XIPHOSURA SENSU STRICTO

Lamsdell, James C., 2013, Revised systematics of Palaeozoic ‘ horseshoe crabs’ and the myth of monophyletic Xiphosura, Zoological Journal of the Linnean Society 167 (1), pp. 1-27 : 18

XIPHOSURA SENSU STRICTO

Kasibelinurus and Willwerathia resolve as a paraphyletic grade in a clade with xiphosurids, and are the only two synziphosurines in the analysis that fall within the revised Xiphosura. Xiphosura sensu stricto is defined by the possession of a cardiac lobe that extends onto the anterior half of the carapace and potentially the possession of opthalmic ridges that curve towards the carapace centre anteriorly and merge with a central cardiac ridge to form a double arch (or ‘m’) shape; although Kasibelinurus amicorum does not appear to possess this ophthalmic ridge morphology the dorsal carapace of the holotype is flattened and these structures may have not been preserved, while other species currently assigned to Kasibelinurus [such as Kasibelinurus (?) randalli ( Beecher, 1902)] clearly show the double arch configuration. Willwerathia is placed as the sister taxon to Xiphosurida despite its unusual morphology, united with the order by its possession of an axial anterior extension to the articulating facet on each tergite.