Tsagkogeorga G, Turon X, Hopcroft RR, Tilak MK, Feldstein T, Shenkar N, Loya Y, Huchon D, Douzery EJ, Delsuc F.

BMC Evol Biol. 2009 Aug 5;9:187. doi:10.1186/1471-2148-9-187

An updated 18S rRNA phylogeny of tunicates based on mixture and secondary structure models.

Phylogenetic tree built with 18S rRNA sequences from 110 species including 4 Oikopleuridae places Oikopleuridae as sister group of Stolidobranchia. Nevertheless, it might be an artefact of AT-richness or long-branch attraction. Many other articles on the topic are cited in this paper.