XIPHOSURA LATREILLE, 1802

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 : 20

XIPHOSURA LATREILLE, 1802 [= MEROSTOMATA DANA, 1852]

Diagnosis: Prosomapoda with a partially reduced tergite of somite VII; appendages of somite VII reduced to chilaria; opisthosoma broadest anteriorly; cardiac lobe extends onto anterior half of carapace; opthalmic ridges merge anteriorly with median ridge to form double arch.

Included taxa: Kasibelinurus Pickett, 1993 ;? Maldybulakia Tesakov & Alekseev, 1998 [= Lophodesmus Tesakov & Alekseev, 1992 ]; Willwerathia Størmer, 1969 ; XIPHOSURIDA Latreille, 1802 .

Remarks: Merostomata was originally proposed by Dana (1852) as consisting solely of Xiphosura but later expanded to include Eurypterida ( Woodward, 1866 –1878). Eurypterida being closer related to Arachnida than Xiphosura necessitates their removal from Merostomata and results in Merostomata being a junior synonym of Xiphosura.

Elleria Raymond, 1944 and Kiaeria Størmer, 1934 probably have a fused thoracetron and are therefore classified as xiphosurids. Maldybulakia has been considered to show similarity to Willwerathia and so is tentatively included within Xiphosura.