The base of the Berriasian, which is aalso the base of the Cretaceous system, has traditionally been placed at the first appearance of fossils of the ammonite species Berriasella jacobi. A global reference profile (a GSSP) for the Berriasian is under active consideration by the International Subcommission on Cretaceous Stratigraphy (ISCS) of IUGS. A range of contender GSSP localities currently being studied in detail by the ISCS's Berriasian Working Group include localities as far apart as Mexico, Ukraine, Tunisia, Iraq and the Russian Far East. Several markers are being employed to refine correlations and to work towards definition of a base for the Berriasian Stage. These include calcareous nanofossils, such as Nannoconus, calpionellids, ammonites, palynological data and magnetostratigraphy, notably the base of chron M18r. The calibration of these markers, especially Nannoconus steinmannii minor, N. kamptneri minor, and Calpionella alpina, within precisely fixed magnetozones gives greater precision in trying to identify the best position for a boundary.
In the Tethys Ocean, the Berriasian consists of four ammonite biozones, from top to bottom (latest to earliest):