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 and
Bashkortostan.[4]
The base of the Asselian Stage is equivalent to the base of the Cisuralian Series and the Permian
System. It is defined as the point in the stratigraphic record where
fossils 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
Kazakhstan.[5] Other reference species which approximate the base of the Asselian include Streptognathodus invaginatus and Str. nodulinearis (conodonts) and Sphaeroschwagerina vulgaris and Sph. fusiformis (
fusilinids).[5]
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),[6] 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.[7][8] In
South China, the Asselian is equivalent to at least the lower part of the regional Zisongian stage of the Chuanshanian Series.[9][10] 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 of Germany.[11][12][13]
^The Nonmarine Permian: Volume 30 of Bulletin of the New Mexico Museum of Natural History and Science, page 48. Editors Spencer G. Lucas, Kate E. Zeigler, 2005
^
abDavydov, V.I.; Glenister, B.F.; Spinosa, C.; Ritter, S.M.; Chernykh, V.V.; Wardlaw, B.R. and Snyder, W.S.; 1998: Proposal of Aidaralash as Global Stratotype Section and Point (GSSP) for base of the Permian System, Episodes 21(1): pp 11–18.