Leptaena is a genus of Brachiopod that existed from the Ordovician up into the Carboniferous and it's look didn't change much in all that time. It's no surprise to find this hardy species in lower Devonian (Eifelian stage) rocks from Germany.
These specimens compare well with those I've blogged about before from the Lower Devonian of Oklahoma, Ordovician of Russia, Lower to middle Devonian of Poland and Silurian of New York.
Both of the above specimens come from the Junkerberg Formation (Devonian, Eifelian stage) near Gerolstein, Germany. Thanks to Nils for the fossils.