EVOLUTION AND SIGNIFICANCE OF THE ORNAMENTATION IN LATE CRETACEOUS PLANKTONIC FORAMINIFERAL GLOBIGERINIFORM TAXA

Santonian-Campanian ornamented planktonic foraminiferal morphotypes are widely recognized in the literature and traditionally accommodated in five genera (Muricohedbergella, Costellagerina, Rugoglobigerina, Archaeoglobigerina and Rugotruncana) based on the position of the primary aperture, wall ornamentation and peripheral margin features. However, SEM images of the primary type species for these genera are available only for Muricohedbergella and Archaeoglobigerina. As a consequence, the taxonomic relevance of some diagnostic characters such as the development of the wall ornamentation and the presence of imperforate peripheral bands have been variably interpreted among authors (i.e., Gandolfi, 1955; Brönnimann & Brown, 1956; Pessagno, 1967; Robaszynski & Caron, 1984; Georgescu & Huber, 2007). Studies of planktonic foraminiferal assemblages from different localities at varies latitudes highlight the occurrence of widely diffuse morphotypes with ornamented walls showing variably developed muricae, pustules and oriented costellae, so that some of the diagnostic characters currently used to discriminate among genera appear inadequate. To perform this study we examined Santonian-Campanian samples collected from different localities as follows: central Pacific Ocean (ODP Leg 198, Hole 1210B), western Australian margin (ODP Leg 122, Hole 762C), coastal Tanzania (TDP Sites 23, 28, 32, 35), and central Italy (Bottaccione Section, Gubbio). Selected specimens from each locality were analyzed using an SEM to characterize the shell microstructure and wall texture. These features have been used to revise the current classification scheme and for reconstructing lineages of descendants based on stratophenetic observations. Combined geochemical analysis (cathodoluminescence, δ18O and δ13C) have been applied on selected specimens showing different preservation to estimate the diagenetic effects on the primary wall features and to identify a possible correlation between ornamentation and habitat selection. Preliminary results suggest that the ornamentation could represent an ecological adaptation, and therefore its importance as a diagnostic character at genus level should be revised.