Area of mixed marine faunas between two major paleogeographical realms, exemplified by the Early Jurassic (Late Sinemurian and Pliensbachian) ammonites in the Alps
Abstract The Late Sinemurian and Pliensbachian ammonite faunas of the Alpine Range yield a good opportunity to study an area of mixed marine fauna. The faunas of the 4 major paleogeographical Alpine units (from Southern Alps to Delphino-Helvetic) are illustrated by fifteen recently investigated outcrops. Moreover, information provided by the Alpine units is compared with both northern and southern surroundings. These data areaanalyzed here through a qualitative and quantitative approach. Two main patterns of distribution are recognizable. The first one, mainly Ammonitina versus Phylloceratina and Lytoceratina, shows a rather gradual decrease of Ammonitina to the South and of Phylloceratina-Lytoceratina to the North. The second pattern, well exemplified if we consider Euroboreal vs Tethyan Ammonitina, corresponds to abrupt faunal changes. The pattern of the first suggests that ecological (depth of water, currents, siliciclastic sedimentation, …) constraints are stronger than competition between the two considered groups. In the second case competition between ammonites seems more important than ecological constraints. At last, redundancies in patterns of faunal distribution reveal that only two paleogeographical areas correspond to important faunal changes. The first one is probably located on the Brianconnais area and the second one is situated between Upper Austroalpine and Southern Alps) areas. If the Brianconnais is classically considered as a possible barrier, the second faunal boundary is more enigmatic because palinspastic reconstructions hardly suggested any physical barrier at this place, except perhaps the Vardar ocean.