Although some pterygotid eurypterids, such as Jaekelopterus or Acutiramus grew to gigantic proportions, it is estimated that the largest species of Erettopterus, E. osiliensis, reached 90 cm (35 in).

Erettopterus was a big eurypterid, with E. osiliensis, the largest species, measuring approximately 90 cm (35 in) in length.

[3] The form of the chelicera within Erettopterus was very variable,[4] but they were generally very long with small curved teeth without serrations.

Its metastoma (a large plate that is part of the abdomen) was very narrow and cordated (heart-shaped) anteriorly deeply notched (V-shaped slit).

[6] The compound eyes were broadly crescentic and convex,[6] and as in the rest of the pterygotioids, they were located in the margin of the carapace.

[11] The English geologist Salter recognized in 1859 that it was possible to divide the genus Pterygotus based on the morphology of the telsons of the species that had been assigned to it.

It is known for multiple poorly conserved specimens discovered in deposits of the Přídolí (Late Silurian) age.

[5] E. laticauda was first described as a variety of E. osiliensis by the same author, but due to telson and metastoma differences, it was elevated to the species range.

E. marstoni (honoring Alfred Marston, responsible for several collections of fishes and eurypterids) is based on incomplete specimens, which together represent an almost complete chelicera.

This feature is also present in E. brodiei, only known from one specimen (FMNH 89411, located in the Field Museum of Natural History) which consists of an uncrushed chela.

Its specific name honors Peter Bellinger Brodie, whose collections of eurypterids have helped the scientific community.

This species is based on a complete and well-preserved free ramus (FMNH 5104, in the Field Museum of Natural History).

The holotype (FMNH 157, housed at the Field Museum of Natural History) consists of an unusually well preserved metastoma.

The metastoma has been defined as roughly ellipsoidal, truncated posteriorly and not excessively cordate at the anterior margin.

[3] In 1966, another species would be included in the genus by Kjellesvig-Waering and Willard P. Leutze based on one chelicera, which is the holotype, two prosomas (head) and one metastoma.

This species differs from others in having narrow lateral lobes, in having coarse striations on the cordated area and in its carinated (keel-like) telson, which is what gives the specific name.

[5] However, it has been questioned whether chelicerae serve as a factor for the distinction between genera since their morphology is dependent on lifestyle and vary throughout ontogeny (the formation and individual development of an organism), although they could be acceptable for the differentiation between species.

[23] The cladogram below published by Braddy et al. (2007) is based on the nine best-known pterygotid species and two outgroup taxa (used as a reference group), Slimonia acuminata and Hughmilleria socialis.

The cladogram also contains 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").

[23] The cheliceral morphology and visual acuity of the pterygotid eurypterids separates them into distinct ecological groups.

The Llandovery-aged deposits of the Kip Burn and Patrick Burn formations in Lesmahagow, where the first fossils of E. bilobus were found, preserve fossils of a large amount of other eurypterids, including Nanahughmilleria lanceolata, Hardieopterus lanarkensis, Eusarcana obesus, Parastylonurus sigmoidalis, Carcinosoma scorpionis and Slimonia acuminata.

Other organisms found in the zone include the gastropod Euomphalopterus, the hyperoartid Jamoytius or the thelodontid Logania.

[27][28] The deposits of the Rootsikula Formation in Saaremaa in which fossils of E. laticauda and E. osiliensis have been found shelter various faunas of eurypterids such as Mixopterus simonsi, Strobilopterus laticeps and Eysyslopterus patteni.

[29] Other fossils have also been found in different places, such as Great Britain, Canada, Scandinavia and the United States.