In the

geologic timescale The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochron ...

, the Asselian is the earliest geochronologic age or lowermost chronostratigraphic stage of the

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

. It is a subdivision of the Cisuralian

Epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

or Series. The Asselian lasted between and million years ago (Ma). It was preceded by the Gzhelian (the latest or uppermost subdivision in the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

) and followed by the Sakmarian.

Stratigraphy

The Asselian Stage was introduced into scientific literature in 1954, when the Russian stratigrapher V.E. Ruzhenchev split it off from the Artinskian. At the time, the Artinskian still encompassed most of the lower Permian – its current definitions are more restricted. The Asselian is named after the Assel River in the southern Ural Mountains of

Kazakhstan Kazakhstan, officially the Republic of Kazakhstan, is a landlocked country primarily in Central Asia, with a European Kazakhstan, small portion in Eastern Europe. It borders Russia to the Kazakhstan–Russia border, north and west, China to th ...

and Bashkortostan. The base of the Asselian Stage is equivalent to the base of the Cisuralian Series and the Permian

System A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...

. It is defined as the point in the stratigraphic record where

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s of the conodont '' Streptognathodus isolatus'' first appear. The global reference profile for the base (the GSSP or golden spike) is located in the valley of the Aidaralash River, near Aqtöbe in the Ural Mountains of

.; 1998: ''Proposal of Aidaralash as Global Stratotype Section and Point (GSSP) for base of the Permian System'', Episodes 21(1): pp 11–18. Other reference species which approximate the base of the Asselian include ''Streptognathodus invaginatus'' and ''Str. nodulinearis'' (conodonts) and '' Sphaeroschwagerina vulgaris'' and ''Sph. fusiformis'' ( fusilinids). The top of the Asselian stage (the base of the Sakmarian stage) is at the first appearance of conodont species '' Mesogondolella monstra''. Asselian biostratigraphy is mostly defined by the evolution of ''Streptognathodus''. There is some debate over the exact sequence of species; one option, according to Henderson (2018), lists the following biozones, from youngest to oldest: *zone of ''Streptognathodus postfusus / Streptognathodus barskovi'' *zone of ''Streptognathodus fusus'' *zone of ''Streptognathodus constrictus'' *zone of ''Streptognathodus sigmoidalis'' *zone of ''Streptognathodus cristellaris'' *zone of ''Streptognathodus glenisteri'' *zone of ''Streptognathodus isolatus''

Regional equivalents

In North America, the regional equivalent of the Asselian is the Nealian stage. The Nealian (lower Wolfcampian) is named after the Neal Ranch Formation in the Glass Mountains of west Texas. In South China, the Asselian is equivalent to at least the lower part of the regional Zisongian stage of the Chuanshanian Series. Both the Nealian and Zisongian are defined by fusilinid assemblages, so their precise correlation to the conodont-based global timescale is uncertain. The Asselian occupies a portion of major terrestrial sediment units in Europe, namely the Autunian series of France and the lower

Rotliegend The Rotliegend, Rotliegend Group or Rotliegendes () is a lithostratigraphic unit (a sequence of rock strata) of latest Carboniferous to Guadalupian (middle Permian) age that is found in the subsurface of large areas in western and central Europe ...

of Germany.

Notable formations

* Altenglan Formation (Germany) * Archer City Formation (Texas, USA) * Council Grove Group (in part) (Kansas / Nebraska, USA) * Dunkard Group (in part) (West Virginia / Ohio / Pennsylvania, USA) * Goldlauter Formation (Germany) * Manebach Formation (Germany) * Meisenheim Formation (Germany) * Muse Formation (France)

References

External links

GeoWhen Database - AsselianUpper Paleozoic stratigraphic chart
at the website of the subcommission for stratigraphic information of the ICS {{Geological history, p, p Permian geochronology .