Echinoderm phylogeny including Xyloplax, a progenetic asteroid

Reconstruction of the phylogeny of the five extant classes of the phylum Echinodermata has proven difficult. Results concerning higher-level taxonomic relationships among echinoderms are sensitive to the choice of analytical parameters and methods. Moreover, the proposal of a putative sixth class based on a small, enigmatic disc-shaped echinoderm, Xyloplax, from the deep seas of the Bahamas and New Zealand in the 1980s further complicated the problem. Although clearly an echinoderm, Xyloplax did not have clear affinity among known groups. Using molecular sequence and developmental data from recently collected Xyloplax adults and embryos we show that, rather than representing an ancient, distinct lineage as implied by its status as a class, Xyloplax is simply a starfish that is closely related to the asteroid family Pterasteridae. Many members of the Pterasteridae and all Xyloplax inhabit deep and polar seas and brood young. Brooding pterasterids and Xyloplax hold their young in spe...