first of the four stages of the Lower Permian (Cisuralian) Series, encompassing all rocks deposited during the Asselian Age (299 million to 294.6 million years ago) of the Permian Period. The Asselian Stage is especially well-developed in the Perm region of Russia. Asselian deposits are shales (shale) and limestones (limestone) interbedded with thin layers of sandstones (sandstone) and conglomerates (conglomerate). They were deposited on the eastern edge of the Russian Platform and the upper slope of the Uralian geosyncline in shallow marine environments. Asselian strata are characterized by the first appearance of the conodont (a primitive chordate with tooth-shaped fossil remains) Streptognathodus isolatus. Important fossil zones for the Asselian Stage, as well as the succeeding Sakmarian Stage, are those containing fusulinids (fusulinid) (single-celled organisms with complex shells) of the genera Sphaeroschwagerina and Pseudoschwagerina and ammonites (ammonoid) of the genus Properrinites. These fossil zones are extensive enough to permit worldwide correlation of Lower Permian strata. The Asselian Stage overlies the Gzelian Stage (Gzhelian Stage) of the Carboniferous Period and is overlain by the Sakmarian Stage.