DETERMINING THE PREY SIZES OF SWEEP-FEEDING EURYPTERIDS VIA ANALYSIS OF APPENDAGE ARMATURE

Eurypterids were a group of aquatic chelicerates that lived throughout most of the Paleozoic. While swimming eurypterids are generally considered to be active predators, the benthic stylonurine eurypterids appear to have had a mode of life similar to modern horseshoe crabs with the exception of two clades, the Stylonuroidea and the Mycteropoidea, both of which independently evolved modifications on their anterior appendages that may have been used for sweep-feeding. Of these two clades, it has been suggested that the mycteropoids were more specialized sweep-feeders; rather than indiscriminately raking through the substrate, their armature included sensory setae which may have allowed them to detect organisms in the subsurface. While the stylonuroids did not survive the Late Devonian mass extinction, the mycteropoids radiated in the Late Devonian and persisted into the late Permian.

Among extant suspension feeders, it has been shown that there is a linear correlation between the average spacing of feeding structures and prey sizes. This relationship was extrapolated to the stylonurine eurypterids in order to estimate the range of prey sizes that they could capture and to determine how prey sizes may have changed over time, particularly during the Late Devonian mass extinction when the majority of eurypterid clades became extinct. It was hypothesized that prey sizes decreased over time as sweep-feeding eurypterids evolved more advanced appendage armature and the success of the mycteropoids was due to their ability to capture smaller prey items.

The majority of eurypterids were found to have similar prey sizes that correlated to a diet of small macrofauna, while adult individuals of one mycteropoid family, the Hibbertopteridae, were capable of capturing mesoplankton. These results suggest that only the hibbertopterids were specialized sweep-feeders. Furthermore, prey size does decrease over time nor mediate survival in the Late Devonian. While the success of the hibbertopterids may have been due to their wide distribution and sweep-feeding mode of life which allowed them to avoid competition with nektonic predators, mycteropoids as a whole likely benefited from the expanding benthos, which was invading freshwater environments in the Devonian and Carboniferous.