Biostratigraphie et paléobiogéographie des Ammonites. Une synthese pour le Lias inférieur de France

Abstract

In the basal Lias, three large families of ammonites follow one another through the Hettangian and the Sinemurian:the Psiloceratidae, the Schlotheimiidae and the Arietitidae. This comprehensive fauna is corresponding to seven stratigraphical chronozones or to an estimated time of 8.5 MA. So in France, are distingued the ubiquitous faunas which are the basic fauna, French or even strictly regional species, and also Tethyan forms which have advanced into the local fauna. Their evolution in course of time strictly depends on their geographical distribution. So from the Lower Hettangian a differenciation between the alpine Psiloceratidae and the north-western European Psiloceratidae appears, showing the presence of two biogeographical provinces. Upon the Euro-western platform, the Paris Basin is wide-communicating with the German sea and also it have some faunal exchanges with Great-Britain during the Lower Hettangian. From time to time Austro-Alpine and typical Italiano-Alpine immigrant faunas advance towards France. The research of relations between these Alpine or north-western forms and the forms now described along the eastern Pacific coast suggests the possible intermittently aperture of a Proto-Atlantique as early as the basis of the Lower Jurassic.