The species "P. monroensis", known from deposits of late Wenlock to Ludlow age in New York State, USA, was suggested to represent a synonym of "Erettopterus osiliensis" by Samuel J. Ciurca Jr. and O. Erik Tetlie in 2007, based upon the similar shape of the eyes and the carapace. Such a reassignment would have implications for other species of "Pterygotus" as well, with "P. impacatus" potentially also representing a synonym of "E. osiliensis". Subsequent studies and lists of eurypterid species have continued to treat "P. monroensis" and "P. impacatus" as distinct species of "Pterygotus".