"P. grandidentatus" is known from a single specimen, the anterior half of a free ramus of a chelicera discovered in the Wenlock-aged beds at Dudley in Worcestershire, England (specimen number I. 3163 in the British Museum of Natural History). It is notable for the stout stem and the unusually long length (1.75 cm, 0.7 in) of the terminal tooth and the unusual thickness of its base. "P. grandidentatus" can easily be distinguished from other species not only be its unusual terminal tooth, but also by the disoriented teeth along the claw, being bent in a variety of different directions. The terminal part of the ramus ends in an arrangement of multiple teeth otherwise only noted in the species "P. waylandsmithi". As this specific part of the ramus is poorly known from other species of "Pterygotus" and "P. waylandsmithi" was reclassified as a species of "Erettopterus" in 2007 the assignment of "P. grandidentatus" to "Pterygotus" is questionable.