19th century A total of 19 valid species have been assigned to
Erettopterus. Most of the species have been found in the
United States or
Great Britain, although fossils have also been found in
Canada,
Scandinavia and
Estonia. The type species and the only species described in Scotland,
E. bilobus, was first found in
Lesmahagow, Scotland, and described by
John William Salter in 1856 as a species of
Himantopterus (a name that is a
junior homonym of
a moth genus), and Salter replaced the genus name with
Erettopterus in 1859, although it would later be referred to as a species of
Pterygotus. It is a very well known and very abundant species that since its original description has not received much attention, so it needs a redescription like other eurypterids to accommode a modern understanding of eurypterid systematics and evolution. The specific name
bilobus refers to the bilobed telson of the species. 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. He divided
Pterygotus into
subgenera, including
Pterygotus (
Erettopterus) for species with a bilobed telson. (
eréttō, rower The other was
E. grandis (
grandis because of the large size of the species) The rami end in a well-developed sharp and triangular point. from the Lochkovian age and
E. holmi from the Late Wenlock age were described in 1934 by
Leif Størmer. Additionally, he described four new English species,
E. marstoni,
E. spatulatus,
E. megalodon and
E. brodiei.
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. The
holotype (BMNH 43790, in the
British Museum of Natural History) consists of a free ramus, and the
paratype (BMNH 43805, in the same museum as the holotype) includes most of the fixed ramus, that is very slender and tapering to the curved distal end. A large tooth is present in the midsection of the ramus. The end of the ramus is broken, but probably ended in a double tooth. 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.
E. spatulatus is remarkable by its shovel-or fan-shaped telson, which is wider than long and gives it the specific name. These telsons are rare, and have only been seen in
E. grandis.
E. serratus was designated a junior synonym of
Pterygotus carmani in 2025. In 1971,
E. serricaudatus and
E. carinatus from the Early Wenlock age in Sweden were described by Kjellesvig-Waering. They were the only pterygotids in the area. The holotype of
E. serricaudatus consists of the fixed ramus with acute termination of a large chelicera. This species is remarkable due to the group of diagonal opposing teeth of the ramus. This species differs from
E. osiliensis (species in which
E. serricaudatus was erroneously included) by its outwardly bowed rami, unlike the straight rami of
E. serricaudatus. In addition, the shape and grouping of the teeth are completely different between them. The holotype of
E. carinatus is the fragment of the base of a telson that measures in length and in width that indicates that the complete telson measured of estimated length, of maximum width. 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. In 2025,
E. carinatus was designated a junior synonym of
E. serricaudatus. 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. Therefore,
Truncatiramus was later recognized as representing a synonym of
Erettopterus. In 2023, the new species
Erettopterus qujingensis was described, from the upper Silurian (Pridolian)
Yulongsi Formation of
Yunnan Province, China. ==Classification==