Stylonurid eurypterids comprise a monophyletic suborder of aquatic chelicerates known from marine, brackish and freshwater environments through the late Ordovician to end Permian. Stylonurids have traditionally been considered to be bottom-dwelling scavengers, however it is now known that two clades independently underwent an evolutionary trend towards a sweep-feeding mode of life. This trend is characterised by the modification of spines on the anterior prosomal appendages and a broadening of the metastoma, however the exact method of sweep-feeding is different in both clades. The Stylonuroidea bear multiple pairs of fixed spines on each podomere and could have used their appendages as dragnets, raking through the substrate surface in a broad arc and sweeping everything in reach towards the chelicerae and coxae for processing (‘indiscriminate sweep-feeding’). Hibbertopteroidea have a single pair of flat, laterally expanded blades on each podomere, each of which was covered in sensory setae. These would have allowed hibbertopteroids to probe the substrate and snatch up any prey they encountered (‘tactile sweep-feeding’). Plotted range data indicates that the two groups of sweep-feeders may have been in competition with one another, as the Hibbertopteroidea only fully diversify as the Stylonuroidea enter their decline. Sweep-feeding can also explain why the Stylonurina persisted through the decline of the Eurypterina during the Devonian; unable to compete for prey with the more manoeuvrable nektonic Eurypterina, 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 eurypterine decline.