Drepanopterus abonensis, a stylonurid eurypterid (Arthropoda : Chelicerata) from the Late Devonian Old Red Sandstone (Famennian) of Portishead, UK, is redescribed. New material reveals D. abonensis to be a primitive sweep-feeder, a precursor to the large, bizarre sweep-feeding hibbertopterids that ranged from the Carboniferous to the end-Permian. The new reconstruction differs drastically from the previous one; D. abonensis was a deep-bodied creature with a length of c.40 cm, shorter than previously considered. Inclusion within a phylogenetic framework resolves D. abonensis at the base of the hibbertopterids, sharing the synapomorphies of a cleft metastoma and blades (blunt modified spines) on the anterior prosomal appendages. A well-resolved stylonurid phylogeny also enable macroevolutionary implications to be addressed. A trend towards sweep-feeding is observed throughout stylonurid evolution, with laurieipterids, hardieopterids and hibbertopterids each independently adopting this feeding mode. This particular ecological niche could explain why the Stylonurina persisted through the Carboniferous whereas the Eurypterina went into decline during the Devonian; unable to compete for prey with their more manoeuvrable nektonic eurypterine brethren, stylonurids adapted to occupy a distinct sweep-feeding habit and so were unaffected by the competition from jawed vertebrate and other invertebrate predators that contributed to the decline of the Eurypterina.