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Parahughmilleria (meaning "near Hughmilleria"Meaning of para- at www.dictionary.com. Retrieved 17 July 2018.) is a genus of eurypterid, an extinct group of aquatic . Fossils of Parahughmilleria have been discovered in deposits of the Devonian and Silurian age in the United States, Canada, Russia, Germany, Luxembourg and Great Britain, and have been referred to several different species. The first fossils of Parahughmilleria, discovered in the Shawangunk Mountains in 1907, were initially assigned to Eurypterus. It would not be until 54 years later when Parahughmilleria would be described.
Parahughmilleria is classified in the family Adelophthalmidae, the only clade in the superfamily Adelophthalmoidea. This clade was characterised by their small size, their parabolic (approximately U-shaped) carapaces and the presence of epimera (lateral "extensions" of the segment) on the seventh segment, among others. Like its relatives, Parahughmilleria possessed reniform (bean-shaped) eyes and spines on its . The largest species was P. major at 12.5 cm (5 in), making it a small-sized eurypterid, although it has been suggested that it and P. hefteri may represent the same species.
It had a semicircular carapace (head plate) with reniform compound eyes placed forward, which is its main characteristic. The metastoma (a large plate that is part of the abdomen) had a deep anterior triangular notch. The telson (the posteriormost division of the body) was wide and lanceolate. The preabdomen (body segments 1 to 7) had a lanceolate shape with a big epimera (lateral "extensions" of the segment) on the 7th tergite (dorsal portion of an arthropod segment). The 3rd tergite was the most wide segment on the body.
Parahughmilleria differs from other more basal members of Adelophthalmidae in its reduced spinosity (being spinous) on the , that along with other factors like the large spatulae (a long, flat piece in the operculum) associated with the genital operculum (plate-like segment which contains the genital aperture), suggests a closer relationship with Adelophthalmus.
In 1950, Kjellesvig-Waering described a new species of Hughmilleria, H. bellistriata. The holotype is the dorsal side of a carapace. In this carapace, the eyes, ocelli and most of its surface with ornamentation are preserved. The prosoma (head) was broad, evenly rounded at the anterior lateral angles and the anterior margin. The lateral and anterior margins were bounded by a thin, rounded and raised rim. The eyes were lateral, reniform (bean-shaped) and intramarginal (occurring within the margin). The ornamentation of the carapace consists of distinct transverse striations along the previous part in front of the eyes. Kjellesvig-Waering noted that this species was different from the other Hughmilleria species. The outline of the carapace, the intramarginal eyes and the small size (7 cm, 2.8 in) did not resemble the other species. As he did with P. maria, he reassigned the species to Parahughmilleria in 1961.
In 1957, L. P. Pirozhnikov described two new species of eurypterids, P. matarakensis and Nanahughmilleria, and erroneously assigned them to the genus Rhenopterus. P. matarakensis is represented by well-preserved carapaces. These were semi-oval and were girded by a narrow edge. The trailing edge was slightly concave inward, towards the anterior part. The eyes were large, at 4 mm (0.16 in) long and 2 mm (0.07 in) wide. They were reniform, and rose slightly from the surface of the carapace. At the closest point between both eyes, there were two round ocelli. The fossils were found at the Matarak Formation in Minusinsk, Siberia. P. matarakensis was assigned to its current genus by Kjellesvig-Waering and Willard P. Leutze.
Later, in 1973, Leif Størmer described two new species, P. major and P. hefteri. Both represent the largest and the smallest species, respectively. P. hefteri is a well-known species that has been found both in Europe and in North America. It had enlarged spines on at least one podomere (leg segment) and a relatively wide swimming leg. Both species had a sharply truncated posterior margin in the metastoma, which only differ by their ratios. P. major had a longer telson with more lateral concave margins and a broader base, a more slender body and narrower paddles than P. hefteri. Størmer also noticed slight differences in the genital appendage. All these differences can be explained by ontogenetic stages, that is, different developmental stages of the animal throughout its life.
In 2012, the Russian paleontologist Evgeniy S. Shpinev described another species, P. longa, from the Khakassia, Russia. Its specific name, from the Latin longus, refers to the unusual length of its prosoma compared to the other species of the genus, about 17 mm (about 0.67 in) long and 18 mm (0.71 in) wide. The holotype and only known specimen (PIN1220/3) consists of an incomplete prosoma and fragments from a swimming leg. In this species, the prosoma was surrounded by a narrow marginal margin. The posterior margin was slightly convex. The eyes were small and, as in other species, reniform. The swimming leg was of Adelophthalmus-type, with the fifth, sixth, seventh and part of the eighth segment known. The seventh segment was very long and narrow, while the eight segment was irregular along the outer edge. This species is distinguished by having a prosoma narrower and longer than others.
In addition, Markus Poschmann transferred Erieopterus to Parahughmilleria in 2015 based on the proportions of paddle segments which were more related to those of P. hefteri. This species is only known by a single specimen that was originally assigned to Eurypterus, but Kjellesvig-Waering classified it to Erieopterus based on the shape of the prosoma lacking of ornamentation and on the highly serrated paddle. Nine years before Poschmann referred E. statzi to Parahughmilleria, he and O. Erik Tetlie had already suggested this change, which was not done because of the need to study the material.
Parahughmilleria and Adelophthalmus form a "derived group" that differentiate them from the other basal (primitive) adelophthalmids. These eurypterids share a series of characteristics such as enlarged spines on at least one podomere in the appendage V, the presence of epimera in the postabdomen and the long spatulae that has been associated with the genital operculum. This clade and Nanahughmilleria have an almost identical carapace, paddle, eye shapes and eye position. However, Parahughmilleria differs from Adelophthalmus in that the telson is generally shorter or that the cuticular sculpture (ornamentation consisting of small, minute, scales across the back) is much thinner, among other characteristics. Another genus in the family, Bassipterus, was morphologically close to Parahughmilleria based on the shape of the metastoma and telson, as well as the preabdomen and postabdomen, slightly differentiated from each other. Some authors have even considered Bassipterus, the type and only species, as a synonym of P. bellistriata.
The cladogram below presents the inferred phylogenetic positions of most of the genera included in the three most derived superfamilies of the Eurypterina suborder of eurypterids (Adelophthalmoidea, Pterygotioidea and the waeringopteroids), as inferred by O. Erik Tetlie and Markus Poschmann in 2008, based on the results of a 2008 analysis specifically pertaining to the Adelophthalmoidea and a preceding 2004 analysis.
The argillaceous (composed of clay-like materials) Devonian deposits of Alken, Germany, where fossils of P. heftari and P. major have been found, contains various eurypterids such as Rhenopterus, Jaekelopterus, Alkenopterus and Vinetopterus. Fossils from other organisms have also been found, including the myriapod Eoarthropleura, the Diploaspis and Heteroaspis and the arachnid Alkenia. In the Silurian deposits of the Temeside Shale, England, where the type species has been discovered, fossils of several organisms have been found, most of them eurypterids such as Erettopterus, Salteropterus and Nanahughmilleria, but also indeterminate species of the Hemicyclaspis and the Logania. Parahughmilleria fossils have also been discovered in the United States, Russia, Luxembourg, Canada and Scotland.
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