page_title: Adelophthalmus
section_title: Internal phylogeny and monophyly
hierarchical_section_title: Adelophthalmus / Classification / Internal phylogeny and monophyly
caption_attribution_description: Carapace of Adelophthalmus mazonensis (Meek and Worthen) from the Herdina collection (H-6); × 1.3.
context_page_description: Adelophthalmus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Adelophthalmus have been discovered in deposits ranging in age from the Early Devonian to the Early Permian, which makes it the longest lived of all known eurypterid genera, with a total temporal range of over 120 million years. Adelopthtalmus was the final genus of the Eurypterina suborder of eurypterids and consisted the only known genus of swimming eurypterids from the Middle Devonian until its extinction during the Permian, after which the few surviving eurypterids were all walking forms of the suborder Stylonurina.
  Fossils of Adelophthalmus have been described from four continents; North America, Europe, Asia, Australia, which indicates that Adelophthalmus might have had a nearly cosmopolitan distribution, one of few eurypterid genera to achieve one besides potentially Pterygotus. The territorial expansion of Adelophthalmus had begun early, with representatives found in both Siberia and Australia during the Devonian, but it first gained its almost cosmopolitan distribution following the amalgamation of the supercontinent Pangaea during the Carboniferous and Permian.
context_section_description: The internal phylogeny and relationships within Adelophthalmus are poorly known, owing to its long history and the large amount of species assigned to the genus, many based on fragmentary remains.
  American paleontologist Victor P. Tollerton suggested in 1989 that some species of Adelophthalmus may be better placed within a new genus in the Slimonidae family of eurypterids, citing their lack of spines, however noted that the then presently available material of Adelophthalmus made it difficult to assess if the legs truly were non-spiniferous. A new genus for non-spiniferous species could be phylogenetically supported, but transferring the new genus to the Slimonidae based on the loss of a feature which seems to have been lost independently in the two groups is not in line with common practice.
  The cladogram below displays the results of a phylogenetic analysis conducted by O. Erik Tetlie and Markus Poschmann in 2008, featuring seven species of Adelophthalmus and excluding other species on the grounds that they were too incompletely known. All characters were treated as unordered and given equal weight. Orcanopterus, part of a clade that also contains Grossopterus and Waeringopterus, was included in the analysis as an outgroup to polarise the characters.
  The results of the analysis showed that all the genera featured (including Adelophthalmus), with the exception of Nanahughmilleria, where the basal species N. patteni was assigned to the new genus Eysyslopterus, were (or had the potential to be) monophyletic. The monophyly of Adelophthalmus was supported by several synapomorphies, including the presence of an anterior triangle on the carapace (the function of which is uncertain), a central circular area of the carapace being raised, the eyes being further away from the margin of the carapace than from the ocelli, an oval metastoma, a long telson and the presence of epimera on the preabdomen.
  A. sievertsi was recovered as more basal than other species, which fits with it also being the earliest known species in the fossil record, mainly due to the broad swimming appendage being similar to the broad appendages of Parahughmilleria and Nanahughmilleria. All other species of Adelophthalmus where this appendage is known possess one that is thinner.
  The analysis left out many fragmentary species of Adelophthalmus, as their character states could not be confidently taken into account, and Adelophthalmus in terms of all the species it is recognized as containing can thus not be fully confidently stated to be monophyletic, more fragmentary species need to be redescribed and more phylogenetic characters need to be confidently established before the status of the genus can be certain.
  Adelophthalmus as it is currently understood may form a monophyletic, and thus phylogenetically valid, group, but that it likely suffers from an under-splitting at the genus level and over-splitting at the species level. It is possible that the large amount of species form two or more distinct clades that could be split into different genera. Though most of the species included in the genus appear to form a monophyletic group, some species have been suggested to represent species of other recognized genera, with A. dumonti supposedly being similar to the obscure Unionopterus in its supposed trapezoid carapace (a feature now known to be incorrect and based on an incorrect illustration) and the large A. perornatus showing ornamentation similar to the one seen in the Hibbertopteridae.
  Many of the more fragmentary species could very well be synonyms of more well known species. In particular, A. imhofi was suggested by Fredrik Herman van Oyen in 1956 to possibly represent a senior synonym of many species, including A. zadrai, A. corneti, A. cambieri, A. pruvosti, A. brasdorensis, A. kidstoni, A. wilsoni and A. sellardsi. Van Oyen's synonymizations were based on ratios of the carapace alone, ignoring other important phylogenetic features as well as possible taphonomic effects on the fossils. Subsequent research has proven the validity of some species, now defi
caption: Fossil carapaces of A. mazonensis.
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