Revised systematics of P alaeozoic ‘horseshoe crabs’ and the myth of monophyletic X iphosura

The monophyly of the class X iphosura is critically re‐examined. For the first time a phylogenetic analysis of a number of synziphosurine and xiphosurid taxa is performed together with representatives of the other chelicerate orders also included as ingroup taxa. X iphosura as currently defined is shown to be paraphyletic, and a revised classification is presented. Previous characteristics used to unite the xiphosurids (possessing a fused thoracetron) and a paraphyletic grade of synziphosurines (retaining freely articulating opisthosomal tergites) include the presence of a cardiac lobe, ophthalmic ridges, an axial region of the opisthosoma, and a reduced first opisthosomal segment. All of these characteristics are, however, here shown to be present in other chelicerate groups, leaving X iphosura without any defining synapomorphies. A number of other characters, including the form of the chelicerae and appendage VII , indicate that xiphosurans may be paraphyletic with respect to a clade consisting of chasmataspidids, eurypterids, and arachnids. What ramifications this has for the evolution of basal chelicerates is briefly discussed, and it is recognized that most of the currently known ‘synziphosurine’ taxa represent offshoots from the main chelicerate lineage with ghost ranges extending into at least the M iddle O rdovician.