Pterygotidae

''. Pterygotid eurypterids, which occur in strata ranging from Late Silurian to Late Devonian in age, '', showcasing the distinct scale-like ornamentation of the pterygotids. Like all other chelicerates, and other arthropods in general, pterygotid eurypterids possessed segmented bodies and jointed appendages (limbs) covered in a cuticle composed of proteins and chitin. In pterygotids, the outer surface of the exoskeletons, ranging in size from small to gigantic, was composed of semilunar scales. The chelicerate body is divided into two tagmata (sections); the frontal prosoma (head) and posterior opisthosoma (abdomen). The appendages were attached to the prosoma, and were characterized in pterygotids by being small and slender and lacking spines. Like other chelicerates, pterygotids possessed chelicerae. These appendages are the only ones that appear before the mouth and take the form of small pincers used to feed in all other eurypterid groups. In the pterygotids, the chelicerae were large and long, with strong well developed teeth on specialised chelae (claws). These specialised chelicerae, likely used for prey capture but differing in the exact role from genus to genus, are also the primary feature that distinguishes members of the group from eurypterids of the other pterygotioid families, Slimonidae and Hughmilleriidae, and other eurypterids in general. == History of research ==

'' and other eurypterids illustrated by Joseph Smit in 1896. Due to their unique features within the Eurypterida, Pterygotidae has attracted a lot of attention ever since their discovery. The first fossils found, discovered by quarrymen in Scotland, were referred to as "Seraphims" by the quarrymen. When describing Pterygotus itself in 1839, Louis Agassiz first thought the fossils represented remains of fish, with the name meaning "winged one", and only recognized their nature as arthropod remains five years later in 1844. By 1859, 10 species (many of which would later be reassigned) had been assigned to Pterygotus. The family Pterygotidae was erected in 1912 by John Mason Clarke & Rudolf Ruedemann to constitute a group for the genera Pterygotus, Slimonia, Hastimima and Hughmilleria. Pterygotus would also designated as containing two "subgenera", Pterygotus (Curviramus) and Pterygotus (Acutiramus) in 1935, differentiated by the curvature of the denticles (teeth) of the chelicerae. In 1986, Paul Selden examined the fossil material of the enigmatic arthropod Necrogammarus and concluded that the specimen represents the infracapitulum and attached palp of a large pterygotid. The fossil likely belongs to either Erettopterus marstoni or Pterygotus arcuatus, both found in the same locality, but the lack of key diagnostic features in the Necrogammarus remains makes assignment to either impossible, and therefore, Necrogammarus is considered an unspecified pterygotid. In 2009, Pterygotus ventricosus was recognized as being distinct from, and far more basal than, other species in its genus and was thus named as the type species of a new genus, Ciurcopterus. Studies of specimens referred to this genus resolved long-standing contentiousness about the precise phylogenetic position of the Pterygotidae, providing evidence in the form of shared characteristics that Slimonia, not Herefordopterus or Hughmilleria as previously thought, was the closest sister taxon of the group. == Evolutionary history ==

The pterygotids were one of the most successful eurypterid groups, with fossilised remains having been discovered on all continents except Antarctica. They are the only eurypterid group with a cosmopolitan distribution. Their remains range in age from 428 reaching their greatest diversity during the Late Silurian, The enlargement and specialisation of the chelicerae within the Pterygotidae has been recognised as one of the two most striking evolutionary innovations within the Eurypterida, besides the transformation of the most posterior prosomal appendage into a swimming paddle (a trait seen in all eurypterids in the Eurypterina suborder). With most of the early vertebrates of the Silurian being just a few decimetres in length and often occurring together with pterygotid eurypterids in freshwater environments, they would seem to represent appropriate prey for the pterygotids, which were large predators with grasping claws. There are few other animals that would present appropriate prey and there are virtually no other predators than the pterygotids that would warrant the evolution of armored protection in their prey. The pterygotids reached their maximum size and number in the Late Silurian and Early Devonian, after which they saw rapid decline during the Devonian. This decline occurred at around the same time as there was an increase in unarmored vertebrates as well as a growth in fish size and the increased migration of fish into marine environments. The Devonian would also see the evolution of significantly faster-moving fish and the evolution of proper jaws. These adaptations, potentially a result of pterygotid predation, would have significantly affected the likelihood of fish representing pterygotid prey and larger predatory fish may even have begun preying on pterygotids and other eurypterids, contributing to their decline and extinction. The arguments of Romer were based on evolutionary trends in both groups and the fossil co-occurrences of both groups but he did not present a detailed analysis. The groups do frequently occur together, with pterygotids present at more than two-thirds of fossil localities where eurypterids and fish are recorded together. There is also a recorded increase in fish diversity at the same time as the eurypterids began to decline in the early Devonian, but available data does not support any direct competitive replacement. Though the pterygotids would be extinct at that point, both fish and eurypterids would decline in the Middle Devonian only to peak again in the late Devonian and to begin another decline in the Permian. Detailed analyses have failed to find any correlation between the extinction of the pterygotids and the diversification of the vertebrates. == Paleobiology ==

