The relationships of the Osteostraci and Galeaspida

The phylogenetic relationships of two monophyletic groups of fossil jawless craniates, the Osteostraci and Galeaspida, are discussed on the basis of new observations. Maximum congruence of derived character distribution implies that the Galeaspida are the sister-group of the Osteostraci + Gnathostomata. This implies that the dorsal naso-hypophysial opening is not a synapomorphy of the Cephalaspidomorphi (Osteostraci + Anaspida + Petromyzontida), but is likely to have appeared independently, at least in lampreys and osteostracans. An outline of the phylogeny of the Galeaspida is proposed on the basis of the published material.