Pittsfordipterus
Pittsfordipterus is a genus of eurypterid, an extinct group of aquatic arthropods. Pittsfordipterus is classified as part of the family Adelophthalmidae, the only clade in the derived Adelophthalmoidea superfamily of eurypterids. Fossils of the single and type species, P. phelpsae, have been discovered in deposits of Silurian age in Pittsford, New York state. The genus is named after Pittsford, where the two only known specimens have been found.
Pittsfordipterus was a basal genus that was distinguished from the more derived adelophthalmids by the specialization of its genital operculum and its long and narrow eyes, being Bassipterus closest relative. With an estimated length of 6 cm, Pittsfordipterus was one of the smallest adelophthalmids.
Description
Like the other adelophthalmid eurypterids, Pittsfordipterus was a small eurypterid. The total size of the largest known specimen is estimated at only 6 cm, making it one of the smallest adelopththalmids and eurypterids overall.Pittsfordipterus had a broad carapace with elongated and narrow eyes placed away from the head margin. In the largest specimen, the carapace was 18 mm wide and 13 mm long. Five parallel lines along the front margin that make up the ornamentation can be seen on the surface of the carapace. In the posterior portion, a series of small irregularly distributed tubercles appear. In the posterior margin, there is a strip of fine triangular scales. The tergites also present three to four parallel lines along the posterior margin, followed by five lines that end in a series of separate and lunate scales.
Its genital operculum is the main characteristic that distinguishes it from the rest of the derived adelopththalmids, showing characteristics indicative of the more basal eurypterid Eurypterus. It possessed two opercular flaps, two protruding extensions lateral to the genital appendage. The genital appendage had a great length, extending beyond the second abdominal plate. It was divided into two joints. The first was approximately hastate and was ornamented with fine scales. It was followed by a tubular joint that lacked ornamentation. The second joint was less broad and long. The distal end widens, with a pair of sharp lateral projections. This gives it a termination finished in three spines similar to those that occur in the genital appendage in Slimonia and Adelophthalmus. The American paleontologist Erik Norman Kjellesvig-Waering predicted that the genital operculum would end up being a feature of great phylogenetic importance at least at the generic level.
History of research
Pittsfordipterus is only known by two well preserved specimens, the holotype and paratype. In 1921, the American paleontologist Rudolf Ruedemann described the species Hughmilleria phelpsae from the Vernon Formation of the New York state. Ruedemann noted several differences between his new species and H. socialis, including the size of the carapace, the position of the eyes further from the margin and the morphology of the genital appendage. Instead, Ruedemann suggested a relationship between H. phelpsae and the species H. shawangunk based on the size of the carapace and the position of the eyes more or less being similar, as well as the same linear ornamentation. However, while in its ventral part, H. shawangunk had the same linear ornamentation, H. phelpsae had imbricate scales similar to those of H. socialis. Even so, he suggested that H. phelpsae could probably represent a late descendant of H. shawangunk.In the description of the genus Parahughmilleria in 1961, Kjellesvig-Waering suggested that H. phelpsae should be classified under this new genus. Three years later, Kjellesvig-Waering decided to assign the same species to the subgenus Nanahughmilleria. In 1966, Kjellesvig-Waering, together with the American paleontologist Kenneth Edward Caster, recognized that H. phelpsae was sufficiently different from the other eurypterids and erected the genus Pittsfordipterus based on the morphology of its genital appendage. The name Pittsfordipterus is translated as "wing from Pittsford", with the first word of the name referring to the type locality and the last word composed of the Greek word πτερόν.
Classification
Pittsfordipterus is classified as part of the family Adelophthalmidae, the only clade within the superfamily Adelophthalmoidea. P. phelpsae was originally described as a species of the genus Hughmilleria, but it was considered different enough to represent a new separate genus in 1966.In 2004, O. Erik Tetlie erected the family Nanahughmilleridae in an unpublished thesis to contain the adelophthalmoids with no or reduced genital spatulae and the second to fifth pair of prosomal appendages of Hughmilleria-type. This family contained Nanahughmileria, Pittsfordipterus and perhaps Parahughmilleria. However, the clade has almost never been used in subsequent studies and lists of eurypterids, and instead, they classify the nanahughmillerids as part of Adelophthalmidae. A derived clade in which Nanahughmilleria is closest to Parahughmilleria and Adelopththalmus is better supported, as well as a basal group consisting of Pittsfordipterus and Bassipterus. This clade is backed by a pair of synapomorphies, relatively long and narrow eyes and a complex termination of the genital appendage. Therefore, Pittsfordipterus is the sister group of Bassipterus.
The cladogram below presents the inferred phylogenetic positions of most of the genera included in the three most derived superfamilies of the Diploperculata infraorder of eurypterids, as inferred by Odd 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.