The Chelicerae of Slimonia (Eurypterida; Pterygotoidea)

Abstract Pterygotid eurypterids are the most speciose eurypterid clade, accounting for almost a fifth of the approximately 250 known species, although it is unclear whether this increase in diversity is due to their increased dispersal ability, shift in predation style to cheliceral-driven prey capture, or some other factor. Determining if the development of their characteristic large chelicerae represents a key trait facilitating increased diversification is hindered by uncertainty regarding the form of the chelicerae in Slimonia, the sister taxon to Pterygotidae. Here I report the discovery of a specimen of Slimonia acuminata preserving the chelicerae in detail and corroborate reports from the 1800s suggesting that the chelicerae of Slimonia were short and robust. The evidence from the new specimen, taken in concert with the morphology of the rest of the animal, indicates that Slimonia was an active predator that captured prey with its robust prosomal appendages. The apparent increase in pterygotid species diversity therefore does indeed seem to be associated with the development of the large chelicerae; however, further work is needed to determine whether taphonomic biases in preservation due to increased sclerotization of the chelicerae or taxonomic oversplitting due to minor changes in denticle morphology are driving this phenomenon.