Revised systematics of Palaeozoic ‘horseshoe crabs’ and the myth of monophyletic Xiphosura

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