Evolution and systematics

The unusual echinoderm class Concentricycloidea (sea daisies) was erected for the genus and species Xyloplax medusi-formis by Baker, et al., (1986) because of its radical departure in morphology from any other known living echinoderm (starfishes, brittle stars, sea urchins, sea cucumbers, and sea lilies). Concentrically arranged skeletal structures and single series of tube feet arranged in a circle were novel in living echin-oderms. Given these special features, it was not considered possible, on the basis of morphology, to accommodate the genus in any existing echinoderm class. Rowe, et al., (1988) did consider, however, that Xyloplax was derived from the Aster-oidea (starfishes), possibly from an ancestral precursor to the order Valvatida. In 1988, a second species, X. turnerae, was discovered which confirmed the unique morphology of the class.

The morphological differences between each of the existing orders of the Asteroidea are quite minor compared with the radical rearrangement of the axial skeleton, water vascular system, and the form of the spermatozoa in the genus Xyloplax. So, where to place this enigmatic taxon within the Echinodermata?

Recently attempts have been made to clarify the morphology and examine the evolutionary history of Xyloplax using cladistic and phylogenetic analyses of DNA sequences.

A case has been made for the Concentricyloidea to be included in the class Asteroidea based on a form of cladistic analysis of rDNA from one species, X. turnerae. Despite regarding Xyloplax as "morphologically enigmatic," the analysis placed Xyloplax as a sister taxon of Rathbunaster, a morphologically completely unrelated genus of the starfish order For-cipulatida. The sequencing has not been subsequently replicated in X. turnerae, and no DNA has yet been successfully sequenced from X. medusiformis. Also, there is a ques tion regarding the effect of initial preservation of some of the samples of X. turnerae in formalin, which usually precludes or confuses accurate DNA analysis. There is an urgent need to apply standard and repeatable phylogenetic analysis of rDNA control region sequences to samples from both X. turnerae and X. medusiformis, and to as wide a range of examples of the Asteroidea as possible, to further elucidate the systematic position of this morphologically highly divergent taxon.

If the Concentricycloidea is to be regarded as a taxon at some level within the Asteroidea, then a re-defining of that class to accommodate the two Xyloplax species is essential. This would also require a revision of the higher systematics of the somasteroids and ophiuroids. Until more fresh material of Xyloplax is collected and examined both morphologically and genetically, and comparisons are made with a much larger number of taxa, the class Concentricycloidea must remain distinct to accommodate the extraordinary unique morphology of the two species of Xyloplax.

The class Concentricycloidea contains one order, Peripo-dida, and one family, Xyloplacidae.

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