DISCERNING THE DIETS OF SWEEP-FEEDING EURYPTERIDS THROUGH ANALYSIS OF MESH-MODIFIED APPENDAGE ARMATURE

Eurypterids were a diverse and ecologically important group of aquatic chelicerates that lived throughout most of the Paleozoic. Two eurypterid clades, the Stylonuroidea and the Mycteropoidea, independently evolved modifications for sweep-feeding on their anterior prosomal appendages. Stylonuroidea range from the Early Silurian to the Early Devonian and had rows of fixed spines on appendages II-IV, which they are considered to have used as dragnets to entangle small prey within the sediment subsurface. Conversely, Mycteropoidea were larger, more specialized sweep-feeders ranging from the Early Silurian to the Late Permian. Most genera had blades on appendages II-III (and IV in Hibbertopteridae) that were broad and blunt but bore numerous sensory setae. Cyrtoctenus, the most specialized genus of sweep-feeders among the mycteropoids, had two rows of flexible filaments extending anteriorly from rachises (modified blades) forming comb-like structures, which were suitable for capturing smaller prey out of suspension. The mycteropoids radiated in the Late Devonian and Carboniferous, and they were one of only two groups of eurypterids that persisted into the late Paleozoic. Their success has been attributed to their sweep-feeding mode of life which allowed them to avoid competition from nektonic predators. Among extant suspension-feeders, it has been shown that there is a linear correlation between the average spacing of feeding structures and prey sizes. Thus, the prey sizes of the stylonuroids and mycteropoids can be estimated from measurements of the spacing between their spines, blades, or filaments, offering insight into how sweep-feeding eurypterid prey sizes may have changed over time, particularly during the Late Devonian mass extinction when the majority of eurypterid clades became extinct.