not voting); a new working group was formed in 2021. In the western part of the ocean of Tethys, the Berriasian consists of four ammonite biozones, from top to bottom (latest to earliest): Thurmanniceras otopeta Subthurmannia boissieri Tirnovella occitanica Berriasella jacobi/Pseudosubplanites grandis The top of the Berriasian stage is defined by the base of the Valanginian, which is fixed at the first appearance of calpionellid species Calpionellites darderi. This is just a little below the first appearance of the ammonite species Thurmanniceras pertransiens. Regional terms used in Russia include "Volgian"(which spans perhaps the latest Kimmeridgian, all the Tithonian and an uncertain amount of the lower Berriasian) and the "Ryazanian" (?upper Berriasian) . References Notes Literature ; (2004): A Geologic Time Scale 2004, Cambridge University Press. External links GeoWhen Database - Berriasian Jurassic-Cretaceous timescale, at the website of the subcommission for stratigraphic information of the ICS Stratigraphic chart of the Lower Cretaceous, at the website of Norges Network of offshore records of geology and stratigraphy 01 Geological ages Cretaceous geochronology