THE INFLUENCE OF PALEOENVIRONMENTAL CHANGE ON DINOFLAGELLATE CYST DISTRIBUTION - AN EXAMPLE FROM THE LOWER AND MIDDLE JURASSIC OF QUERCY, SOUTHWEST FRANCE

The stratigraphical distributions of Lower and Middle Jurassic (Late P liensbachian-Early Aalenian) dinoflagellate cysts are described from s everal outcrops in northern Quercy (southwest France). During this int erval the dinoflagellate cyst assemblages underwent significant change s in their palaeogeographical affinities. The Late Pliensbachian dinof lagellate cyst assemblages exhibited mixed characteristics of both the Boreal and Tethyan realms. The associations are of Boreal affinities, however, during the Late Toarcian. These changes are interpreted as a direct response to regional palaeogeographical modifications. Palaeoe cological information on this succession has been obtained by quantita tive analyses of the dinoflagellate cyst floras. The description of tw o new species, Nannoceratopsis globiformis and Nannoceratopsis magnico rnus, provides additional information on the evolution of this importa nt Jurassic genus. These two new species are confined to the Upper Pie nsbachian and are thought to represent a small isolated population whi ch exhibits rapid evolution. They probably evolved from Nannoceratopsi s deflandrei subsp. deflandrei and possibly gave rise to the speciatio n event in this genus during the Late Toarcian.