Phylloceratina ammonoids in the Iberian Basin during the Middle Jurassic: a model of biogeographical and taphonomic dispersal related to relative sea-level changes

Abstract

Phylloceratina and Lytoceratina ammonoids in the Middle Jurassic of the Iberian Chain (E. Spain) represent less
than 1% of the whole of Ammonoidea. There are two intervals, however, in which their occurrence is remarkably
constant: within the Upper Bajocian and at the Lower/Middle Callovian boundary. These two dispersal episodes of
Phylloceratina and Lytoceratina into the Iberian Basin are regarded to reflect changes in their palaeoecological and
taphonomical behaviour, as a consequence of regional sea-level changes during the Middle Jurassic. A relative rise
during the Late Bajocian favoured the immigration of juvenile phyllocerataceans. Phylloceras and Adabofoloceras
immigrations gave rise to monospecific assemblages, where they soon died. They did not breed or ontogenically
develop in this basin. In contrast, phyllocerataceans recorded at the Lower/Middle Callovian boundary constitute
polyspecific assemblages, dominated by adult individuals. These Callovian assemblages were formed by nekroplanktonic
drift, related to a relative sea-level fall and general homogenization of the shelf bottom, hence favouring the
concentration of remains of organisms from more open marine and oceanic areas.