Pterygotidae

Pterygotidae (the name deriving from the type genus Pterygotus, meaning 'winged fish') 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 (6.6 ft) in length. Their fossilized remains have been recovered in deposits ranging in age from 428 to 391 million years old (Late Silurian to Middle Devonian). 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 (the posteriormost segment of the body) likely used as rudders to provide additional agility and enlarged chelicerae (frontal appendages) with claws. These claws were robust and possessed teeth which would have made many members of the group formidable predators. 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. Pterygotid eurypterids, which occur in strata ranging from Late Silurian to Middle Devonian in age, ranged in size from quite small animals, such as Acutiramus floweri at 20 cm (7.9 in), 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 (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. The telson (the posteriormost segment of the body) was expanded and flattened with a small median keel. 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). 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. 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 fish', and only recognized their nature as arthropod remains five years later in 1844.