Parahughmilleria (meaning "near Hughmilleria"[1]) is a genus of eurypterid, an extinct group of aquatic arthropods. 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 appendages. 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.
Like other adelophthalmid eurypterids, Parahughmilleria was a small-sized eurypterid. The largest species, P. major, only reached 12.5 cm (5 in),[2] and the smallest species, P. hefteri, measured only 6 cm (2.4 in),[3] although it has been suggested that these species form different ontogenetic stages (different developmental stages of the same animal throughout its life) of each other.[2] If so, the smallest species would be P. maria and P. bellistriata, both at 7 cm (2.8 in).[3]
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.[4] 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.[2]
Parahughmilleria differs from other more basal members of Adelophthalmidae in its reduced spinosity (being spinous) on the appendages, 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.[5]