Delsuc F, Brinkmann H, Chourrout D, Philippe H.

Nature. 2006 Feb 23;439(7079):965-8.

Tunicates and not cephalochordates are the closest living relatives of vertebrates.

Phylogenetic study of 146 genes in 28 species places tunicates next to vertebrates (together forming the “Olfactores” clade proposed earlier by others), and suggests that cephalochordates may be closer to echinoderms, questionning the monophyly of chordates. This was refuted by Bourlat et al. in 2006. Oikopleura is basal to other tunicates (and was not included in Bourlat et al).