Pterygotus is an extinct genus of giant predatory eurypterid, a group of extinct aquatic arthropods. Fossils of Pterygotus have been discovered in deposits ranging in age from Middle Silurian to Late Devonian, and have been referred to several different species. Fossils have been recovered from five continents; Australia, Asia, Europe, North America and South America, which indicates that Pterygotus might have had a nearly cosmopolitan (worldwide) distribution. The type species, P. anglicus, was described by Swiss naturalist Louis Agassiz in 1839, who gave it the name Pterygotus, meaning "winged one". Agassiz mistakenly believed the remains were of a giant fish; he would only realize the mistake five years later in 1844.
Pterygotus was among the largest eurypterids. Isolated fossil remains of a large chelicera (frontal appendage) suggests that the largest known species, P. grandidentatus, reached a body length of . Several other species, notably P. anglicus at was similarly gigantic. Pterygotus was surpassed in size by other giant eurypterids. Acutiramus was able to surpass 2 metres (6.6 ft), and Jaekelopterus could reach 2.6 metres (8.5 ft). Many species were considerably smaller than the largest species, such as P. kopaninensis at . Pterygotus may have weighed around 30 kilogramms.
Like its close relative Jaekelopterus, Pterygotus was a large and active predator noted for its robust and enlarged cheliceral claws that would have allowed it to puncture and grasp prey and a visual acuity (clarity of vision) comparable to that of modern predatory arthropods.
Description With the largest species, P. grandidentatus, reaching a body length of , Pterygotus was among the largest known eurypterids to have existed, though some of its close relatives (such as Acutiramus and Jaekelopterus) surpassed it in length. Though there were a few gigantic species, many species were considerably smaller in size. The smallest species, P. kopaninensis, measured just in length.
Pterygotus is classified as part of the pterygotid family of eurypterids, to which it lends its name, a group of highly derived eurypterids of the Silurian to Devonian periods that differ from other groups by a number of features, perhaps most prominently in the chelicerae (the first pair of limbs) and the telson (the posteriormost division of the body). The chelicerae of the Pterygotidae were enlarged and robust, clearly adapted to be used for active prey capture and more similar to the claws of some modern crustaceans, with well developed teeth on the claws, than to the chelicerae of other eurypterid groups. Unlike most of the rest of the body, which was covered in a scale-like ornamentation like other pterygotid eurypterids, the claws lacked any type of ornamentation. Additionally, the end points of the claws were round and curved unlike the sharp points present at the ends of the claws of the related Erettopterus. The pterygotid telsons were flattened and expanded, likely used as rudders when swimming. Their walking legs were small and slender, without spines, and they were likely not capable of walking on land.
Pterygotus is distinguishable from other pterygotids by the curved distal margin of the chelae (claws). The prosoma (head) is subtrapezoidal (a trapezoid with rounded corners), with compound eyes located near the edge of the front corners. The telson has a pronounced dorsal carina (or keel) running down its center, terminating in a short spine.
Size The Pterygotidae includes the largest known arthropods to have ever lived, with several species surpassing two metres in length (such as Jaekelopterus rhenaniae at and Acutiramus bohemicus at ). Though Pterygotus was not the largest of the pterygotids, several species were large, surpassing in length.
The largest known species was P. grandidentatus, with the largest known isolated chelicerae fragments suggesting a length of . P. anglicus, the type species, grew to in length, based on a large tergite discovered by Henry Woodward at some point between 1866 and 1878. Measuring just over in length and in width, the tergite suggests a eurypterid with a full length of from the beginning of the carapace to the end of the telson, if the extended chelicerae are counted (normally they are not) the total length would exceed . P. carmani, from the Devonian of Ohio, likely reached lengths in excess of .
The species P. cobbi (), P. barrandei () and P. denticulatus () also exceeded 1 metre in length. Smaller species include P. floridanus at , P. arcuatus at , and the smallest known species, P. kopaninensis, at in length.
History of research
Initial finds
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, which translates to "winged one". Agassiz believed that they were the fossil remains of a large fish. Note that this work incorrectly translates Pterygotus as "winged fish", but there is no "fish" element in the name. 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.
The new Scottish fossils were named as the species P. anglicus in 1849, which remains the most extensively known species of Pterygotus, distinguished from subsequently discovered species by possessing curved terminal teeth and the primary and intermediate teeth being inclined slightly backwards.
