FIRST RECORD OF CHITINOZOANS FROM THE ORDOVICIAN-SILURIAN BOUNDARY BEDS OF MAURITANIA - PALAEOGEOGRPHIC IMPLICATIONS

In the Hodh area (SE Mauritania), siltstones and shales overlying glac ial deposits yield numerous chitinozoans. They provide a high resoluti on biostratigraphy for these Upper Hirnantian and Lower Rhuddnian depo sits. Glacial evidence disappears in the Upper Hirnantian along with t he development of the first marine faunas indicative of an opening tow ard the Rheic Ocean. This opening appears to be younger in the Hodh th an in the northern Sahara Basins. The exceptional development of the H irnantian and of part of the Rhuddanian in the Hodh area, compared wit h the highly condensed sequences representing the contemporaneous depo sits in the northern Sahara Basins, suggests an end-glacial transgress ive event followed by a northward progradational event. (C) Academic d es sciences/Elsevier, Paris.