Utility and precision of Silurian graptolite biozones

The various past uses of the assemblage-zone and biozone concepts are reviewed and a modern definition of biozone is given. Silurian biozones are related to the recentideas on standard reference (graptolite) zones (SRZs) and primary biozones (PBs). Use of evolving lineages, interregna, numerical notation systems, and chronozones are discussed. Precision in correlation of the Silurian biozones is described in terms of the Ordovician-Silurian boundary and the Wenlock-Ludlow (Silurian) boundary. A number of recommendations are made with respect to all zonal concepts, essentially giving prime importance to the use of biozones. □ Biozones, standard reference zones, primary biozones, chronozones, graptolites.