The Kasimovian is subdivided into three conodont biozones: The base of the Gzhelian is at the first appearance of Streptognathodus zethus.
It is defined as the place in the stratigraphic record where fossils of the species 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.
The Asselian contains five conodont biozones: The base of the Sakmarian stage is laid with the first appearance of Streptognathodus postfusus in the fossil record.
The top of the Sakmarian (the base of the Artinskian) is defined as the place in the stratigraphic record where fossils of conodont species Sweetognathus whitei and Mesogondolella bisselli first appear.