Cheliceral claws (cheliceral claw) of Pterygotus. The function of pterygotid chelicerae was likely the same as the chelicerae of other eurypterids as well as those of other arthropods, such as crustaceans and xiphosurans; the capture and cutting of food into smaller pieces and transport of food into the mouth as well as defense. Though most other eurypterid families had simple pincers, the Pterygotidae is the only eurypterid family to possess enlarged and robust chelicerae with claws and teeth, showing unique adaptations to defense and/or prey capture. and may be vital to the evolution of arthropod gigantism as a light-weight build decreases the influence of size-restricting factors. == Classification ==

''. Taxonomy Since its creation by John Mason Clarke & Rudolf Ruedemann in 1912, the phylogenetic status of the Pterygotidae has changed several times. Leif Størmer considered the group to represent a family within the eurypterid superfamily Eurypteracea. Both Størmer (in 1974) and Erik N. Kjellesvig-Waering (in 1964) would come to consider the pterygotids as distinctive enough, due to their uniquely enlarged chelicerae, to warrant the status of a separate suborder, which was dubbed the "Pterygotina". == Paleoecology ==

, including Dolichopterus, Eusarcana, Stylonurus, Eurypterus and Hughmilleria. Pterygotus can be seen in the center-left.|left Traditionally interpreted as visual and active predators as a group, recent studies on the cheliceral morphology and visual acuity of the pterygotid eurypterids have revealed that it is possible to separate them into distinct ecological groups. The primary method for determining visual acuity in arthropods is by determining the number of lenses in their compound eyes and the interommatidial angle (abbreviated as IOA and referring to the angle between the optical axes of the adjacent lenses). The IOA is especially important as it can be used to distinguish different ecological roles in arthropods, being low in active predators. Despite morphological similarities within the group, the ecology of the pterygotids differed greatly from genus to genus. The vision of Erettopterus was similar to that of the more basal pterygotoid Slimonia and more acute than the more derived Acutiramus, though it was not as acute as the vision of the apex predators Jaekelopterus and Pterygotus or modern actively predatory arthropods. Additionally, the chelicerae of Erettopterus suggest that it was a generalised feeder rather than a highly specialised predator. The claws in Erettopterus are enlarged, as in other pterygotids, though the differentiated denticles and paired distal teeth mean that they were likely not used for specialised feeding, but solely for grasping. Though the number of lenses in its compound eyes is comparable to more derived members of the group, its morphology suggests that it was not as active, nor as specialised as Pterygotus or Jaekelopterus. The eyes of Acutiramus were low in visual acuity (with few lenses in the compound eyes and high IOA values), inconsistent with the traditionally assumed pterygotid lifestyle of "active and high-level visual predators". The IOA values of Acutiramus changed during ontogeny but in a way opposite to other pterygotids. Vision becomes less acute in larger specimens, whilst vision tends to get more acute in adults in other genera, such as in Jaekelopterus. Pterygotids may thus have been almost equally visually acute early in their life cycle, becoming more differentiated during growth. The chelicerae of Acutiramus likely served as slicing or shearing devices, adding to the evidence that it would have occupied a distinct ecological niche. A significantly less active predator, Acutiramus might have been a scavenger or ambush predator, feeding on soft-bodied animals. Both Jaekelopterus and Pterygotus have a very high visual acuity, which researchers could determine by observing low IOA values and large numbers of lenses in their compound eyes. The chelicerae of these genera were enlarged, robust and possessed a curved free ramus and denticles of different lengths and sizes, all adaptations that correspond to strong puncturing and grasping abilities in extant scorpions and crustaceans. These genera likely represented active and visual apex predators. ==See also==