"P. impacatus," recovered from deposits of Ludlow age at Kielkond in Saaremaa, Estonia, is represented by a holotype specimen consisting of a fragmentary free ramus of a chelicera that preserves some diagnostic and well-preserved details (specimen number 7059/7 housed in the Museum of Comparative Zoology). Alongside this specimen, important specimens referred to the species include the paratype (No. 7059/3, a fragmentary chelicera). This specimen includes several of the features that are diagnostic of "P. impacatus", such as upright teeth following the thick and long teeth of the terminal part of the claw. Particularly of diagnostic value is that there are teeth present at the point where the terminal teeth first begin. The central tooth of the free ramus is unusually large in this species. The terminal tooth measures 2.3 cm (1 in) in length. The rami of "P. impacatus" are ornamented with large and pointed pustules (elevations in the skin), and this feature helps distinguish specimens of "P. impacatus" from other pterygotids in the fossil sites where its remains are found. In 2007, O. Erik Tetlie cast doubt on the assignment of "P. impacatus" to "Pterygotus" as the shape of the eyes and carapace was similar to how these body parts are shaped in "Erettopterus".