The target of this paper is to study the succession of
the Pliensbachian calcareous nannofossil assemblages recovered
from the Santotis section (Northern Spain) in order to calibrate
the biohorizons against the ammonite zones. In this section, one
hundred nineteen marly and carbonate-dominated levels have
been recognized and forty-one of them have been sampled. The
semiquantitative analysis has been carried out on smear slides
and reveals that the majority of the assemblages show a bad
to moderate degree of preservation and the species abundance
varies from rare to frequent. In the studied section, the most
relevant Pliensbachian composition changes of the assemblages
are the first appearances of the genera Similiscutum (Jamesoni
Zone) and Lotharingius (Stokesi Zone). The assemblages
further change due to the appearance of the species belonging
to the genus Biscutum. The succession of the assemblages
have allowed the identification of two main events, the FOs
of Similiscutum cruciulus (Jamesoni Zone) and Lotharingius
hauffii (Stokesi Zone) as well as four secondary events, the FOs
of Biscutum novum (Davoei Zone), Biscutum grandis (Stokesi
Zone), Biscutum finchii (Stokesi Zone) and Lotharingius barozii
(Stokesi Zone). Assemblages and biohorizons have allowed the
identification of the upper part of the NJ3 Crepidolithus crassus
Zone, the complete NJ4 Similiscutum cruciulus Zone and the
almost complete NJ5 Lotharingius hauffii Zone
