Eurypterid
History of study

The first known eurypterid specimen was discovered in the Silurian-aged rocks of New York, to this day one of the richest eurypterid fossil locations. Samuel L. Mitchill described the specimen, discovered near Westmoreland in Oneida county in 1818. He erroneously identified the fossil as an example of the fish Silurus, likely due to the strange, catfish -like appearance of the carapace. Seven years later, in 1825, James E. DeKay examined the fossil and recognized it as clearly belonging to an arthropod. He thought the fossil, which he named Eurypterus remipes, represented a crustacean of the order Branchiopoda, and suggested it might represent a missing link between the trilobites and more derived branchiopods. The name Eurypterus derives from Greek eurús (εὐρύς 'broad, wide') and pteron (πτερόν 'wing').

In 1843, Hermann Burmeister published his view on trilobite taxonomy and how the group related to other organisms, living and extinct, in the work Die Organisation der Trilobiten aus ihren lebenden Verwandten entwickelt. He considered the trilobites to be crustaceans, as previous authors had, and classified them together with what he assumed to be their closest relatives, Eurypterus and the genus Cytherina, within a clade he named "Palaeadae". Within Palaeadae, Burmeister erected three families; the "Trilobitae" (composed of all trilobites), the "Cytherinidae" (composed only of Cytherina, an animal today seen as an ostracod) and the Eurypteridae (composed of Eurypterus, then including three species).

The fourth eurypterid genus to be described (following Hibbertopterus in 1836 and Campylocephalus in 1838, not identified as eurypterids until later), out of those still seen as taxonomically valid in modern times, was Pterygotus (lit. ' winged one '), described by Louis Agassiz in 1839. Pterygotus was considerably larger in size than Eurypterus and when the first fossils were discovered by quarrymen in Scotland they were referred to as " Seraphims " by the workers. Agassiz first thought the fossils represented remains of fish, only recognizing their nature as arthropod remains five years later in 1844.

In 1849, Frederick M'Coy classified Pterygotus together with Eurypterus and Belinurus (a genus today seen as a xiphosuran) within Burmeister's Eurypteridae. M'Coy considered the Eurypteridae to be a group of crustaceans within the order Entomostraca, closely related to horseshoe crabs. A fourth genus, Slimonia, based on fossil remains previously assigned to a new species of Pterygotus, was referred to the Eurypteridae in 1856 by David Page.

Jan Nieszkowski's De Euryptero Remipede (1858) featured an extensive description of Eurypterus fischeri (now seen as synonymous with another species of Eurypterus, E. tetragonophthalmus), which, along with the monograph On the Genus Pterygotus by Thomas Henry Huxley and John William Salter, and an exhaustive description of the various eurypterids of New York in Volume 3 of the Palaeontology of New York (1859) by James Hall, contributed massively to the understanding of eurypterid diversity and biology. These publications were the first to fully describe the whole anatomy of eurypterids, recognizing the full number of prosomal appendages and the number of preabdominal and postabdominal segments. Both Nieszkowski and Hall recognized that the eurypterids were closely related to modern chelicerates, such as horseshoe crabs.

In 1865, Henry Woodward described the genus Stylonurus (named and figured, but not thoroughly described, by David Page in 1856) and raised the rank of the Eurypteridae to that of order, effectively creating the Eurypterida as the taxonomic unit it is seen as today. In the work Anatomy and Relations of the Eurypterida (1893), Malcolm Laurie added considerably to the knowledge and discussion of eurypterid anatomy and relations. He focused on how the eurypterids related to each other and to trilobites, crustaceans, scorpions, other arachnids and horseshoe crabs. The description of Eurypterus fischeri by Gerhard Holm in 1896 was so elaborate that the species became one of the most completely known of all extinct animals, so much so that the knowledge of E. fischeri was comparable with the knowledge of its modern relatives (such as the Atlantic horseshoe crab). The description also helped solidify the close relationship between the eurypterids and other chelicerates by showcasing numerous homologies between the two groups.

In 1912, John Mason Clarke and Rudolf Ruedemann published The Eurypterida of New York in which all eurypterid species thus far recovered from fossil deposits there were discussed. Clarke and Ruedemann created one of the first phylogenetic trees of eurypterids, dividing the order into two families; Eurypteridae (distinguished by smooth eyes and including Eurypterus, Anthraconectes, Stylonurus, Eusarcus, Dolichopterus, Onychopterus and Drepanopterus) and Pterygotidae (distinguished by faceted eyes and including Pterygotus, Erettopterus, Slimonia and Hughmilleria). Both families were considered to be descended from a common ancestor, Strabops. In line with earlier authors, Clarke and Ruedemann also supported a close relationship between the eurypterids and the horseshoe crabs (united under the class Merostomata) but also discussed alternative hypotheses such as a closer relation to arachnids.
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