James C. Lamsdell¹ & Martin Stein^2
1Department of Geology and Paleontological Institute, University of Kansas, Lawrence, KS, USA
²Museum of Natural History, Copenhagen University, Copenhagen, Denmark

Recent restudy of a number of fossil arthropods has revealed inconsistencies in the treatment of the articulation devices between their trunk segments: for example the definition of an ‘articulating half-ring’ is rather liberally applied. A number of different articulation morphologies are identified; an anterior axial recess with attachment to
the preceding tergite via arthrodial membrane, an anterior articulating ridge or shelf constraining overlap with the preceding tergite, a posterior articulating ridge or shelf constraining overlap with the succeeding tergite, a transverse articulating boss, or an articulating half-ring and furrow, as well as telescope-like joining without obvious articulating devices other than arthrodial membrane and possibly pivot joints. The anterior articulating ridge morphology appears particularly widespread among arthropods, appearing to be part of the ground pattern for Artiopoda, Megacheira and Chelicerata. However, some synziphosurines (currently considered the paraphyletic stem-lineage
to Xiphosurida) have been described as possessing articulating half-rings, and the ramifications of this are considered in light of a number of other characters. A new interpretation of basal chelicerate relationships is considered where Xiphosura is para- or even polyphyletic, with synziphosurines representing a basal grade to a clade consisting of Xiphosurida, Chasmataspidida, Eurypterida and Arachnida; further characters supporting this topology are reviewed.