A new genus Crassirensselaeria (Terebratulida, Rhenorensselaeriidae) from the Siegenian (middle Lower Devonian) of the Rheinisches Schiefergebirge (= Rhenish Slate Mountains, Germany ) with the type-species Crassirensselaeria crassicosta is erected. Exceptionally well-preserved material of this taxon has recently been obtained from a temporary outcrop at an ICE (= InterCityExpress) railway construction site close to Aegidienberg near Bonn. As representatives of Crassirensselaeria have hitherto been included into the terebratulid genus Rhenorensselaeria; they are compared with the two Lower Devonian species of that genus from the Rheinisches Schiefergebirge: Rhenorensselaeria strigiceps and Rh. demerathia. Crassirensselaeria is chiefly distinguished from these by more globular shells, the presence of very strong plications, long and strong dental plates, different ventral muscle field, and thickened cardinalia. Representatives of the new genus are excellent index fossils for the Siegenian of the Rheinisches Schiefergebirge, even in deposits of restricted-marine and probably brackish environments. New material of articulated specimens in life position shows that both rhenorensselaeriid genera lived in clusters with the longitudinal axes of the shells oriented steep or perpendicular to the seafloor. The new genus is abundant in the Rheinisches Schiefergebirge ( Germany ) and the northern Ardennes ( Belgium ), it also occurs in the Hrubý Jeseník Mountains ( Czech Republic ). Its occurrence in Cornwall ( Great Britain ), however, is still questionable.

Key words: Brachiopoda, Terebratulida, Crassirensselaeria, Rhenorensselaeria, Rheinisches Schiefergebirge, Devonian.

Mena Schemm-Gregory Mena.Schemm-Gregory@senckenberg.de and Ulrich Jansen Ulrich.Jansen@senckenberg.de, Senckenberg Research Institute, Paläozoologie III, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany.