Fig. 2.

Phylogeny of the basal vertebrate lineages. It is generally agreed that gnathostome (jawed vertebrates) and hagfish lineages diverged from one another following a deep split of the ancestral craniate lineage. The relative position of lamprey is not definitively resolved but the lamprey lineage is generally considered to have diverged from one of the two lineages shortly after the primary divergence. Molecular data suggest shared ancestry of lampreys and hagfish, whereas morphological data suggest shared ancestry of lampreys and gnathostomes.