A new sea spider (Arthropoda: Pycnogonida) from the Lower Devonian Hunsrück Slate (Germany) is described as Pentapantopus vogteli gen. et sp. nov. This is the fifth pycnogonid species known from this exceptional fossil Lagerstätte. The most conspicuous character of the new species is the presence of five pairs of walking legs. This character, in concert with a reduced abdomen, indicates a phylogenetic position of P. vogteli among the crown group pycnogonids. P. vogteli extends the knowledge of fossil pycnogonid body plans and underlines the significance of the Hunsrück Slate, as this locality shows the highest diversity of sea spiders for the entire fossil record so far.

The present paper is the first scanning electron microscope (SEM) description of a protonymphon larva of an Ammotheidae. The morphology of the first larval instar of Achelia assimilis is described in detail and illustrated for the first time. The morphological characters are compared to previous larval descriptions of other pycnogonid species. The larvae are integrated into Bain's classification of larval types within the pycnogonids (Bain, 2003a) as a ‘typical protonymphon’. Larvae were obtained from ovigerous males, caught in Punta Huinay, Huinay, Chile and analysed with light microscope and SEM. Descriptions of pycnogonid protonymphons at a species-specific level in the future will contribute to a deeper understanding of larval pycnogonid taxonomy at the level of differential diagnoses.