Prosomapoda

 ( Middle Ordovician First Appearances ) 

Prosomapoda is a clade of Chelicerata including the groups Xiphosura (horseshoe crabs) and Planaterga (a group comprising Bunodidae, Pseudoniscidae, Chasmataspidida, Eurypterida and Arachnida), as well as several basal Synziphosurina genera. The clade is defined by the lack of exopods (outer branches) of prosomal appendage II-V in the adult instar, where in contrast the exopods of appendage II-V are well-developed in the non-prosomapod euchelicerates Offacolus and Dibasterium.