P. problematicus was also used as the designation for an incomplete chelicera discovered in the Welsh Borderland of western England by John William Salter in 1852 but is in modern times considered a nomen vanum ("baseless name") as the species is impossible to define. The fossil material with which it was described is undiagnostic and insufficient to establish any meaningful characteristics and as such many fragmentary pterygotid fossils have been referred to it throughout its long history, rendering it effectively synonymous with the family Pterygotidae. More often than not, these fragments consist of patches of pterygotid integument preserving the scale-like ornamentation characteristic of the group which researchers have wrongfully believed was characteristic of only Pterygotus or P. problematicus. As such ornamentation is known from every pterygotid genus it can not be used as a diagnostic feature of a single species. Though P. problematicus is the earliest name used for a species of Pterygotus, it is not considered the type species as the name is no longer in use. Instead P. anglicus, based on a number of diagnostic features and properly illustrated in its description by Agassiz in 1844, is considered the type species of Pterygotus.
Two further species that remain assigned to the genus to this day would be described from England during the 19th century; P. ludensis of Pridoli (Late Silurian) age and P. arcuatus of Ludlow (Late Silurian) age, along with a North American species, P. cobbi, from the Pridoli of the United States and Canada. P. arcuatus was originally described from a syntypic series (a series of specimens out of which a particular holotype is not designated) of fossil remains by John William Salter, containing five separate fossil specimens. In 1961, 102 years after its description, Erik N. Kjellesvig-Waering noted that only one of these specimens (Number 89587 of the Geological Survey and Museum of London, a free ramus of a chelicera) might truly be referrable to this distinct species as the other fossils (tergites, coxae and indeterminable fragments) might actually represent fossils of other species due to not being diagnostic enough. P. ludensis, described by Salter in 1859, can be distinguished from other species by the more developed and prolonged keel (or ridge) running along the center of the telson from its beginning to the tail spike.
The rare species P. cobbi, described by James Hall in 1859 based on fossils recovered in New York, USA, was the first species of Pterygotus to be described from outside of Scotland and expanded the known range of the genus considerably. Hall described this new species alongside two other North American species; P. macrophthalmus (now referred to Acutiramus) and P. osborni (later synonymized with P. macrophthalmus). The distal tooth of the free ramus (the part of the claw that moves) was less prominent than in other species, which has been noted as similar to the distal tooth in the free ramus of Acutiramus cummingsi. Although P. cobbi is based on poor fossil material, only known from a free ramus, it remains recognized as a distinct species on the account of being more similar to certain species discovered in the Czech Republic (such as P. barrandei) than it is to other pterygotid species discovered in the P. cobbi locality (such as Acutiramus macrophthalmus).
Creation of subgenera and the Pterygotidae
By 1859, 10 species had been assigned to the genus,Dunlop, J. A., Penney, D. & Jekel, D. 2018. A summary list of fossil spiders and their relatives. In World Spider Catalog. Natural History Museum Bern and John William Salter recognized that it was possible to divide these species into subgenera based on the morphology of the telsons. Salter erected the subgenus Pterygotus (Erettopterus) for species with a bilobed telson. Further subgenera would be named as more differences were noted between the species, such subgenera include Pterygotus (Curviramus) and Pterygotus (Acutiramus), named in 1935 based upon features of the denticles (teeth) of the chelicerae. Pterygotus (Curviramus) was later recognized as synonymous with Pterygotus (Pterygotus) by Leif Størmer the same year, and Erettopterus and Acutiramus would be recognized as separate, but closely related, genera (Erettopterus by Erik N. Kjellesvig-Waering in 1961, and Acutiramus by Størmer in 1974).
In 1912, the family Pterygotidae was erected by John Mason Clarke & Rudolf Ruedemann in 1912 to include the eurypterid genera Pterygotus, Slimonia, Hughmilleria and Hastimima. The three latter genera would be reclassified as members of the Hughmilleriidae by Erik N. Kjellesvig-Waering in 1951, leaving Pterygotus and its former subgenera as the sole pterygotid eurypterids.
Further discoveries Though early discoveries of Pterygotus were confined to England and North America, fossil finds throughout the 20th century would help establish that Pterygotus as a genus achieved a nearly cosmopolitan distribution. The first eurypterid to be discovered in Australia was Pterygotus australis, whose fossils were found in the Ludlow age Melbourne Group of the Dargile Formation. The fossils referred to P. australis, consisting of four fragments making up about half of a segment that were discovered during the process of excavations beneath Melbourne during the construction of new drainage works for the city in 1899. The fragmentary fossils closely resemble fossils of Erettopterus bilobus (classified as a species of Pterygotus at the time), which might make their assignment to Pterygotus questionable. In 2020, the species was marked as a nomen dubium (a dubious species) due to the lack of sufficient diagnostic material to separate P. australis from the rest of the pterygotids.