Like father, like son? Not amongst the eurypterids (Chelicerata) from Beartooth Butte, Wyoming

The eurypterid Strobilopterus princetonii from the Lower Devonian Beartooth Butte Formation of Wyoming is redescribed, and previously unknown morphological features, including shape and position of eyes and ocelli are identified, and the prosomal appendages are described in more detail. The monotypic Erieopterus latus, based on a much smaller specimen from the same locality, shares a large number of characters with S. princetonii, and must be interpreted as belonging to the genus Strobilopterus. It is here interpreted as a juvenile S. princetonii, but exhibits extreme ontogenetic variation in the morphology of the swimming leg. The other monotypic eurypterid from the locality, Dorfopterus angusticollis, remains mysterious. It might be a S. princetonii telson, but evidence is inconclusive. The name of the Devonian eurypterid genus Syntomopterus is preoccupied for a beetle (Coleoptera: Insecta) and the replacement name Syntomopterella is proposed. The phylogenetic position of S. princetonii is problematic since it lacks unequivocal synapomorphies with other eurypterids, but the genus is showing more similarities to Syntomopterella, Erieopterus and Buffalopterus than it does to Dolichopterus, contrary to earlier suggestions.