Note on the morphology and systematic position of Alkenopterus burglahrensis (Chelicerata: Eurypterida: Eurypterina) from the Lower Devonian of Germany

The holotype of the eurypterid Alkenopterus burglahrensis from the Lower Devonian (Siegenian) of the Westerwald area (Rhineland-Palatinate, SW Germany) is re-examined. In contrast to a previous concept, this species has flattened distal podomeres seven and eight and a movable flattened spine on the seventh podomere of the sixth prosomal appendage (podomere VI-7a). Based on the presence of such a slender paddle, A. burglahrensis has to be regarded as a basal ‘swimming eurypterid’ (Eurypterina) and is formally transferred here to the family Onychopterellidae, extending the stratigraphical range of this family into the Lower Devonian.