sweep-feeding strategies in stylonurid eurypterids
James C. Lamsdell
4 Hardings Close, Iver Heath, Bucks, SL0 0HL
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.