Les grandes lignes de la biogeographie des heterostraces du silurien superieur-devonien inferieur dans le domaine nord-atlantique

Abstract

The Upper Silurian—Lower Devonian heterostracan-bearing localities from the northeast Appalachians, British Isles, northern France, southern Belgium, western Germany, Spain and Algeria are here reviewed. New specimens from the Lower Devonian of Spain and the Upper Silurian of Algeria are described and figured. Four heterostracan facies can be defined. Facies I and II A are clearly associated with the near-shore zones of the North Atlantic continent: their heterostracans occur in the Old Red Sandstone series [Facies I (northern Ireland and Scotland) has alone been re-invaded by Upper Devonian heterostracans]. The heterostracans from Facies II B are found in grey slates from the same continental shelf (Cornwall and southern Rhenish Massif). Facies III (still badly defined) seems to be part of the islands between the North Atlantic continent and Gondwanaland. Some palaeobiological problems such as migrations, extinctions and adaptations are considered in this palaeogeographical scheme.