Ammonites et paléoécologie: Étude quantitative d'associations du Tithonien inférieur de la Téthys occidentale

Abstract

We collected ammonite faunas from Early Tithonian levels of different localities belonging to the Western Tethys area. These are: marches Apennines (Central Italy), Bakony and Gerecse Mountains (Hungary) on the Apulian block; Ardèche (South-East France), on the southern margin of the european platform; Rogoznik area (Pieniny klippen belt, Poland), which was part of the intraoceanic Czorsztyn ridge, close to the southern european margin. We followed the Ziegler's faunal spectra quantitative method which is considered to be a very useful tool in paleobiogeography and paleoecology of ammonites. Functional interpretation on the basis of shell morphology and sutural complexity is supported by the quantitative data. Phylloceratina, Lytoceratina, Bochianitidae and Haploceratidae (except Pseudolissoceras) are considered as nektopelagic forms. The Haploceratid genus Pseudolissoceras and most of the species of the typical mediterranean Simoceratidae family are characterized by simplified suture lines and morphological characters usually seen in the shallow water forms. These taxons are considered as nektobenthic forms. Haploceras s. str., Ptychophylloceras and Periphinctids characterize the fauna from Ardèche; the proper mediterranean elements are present in some levels only, thus representing mediterranean faunal spreads. In the Marches Appennines the nektopelagic forms are abundant but nektobenthic forms are relatively frequent. In Rogoznik and Bakony sections the nektopelagic group is the prevailing one; simplified suture bearing forms are absent or extremely rare. Possible ecologic competitions between nektopelagic taxons can ben inferred on the basis of the faunal spectra: these forms must live in a similar level of the water column. Mediterranean faunal spreads similar to those rocognized in Ardèche and in Franconia are recognized in Rogoznik. However, there are no major differences between the faunas of the two western tethyan margins. These are actually the result of the influence of ecologic factors rather than pure paleogeographic causes.