Pterygotidae
Pterygotidae is a family of eurypterids, an extinct group of aquatic arthropods. They were members of the superfamily Pterygotioidea. Pterygotids were the largest known arthropods to have ever lived with some members of the family, such as Jaekelopterus and Acutiramus, exceeding 2 metres in length. Their fossilized remains have been recovered in deposits ranging in age from 428 to 372 million years old.
One of the most successful groups of eurypterids, the pterygotids were the only eurypterid family to achieve a truly worldwide distribution. Several evolutionary innovations made the pterygotids unique among the eurypterids, with large and flattened telsons likely used as rudders to provide additional agility and enlarged chelicerae with claws. These claws were robust and possessed teeth which would have made many members of the group formidable predators. 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".
Studies on the cheliceral morphology and compound eyes of the pterygotids have revealed that the members of the group, despite overall morphological similarities, were highly divergent in their ecological roles. Pterygotid ecology ranged from generalized predatory behaviour in basal members of the group, such as Erettopterus, to active apex predators, such as Jaekelopterus and Pterygotus, and ambush predators and scavengers, such as Acutiramus .
Some researchers have suggested that the pterygotid eurypterids evolved in something akin to an "arms race" with early vertebrates, that the evolution of heavy armor in the ostracoderms could be attributed to pressure from pterygotid predation and that later pterygotid decline could be attributed to subsequent evolutionary trends in fish. This hypothesis is mostly considered as far too simplistic of an explanation by modern researchers. Detailed analyses have failed to find any correlation between the extinction of the pterygotids and the diversification of the vertebrates.
Description
Pterygotid eurypterids, which occur in strata ranging from Late Silurian to Late Devonian in age, ranged in size from quite small animals, such as Acutiramus floweri at 20 cm, to the largest known arthropods to have ever lived. Several species reached and exceeded 2 metres in length, the largest known species including Jaekelopterus rhenaniae at 2.5 metres and Acutiramus bohemicus at 2.1 metres.Like all other chelicerates, and other arthropods in general, pterygotid eurypterids possessed segmented bodies and jointed appendages 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 ; the frontal prosoma and posterior opisthosoma. The appendages were attached to the prosoma, and were characterized in pterygotids by being small and slender and lacking spines. The telson was expanded and flattened with a small median keel. The posterior margin of the telson forms a short spine in some genera and is indented in others.
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. 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
Due to their unique features within the Eurypterida, the 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 had been assigned to Pterygotus. John William Salter recognized that it was possible to divide Pterygotus based on the morphology of the telsons of the species that had been assigned to it. He divided Pterygotus into subgenera, erecting Pterygotus for species with a bilobed telson.
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 and Pterygotus in 1935, differentiated by the curvature of the denticles of the chelicerae. The same year, Leif Størmer named a new pterygotid genus, Grossopterus, and split Pterygotus into two other subgenera, Pterygotus and Pterygotus, designating Pterygotus as a junior synonym of Pterygotus and not recognizing Pterygotus. A division into three subgenera of Pterygotus was proposed by Ferdinand Prantl and Alois Přibyl in 1948, retaining P. and P. but also restoring P. to subgenus level.
Erik N. Kjellesvig-Waering emended the family in 1951, when the genera Hastimima, Hughmilleria, Grossopterus and Slimonia were referred to their own family, the Hughmilleriidae, which left Pterygotus as the only genus within the Pterygotidae. In 1961, Kjellesvig-Waering raised Erettopterus to the level of its own genus, recognizing two subgenera of Pterygotus; P. and P., as well as two subgenera of Erettopterus; E. and E.. Kjellesvig-Waering placed the primary taxonomical value on the morphology of the telson, considering potential differences in the chelicerae and metastoma to be secondary in importance.
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. These supposed differences would later turn out to be false, but briefly prompted Jaekelopterus to be classified within a family of its own, the "Jaekelopteridae". The error with the genital appendage was later discovered and rectified, making Jaekelopterus a member of the Pterygotidae once more. In 1974, Størmer raised the Pterygotus subgenera Acutiramus and Truncatiramus to the level of separate genera. Truncatiramus has later been recognized as representing a synonym of Erettopterus.
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. However, in a 2025 paper Ciurcopterus was separated out from Pterygotidae and placed in a new family, Ciurcopteridae.
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 to 372 million years old, reaching their greatest diversity during the Late Silurian, a period in time when other eurypterid groups became increasingly diverse as well. 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.The most primitive and basal pterygotid, Ciurcopterus, preserves a mixture of characteristics that are reminiscent of Slimonia, which is often interpreted as a sister-taxon of the Pterygotidae, as well as more derived pterygotids. 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.
Potential influence in vertebrate evolution
suggested in 1933 that early vertebrate evolution might have been heavily influenced by pterygotid predation. Early vertebrates of the Late Silurian and Devonian are often heavily armored, and it is likely that this represents an ancestral vertebrate trait that was later lost or reduced, rather than something that evolved separately in several groups at the same time. Some researchers have suggested that the armor was to protect against hitting rocky surfaces in fast flowing streams, but Romer pointed out that there is no such armor protection in modern fish that live in that type of environment. Instead, Romer stated that the only reasonable explanation for the armor was as "a protection against living enemies".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.