The position of the Lower–Middle Cambrian boundary, as classically used, is an issue still under discussion by the International Subcommission on Cambrian Stratigraphy (ISCS), and no level has been established yet. At present, there are two oryctocephalid trilobite species–based Global Standard Stratotype-section and Point (GSSP) proposals in the literature, Oryctocephalus indicus and Ovatoryctocara granulata. These two species have not yet been found in the Mediterranean subprovince. For this reason, other correlation tools that approximate these levels are needed.

A complete chronostratigraphy for the Lower and Middle Cambrian Series of Iberia was proposed by Sdzuy (1971a,b). Recently Geyer and Landing (2004) made a new proposal for the Lower–Middle Cambrian chronostratigraphy of West Gondwana. They proposed the Agdzian Stage. This stage is more or less equivalent to the Bilbilian and Leonian Stages of Spain. The main problem with the Agdzian Stage is that its boundaries are not correlative with a potential global Cambrian Series boundary. Those levels would be in the middle part of the Agdzian Stage. In order to make a more accurate correlation, we prefer to use the previous Spanish scale and try to clarify the correlation between different sequences of this time interval in the Mediterranean subprovince.

Here we present a summary of biostratigraphical data from the Bilbilian and Leonian Stages, the boundary of which is placed at the first appearance datum (FAD) of Acadoparadoxides mureroensis. This, the classical Lower–Middle Cambrian boundary in the Paradoxides realm, is constrained by the data of various areas of the Mediterranean region. A revised correlation chart comparing the Mediterranean and other regions is presented. The position of the Bilbilian–Leonian boundary roughly coincides with the two GSSPs proposed by the ISCS.