayrshire, and by the Ribband Series of slates and shale in Wicklow and Wexford. It may be mentioned here that the "Llanvirn" Series of H. Hicks was equivalent to the bifidus[ clarification needed ] shale and the Lower Llandeilo Series. [1]

Geochronology

In the geologic timescale, the "Arenig" or Arenigian refers to an age of the Lower Ordovician epoch, between 478.6 ± 1.7 and 471.8 ± 1.6 million years ago, contemporary with the more recently proposed Floian by the ICS, [2] based on a section in Sweden (Diabasbrottet quarry) and with the same boundaries. The Arenigian and Floian are the upper part of the Lower Ordovician and follow the Tremadocian (Gasconadian in North America) which is the lower part. Either is followed by the Middle Ordovician ICS Dapingian or by the Llanvirnian of older chronologies. The Arenigian and equivalent Floian are represented in North America by the upper three stages of the Canadian which is followed by the Middle Ordovician Whiterockian which is the lower part of the now shortened Chazyan.

Events

The Arenig group was deposited during a sudden worldwide rise in sea level resulting in widespread marine transgression. The early Ordovician surge in marine diversity also began around this time. [3]

Brachiopod fauna

Incertae sedis brachiopods of the Floian [4]
Acrotretida of the Floian [4]
Lingulida of the Floian [4]
Orthida of the Floian [4]
Paternida of the Floian
Pentamerida of the Floian

Strophomenida

Strophomenida of the Floian
Trimerellida of the Floian

Cephalopoda

Actinocerida

Upper

The following is a list of Actinocerid genera whose fossils are geochronologically found first in upper Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

Orthocerida

Orthocerids of the Floian [5]
Barrandeocerida of the Floian [5]
Ellesmerocerida of the Floian [5]
Endocerida
Lower

The following is a list of Endocerid genera whose fossils are geochronologically found first in lower Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

Upper

The following is a list of Endocerid genera whose fossils are geochronologically found first in upper Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

Intejocerida of the Floian
Oncocerids of the Floian
Nautiloids of the Floian
Tarphycerida
Lower
Upper

Trilobite fauna

Trilobites of the Floian [6]
Agnostida of the Floian' [6]
Asaphida of the Floian [6]
Asaphus Naturhistorisches Museum Vienna Dez 2006 073.jpg
Asaphus
Corynexichida of the Floian [6]
Lichida of the Floian [6]
Odontopleurida of the Floian [6]
Phacopida of the Floian
Colpocoryphe grandis Colpocoryphe grandis.3 - Fosil.JPG
Colpocoryphe grandis
Proetida of the Floian
Ptychopariida of the Floian [6]

Related Research Articles

Dimeropygiella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

<i>Gyrometopus</i>

Gyrometopus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 479 to 472 million years ago.

Stenorhachis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

Tungtzuella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

Bolbocephalus is a genus of proetid trilobites in the family Bathyuridae. Species lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago, in marine strata of the United States, Canada, and Greenland.

Benthamaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago.

Aspidaeglina is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 473 to 470 million years ago.

Lachnostoma is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago.

Trigonocercella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago.

Hunnebergia is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago.

Merlinia is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago. These trilobites were named after the Welsh wizard Merlin, as the fossilised tails (pygidia) of trilobites were often mistaken for petrified butterflies, and were hence linked with mythological tales involving Merlin.

Psilocara is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago.

Circulocrania is an extinct genus of trilobites in the family Cyclopygidae. The genus lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

Lyrapyge is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

Megalaspidella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

<i>Megalaspides</i> Extinct genus of trilobites

Megalaspides is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

Ningkianolithus is an extinct genus of trilobites in the family Trinucleidae. The genus lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago.

Paraptychopyge is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

<i>Pytine</i> Extinct genus of trilobites

Pytine is an extinct genus of asaphid trilobites. Species lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago. Various species are found in the Svalbard, Valhallfonna Formation, Olenidsletta, Member, of Spitzbergen, Norway, the Megistaspis (Paramegistaspis) planilimbata Zone of the 'Shumardia Shale' of Sweden, Jujuy Province, Argentina, early Arenig-aged strata of Jiangxi province, China, and Darriwilian-aged strata in Western Hunan province, China. The type species, P. graia, has seven thorax segments, and lacks the rapier-like glabellar spine, that occurs in many other raphiophorids. The Chinese species, by contrast, have only six thoracic segments. So far, only the type species, and one of the Chinese species, P. laevigata, are known from complete specimens.

Sycophantia is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago.

References

  1. 1 2 Wikisource-logo.svg One or more of the preceding sentences incorporates text from a publication now in the public domain : Howe, John Allen (1911). "Arenig Group". In Chisholm, Hugh (ed.). Encyclopædia Britannica . 2 (11th ed.). Cambridge University Press. p. 452.
  2. ICS; see Ordovician
  3. "Archived copy". Archived from the original on 2007-11-13. Retrieved 2007-12-11.CS1 maint: archived copy as title (link)
  4. 1 2 3 4 Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (entry on brachiopoda)". Bulletins of American Paleontology. 364: 560. Archived from the original on 14 October 2004. Retrieved 27 April 2008.
  5. 1 2 3 Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (entry on cephalopoda)". Bulletins of American Paleontology. 364: 560. Archived from the original on 7 May 2008. Retrieved 17 April 2008.
  6. 1 2 3 4 5 6 7 Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (entry on trilobita)". Bulletins of American Paleontology. 364: 560. Retrieved 19 April 2008.