Pterygotidae (the name deriving from the type genus Pterygotus, meaning "winged one") is a family of eurypterids, an extinct group of aquatic arthropods.

Several evolutionary innovations made the pterygotids unique among the eurypterids, with large and flattened telsons (the posteriormost division of the body) likely used as rudders to provide additional agility and enlarged chelicerae (frontal appendages) with claws.

The strange proportions and large size of the pterygotid eurypterids led to the quarrymen who discovered the first fossil remains of the group to give them the common name "Seraphims".

[4] 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.

The posterior margin (tip) of the telson forms a short spine in some genera (Pterygotus and Acutiramus) and is indented (giving a bilobed appearance) in others (Erettopterus).

[10] 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.

[8] Kjellesvig-Waering placed the primary taxonomical value on the morphology of the telson, considering potential differences in the chelicerae and metastoma (a large plate that is part of the abdomen) to be secondary in importance.

[10] Jaekelopterus, previously designated as a species of Pterygotus, was separated into a distinct genus in 1964 based on the supposed different segmentation of the genital appendage.

[9] 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.

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.

[12] 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).

The appendages were similar to those of Slimonia but the carapace clearly belonged to a pterygotid, further suggesting a close relationship between the Pterygotidae and the Slimonidae within the Pterygotioidea superfamily.

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.

Based on the feeding process seen in modern arthropods with chelicerae, one of the claws would hold the prey while the other would cut off pieces and transport it to the mouth with continuous and simple movements.

The posterior margin (tip) of the telson form a short spine in some genera, such as Pterygotus and Acutiramus, and is indented (giving a bilobed appearance) in Erettopterus.

[6] An alternate hypothesis first proposed by C. D. Waterston in 1979 postulates that the median keel and the telson at large was used to steer the body, working more like a vertical and horizontal rudder than a tail fluke.

[4] Except the cheliceral claws, which are robust and heavily sclerotized, a majority of fossilized large pterygotid body segments are unmineralized and thin.

Even the plates that form the surface of the abdominal segments, the stergites and sternites, are preserved as paper-thin compressions which suggests that pterygotids were very light-weight in construction.

Giant eurypterids of other lineages, notably the deep-bodied walking forms of the Hibbertopteridae, such as the almost 2 metre long Hibbertopterus, might have rivalled the pterygotids in weight, if not surpassed them.

[6][16] 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".

Yet, a comprehensive species-level phylogenetic analysis has proven impossible due to the large amount of species based on scant and fragmentary fossilised material.

The cladogram also contains the primary unifying characteristics for the various clades, as well as the maximum sizes reached by the species in question, which have been suggested to possibly have been an evolutionary trait of the group per Cope's rule ("phyletic gigantism").

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.

[18] 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.