Arenig

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Categories: Geochronology stubs | Ordovician

In geology, the Arenig group is the name applied to the lowest stage of the Ordovician System. The term was first used by Adam Sedgwick in 1847 with reference to the "Arenig Ashes and Porphyries" in the neighborhood 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. 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 shale and the Lower Llandeilo Series.

In the geologic timescale, the Arenig is the age of the Lower Ordovician epoch of the Ordovician period of the Paleozoic era of the Phanerozoic eon that is comprehended between 478.6 ± 1.7 Ma and 471.8 ± 1.6 Ma (million years ago), approximately. The Arenig age succeeds the Tremadocian age and precedes the Stage III age.

The Arenig age began with a worldwide sudden rise in sea level rise in sea level (marine transgression). The early ordivician surge in marine diversity also began around the begging of the Arenig age.^[1].

Cephalopod Fauna

Nautiloids

Image:Cameroceras trentonese.jpg

An illustration of a variety of fossil nautiloid genera, although not necessarily ones contemporary with the Arenig stage.

Nautiloids are a group of marine mollusks in the subclass Nautiloidea, which all possess an external shell, the best-known example being the modern nautiluses. They flourished during the early Paleozoic era, where they constituted the main predatory animals, and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, but only a handful of species survive to the present day.

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.

Anarcestida

Eobactrites

Barrandeocerida

Lower

The following is a list of Barrandeocerid 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.

Plectoceras

Ellesmerocerida

Ellesmerocerida are the ancestral nautiloid stock which comprise all Late Cambrian and lowest of the Ordovician nautiloids from which the other orders evolved. Ellesmerocerids continued through the Early Ordovician but became an increasingly minor part of cephalopod faunas during the rest of the period. Ellesmerocerid families evolved from the ancestral Plectronoceratidae, one of two families found in the Upper Cambrian. The principal group, the Ellesmeroceratina have ventral siphuncles with thick, well calcified and layered connecting rings ,especially in primitive forms in which diaphragms are rather common. The predominant Ellesmeroceratidae, consists of laterally compressed ,straight or endogastric shells with short phragmocones and rapidly expanding apices. Some were compressed, others more circular in cross section. Some were straight while others were cyrtoconic.

Lower

The following is a list of Ellesmocerid 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 Ellesmocerid 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.

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.