TIES THAT BIND: SEGMENT ARTICULATIONS IN ARTHROPODS AND THEIR IMPLICATIONS FOR BASAL CHELICERATE PHYLOGENY

Ever since Henry Woodward proposed to unite horseshoe crabs, eurypterids and trilobites as Crustacea within the group Merostomata in 1866 there has been much debate surrounding the affinities of the group, the relationship of its constituent taxa and its validity as a monophylum. It is now accepted that ‘merostomes’ (comprising xiphosurans, eurypterids and chasmataspidids) are a paraphyletic grouping of basal chelicerates that form the stem lineage to Arachnida, but the phylogenetic topology of this part of the chelicerate tree is still unclear and several groups as currently defined (including Xiphosurida and Eurypterida) lack any convincing synapomorphies. Recent restudy of a number of fossil arthropods has revealed inconsistencies in the treatment of the articulation devices between their abdominal segments. For example the definition of an ‘articulating half-ring’ is rather liberally applied. When the morphology of these articulation devices is compared across arthropod groups several trends become apparent, one being that for several groups (such as Artiopoda, Megacheira and Chelicerata) the ground pattern for articulations appears to involve a narrow transverse ridge on the tergite preceded by a smooth articulation facet. 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 ‘merostome’ 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. Eurypterids are considered monophyletic, and the potential autapomorphy of having the opercular plates of somites VIII and IX fused into a genital operculum is identified, while xiphosurids are united by the possession of a fused thoracetron. Diploaspid chasmataspidids are considered to represent a monophyletic clade, however the monophyly of the order is still unclear due to their extreme morphological differences from the Ordovician Chasmataspis. Finally, what ramifications this revision might have on the arachnid ground pattern will be discussed in brief.