The first fossils of "Pterygotus" were found in deposits of Lochkovian-Pragian (Early Devonian) age by quarrymen in Scotland and western England, who referred to the large fossil remains as "Seraphims". Louis Agassiz, a Swiss-American biologist and geologist, described the fossils in 1839 and named it "Pterygotus" in the belief that they were the fossil remains of a large fish; ""Pterygotus"" translating to "winged fish". The specimens described by Agassiz from England were referred to a species he dubbed "Pterygotus problematicus". Agassiz first recognized the true nature of the fossils as arthropod remains five years later in 1844 after having examined more complete fossils recovered in the Old Red Sandstone of Scotland. Although recognizing the fossils of "Pterygotus" as arthropod was closer to its modern phylogenetical position, Agassiz would consider "Pterygotus" to represent a crustacean of the Entomostraca subclass. Although Frederick M'Coy did note that "Pterygotus" resembled the Limulidae and the previously discovered eurypterid "Eurypterus" in 1849, he classified both "Eurypterus" and "Pterygotus" as crustaceans.