The stratigraphical distributions of Lower and Middle Jurassic (Late Pliensbachian-Early Aalenian) dinoflagellate cysts are described from several outcrops in northern Quercy (southwest France). During this interval the dinoflagellate cyst assemblages underwent significant changes in their palaeogeographical affinities. The Late Pliensbachian dinoflagellate 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. Palaeoecological information on this succession has been obtained by quantitative analyses of the dinoflagellate cyst floras. The description of two new species, Nannoceratopsis globiformis and Nannoceratopsis magnicornus, provides additional information on the evolution of this important Jurassic genus. These two new species are confined to the Upper Piensbachian and are thought to represent a small isolated population which exhibits rapid evolution. They probably evolved from Nannoceratopsis deflandrei subsp. deflandrei and possibly gave rise to the speciation event in this genus during the Late Toarcian.