DEFINITION OF THE AALENIAN-BAJOCIAN STAGE BOUNDARY

The Global boundary Stratotype Section and Point (GSSP) for the Bajoci an Stage, formally defined at the base of bed AB11 of the Murtinheira section at Cabo Mondego (Portugal), has been ratified by the IUGS. Mul tidisciplinary biostratigraphical data, mainly based on ammonite and c alcareous nannofossil assemblages, assure worldwide correlations; magn etostratigraphic data increase the correlation power The position of t he boundary coincides with the first occurrence of the ammonite assemb lage characterized by Hyperlioceras mundum and related species (H. fur catum, Braunsina aspera, B. elegantula). The boundary lies just below the nanno-horizon of the entry of Watznaueria communis and W. fossacin cta, and closely corresponds with an inversion to normal polarity corr elated with the up-to-date Jurassic magnetic polarity time scale. An A uxiliary Stratotype Point (ASP) is also selected at the base of bed U1 0 of the Bearreraig Bay section on the Isle of Skye, Scotland, as the complementary reference for the palaeobiological key of the Bajocian l ower boundary i.e. the evolutionary transition Graphoceras-Hyperliocer as.