Fig. 4

Phylogeny showing the fossil groups discussed in this paper and their relationship to extant gnathostomes and cyclostomes. Double-headed arrows indicate the extent of groups. A stem group contains only fossil taxa; a crown group is defined on the basis of living taxa, but can also include fossils. Heterostracans, galeaspids, osteostracans and placoderms are thus stem gnathostomes but crown vertebrates