is very distinctive and though its function remains unknown (possibly used for additional balancing), it was likely not used as a weapon in the same way the telson of Slimonia was. The flattened portion is trigonal and smaller than that of Slimonia but the telson spike is far longer, forming something akin to a stem with knobs running alongside it and ending in a tri-lobed organ unseen in any other eurypterid. Classification Slimonid eurypterids are classified as part of the Pterygotioidea superfamily, within the Diploperculata infraorder and Eurypterina suborder. The Slimonidae is often interpreted as a sister-taxon (the most closely related group) to the Pterygotidae. The other Pterygotioid family, the Hughmilleriidae, has also been interpreted as the most closely related sister-taxon to the pterygotids. The discovery of Ciurcopterus, currently the most primitive known pterygotid, allowed researchers to study its features which showed that the primitive pterygotid combined characteristics of more derived members of its own family and Slimonia. In particular, the similar appendages shared between the two genera suggested that the Slimonidae was the most closely related group to the Pterygotidae. Hughmilleriid eurypterids are thus seen as a group more basal than both the slimonids and the pterygotids. The cladogram below is based on the conclusions drawn by O. Erik Tetlie (2004) on the phylogenetic positions of Herefordopterus, Salteropterus and the Pterygotioidea at large following his redescriptions of various eurypterids from Herefordshire, England, including Salteropterus. The partial lack of spines on the appendages of both slimonid genera unites them as a group and showcases that they are more derived than the hughmilleriids, on which spines appear on four to five of their podomeres (leg segments). See also List of eurypterid genera Timeline of eurypterid research Hughmilleriidae Pterygotidae References Silurian animals Pterygotioidea Prehistoric arthropod families