Phylogeny and palaeoecology of the Adelophthalmoidea (Arthropoda; Chelicerata; Eurypterida)

A phylogenetic analysis of the monophyletic superfamily Adelophthalmoidea suggests that there is a basal, hitherto unrecognised genus, Eysyslopterus gen. nov., in the clade, on the basis of carapace shape, anterior ornament and eye position. The two comparatively poorly known genera Pittsfordipterus and Bassipterus form a relatively basal clade and are united by their shared possession of elongated eyes and a type A genital appendage consisting of two articles forming a complex distal termination. The genera Parahughmilleria and Nanahughmilleria are represented in the analysis by only one well‐known species each and the monophyly of these two genera remains untested. The geologically long‐lasting Adelophthalmus is monophyletic and supported by a number of synapomorphies. The clade probably had a marine origin. Most known representatives are brackish (estuarine‐deltaic) or freshwater (laminated lacustrine‐fluvial) inhabitants, but with a marine influence commonly encountered within the sequences. The fossil record of the clade is relatively poor with a Relative Completeness Index value of between 31 and 34%. However, the fit between phylogeny and stratigraphical occurrences of the taxa analysed is very good (Stratigraphic Consistency Index = 0.73; Gap Excess Ratio = 0.96–0.97). The sister group of Adelophthalmus, the genus Parahughmilleria, is reported for the first time from the Silurian of Lesmahagow, Scotland and these specimens are questionably referred to P. cf. hefteri, previously known only from the Siegenian and Emsian of western Germany and Luxembourg.