Abstract-The evolution of the ammonoid fauna in the Tournaisian and at the Tournaisian-Visean boundary
is discussed.  The study of the dynanmics of the species diversity revealed two peaks (in the Early Tournaisian
and at the Tournaisian-Visean boundary).  The comparison of these events with eustatic fluctuations showed no
obvious correlation between the transgressive-regressive cycles and increase in the taxonomic diversity of
ammonoids. The morphogenetic radiations were apparently to a arge extent connected to the appearance of
diverse sedimentological settings and, hence diverse ammonoids habitats. Changes in sedimentary settings
could have occurred both early in the Early Tournaisian transgression and during the regression at the Tournai-
sian-Visean boundary.