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

Discerning the Diets of Sweep-Feeding Eurypterids Through Analyses of Mesh-Modified Appendage Armature Emily Hughes 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 for sweep-feeding on their anterior appendages. 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 sweepfeeding stylonuroid and mycteropoid eurypterids in order to estimate the range of prey sizes that they could capture. The majority of eurypterids are found to have armature spacing within a range of 2.5mm to 4.6mm, which corresponds to a prey size range of 3.6mm to 58mm. This suggests that the armature of the sweep-feeding eurypterids was optimally suited for capturing small macrofauna. Benthic macroinvertebrates such as crustaceans, mollusks, and worm-like organisms would have been manageable prey items. The mycteropoid Cyrtoctenus was the most specialized genus of sweep-feeders, and its appendage armature consisted of two rows of flexible filaments extending anteriorly from rachises forming comb-like structures. The rachises were associated with moveable spines on the anterior-ventral sides of the podomeres, which would scrape the filaments and eject trapped food particles into the oral region. Cyrtoctenus had the smallest estimated prey size among eurypterids, and its inter-filament spacing suggests its diet consisted of mesoplankton. 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 was likely due to their wide distribution and sweep-feeding mode of life, which allowed them to avoid competition with nektonic predators while capitalizing on the expanding benthos, which was invading freshwater environments in the Devonian and Carboniferous. It has been proposed that several of the mycteropoid genera are synonyms. In particular, Hibbertopterus may be a juvenile form of Cyrtoctenus in which case the comb-like rachises did not develop until the late stages of ontogeny, and juveniles retained broad blades suited to probing for prey in soft sediment but incapable of capturing small prey out of suspension. In order to evaluate the likelihood of synonymy, the ornamentation of several mycteropoids was compared and found to be largely consistent in Hibbertopterus, Dunsopterus, and Cyrtoctenus. The strong morphological similarities between Hibbertopterus and Dunsopterus in particular warrant their synonymization, and because Dunsopterus is paraphyletic with respect to Cyrtoctenus, all three genera can be synonymized. The overall form and ornamentation of the moveable blades of Hibbertopterus suggests that these blades are the precursors to the moveable fingers of Cyrtoctenus. The comb rachises of Cyrtoctenus are much thinner than the moveable fingers and lack folliculated ornamentation, suggesting that they are not derived from the same moveable blades, but rather from fixed posterior-ventral spines. This ontogenetic niche shift would have resulted in distinct prey sizes between juveniles and adults which would have increased survival rates by reducing intraspecific competition.