The eurypterid Adelophthalmus sievertsi (Chelicerata : Eurypterida) from the Lower Devonian (Emsian) Klerf Formation of Willwerath, Germany

The eurypterid Rhenopterus sievertsi Stormer from the Lower Devonian of Germany is redescribed. Based on new morphological data, including the possession of prosomal limbs of Adelophthalmus-type and spatulae on the genital operculum, the species is transferred to Adelophthalmus Jordan, in Jordan and von Meyer and thus is the oldest representative of this geographically and stratigraphically widespread genus. Eurypterus? trapezoides Stormer is recognized as a junior synonym of A. sievertsi.