page_title: Strabops
section_title: History of research
hierarchical_section_title: Strabops / History of research
context_page_description: Strabops is a genus of strabopid, an extinct group of arthropods. Strabops is known from a single specimen from the Late Cambrian of the Potosi Dolomite, Missouri, collected by a former professor, Arthur Thacher. It is classified in the family Strabopidae of the monotypic order Strabopida, a group closely related to the aglaspidids with uncertain affinities. The generic name is composed by the Ancient Greek words στραβός, meaning "squinting", and ὄψῐς, meaning "face".
  The history of Strabops has been turbulent and confusing since its original description by Charles Emerson Beecher, who classified it as a eurypterid. Many authors do not agree with this and have classified Strabops and its allies as part of the Aglaspidida order, while others classify them in their own order. Although the latter is the taxonomic position currently accepted, other paleontologists prefer to simply omit the strabopids from their analyzes due to the poor preservation of their fossils. In addition, it has been suggested that the closely related Paleomerus represents a synonym of Strabops, which are uniquely differentiated by the size of the telson and the position of the eyes.
context_section_description: Strabops is known by one only well preserved specimen (YPM 9001, housed at the Peabody Museum of Natural History). It was found by Arthur Thacher, a former professor at the Washington University, in the Potosi Dolomite of the St. Francois County, Missouri. The specimen was sent to Yale University, Connecticut. The American paleontologist Charles Emerson Beecher described it as the only Cambrian eurypterid, Strabops thacheri, the generic name derived from the Ancient Greek words στραβός (strabós, squinting) and ὄψῐς (ópsis, face) and refers to the inward turning eyes. He considered Strabops different enough from the other eurypterids to erect a new genus, although he did not assign it to any family. In 1912, the American paleontologists John Mason Clarke and Rudolf Ruedemann assigned Strabops to the Eurypteridae family, as well as affirming the possession of a twelfth segment and changing the position of the eyes from anterolateral (in the middle of the front) to lateral.
  In his book Cambrian Merostomata of 1939, the American paleontologist and geologist Gilbert Oscar Raasch considered the descriptions of the joint authorship of 1912 erroneous and agreed with Beecher on all aspects of his description, except for some reservations about the ocelli. Despite its initial classification as a eurypterid, Raasch admitted that the description of Strabops concurred with the old thought of what an aglaspidid was. It is possible that Beecher was unaware of the similarity between it and Aglaspis because of the distorted illustration of the latter by the American zoologist and paleontologist Robert Parr Whitfield. Therefore, Raasch placed Strabops under the family Aglaspididae in the order Aglaspida.
  In 1971, the Swedish geologist and paleontologist Jan Bergström tentatively removed Strabopidae (at that time containing Strabops and Neostrabops) and Paleomeridae (only containing Paleomerus) from the order Aglaspidida based on the fact that the head tagma was too short to accommodate the six pairs of appendages then assumed to be present in aglaspidids. Instead, he classified them in an uncertain order in the Merostomoidea class together with the emeraldellids. Ironically, Bergström speculated that the number of pairs of appendages present in the three genera could be fewer than seven, as well as including a possible antennal segment. This is currently observed as an overestimate. A study published by Derek Ernest Gilmor Briggs et al. in 1979 has shown that Aglaspis spinifer had between four and five pairs of appendages, but not six, weakening Bergström's argument.
  In 1997, Bergström and Hou Xianguang, a Chinese paleontologist, completely removed Strabopidae (recognizing Paleomeridae as a junior synonym), as well as the family Lemoneitidae (containing Lemoneites), from the order Aglaspidida to erect a new order, Strabopida, this time suggesting a number of no more than two pairs of appendages. However, this new clade (group) remained under the similarly-named Aglaspida subclass. A year later, the British paleontologists Jason Andrew Dunlop and Paul Antony Selden eliminated Strabopida from the suborder Aglaspidida and classified them as the sister taxa of the latter based on the lack of aglaspidid apomorphies (distinctive characteristics), such as the lack of genal spines (a spine placed in the posterolateral part of the prosoma). Other authors have reinforced this argument by the trapezoidal telson form of Paleomerus and Strabops in contrast to the long styliform telson of the aglaspidids. However, some authors prefer to represent the taxonomic position of the strabopids as uncertain due to the poor preservation of their fossils.
caption: Size comparison of Strabops and both species of Paleomerus