Phylogenetic relationships among extant classes of echinoderms, as inferred from sequences of 18S rDNA, coincide with relationships deduced from the fossil record.

In spite of the rich fossil record and multiple descriptions of morphological and embryological characteristics, the origin and subsequent evolution of echinoderms remain highly controversial issues. Using sequence data derived from 18S rDNA, we have investigated the phylogenetic relationships among five extant classes of echinoderms--namely, crinoids, asteroids, ophiuroids, echinoids, and holothurians. Almost complete sequences of 18S rDNA were determined for one species in each class, and phylogenetic trees were constructed both by the neighbor-joining method and by the maximum-likelihood method, with a hemichordate as an outgroup. The trees constructed by these methods support the hypothesis that the phylum Echinodermata can be subdivided into two subphyla, Pelmatozoa and Eleutherozoa. The class Holothuroidea, which has been the subject of debate with respect to whether the members are primitive or advanced echinoderms, did not occupy a primitive position but had an affinity for the class Echinoidea. Since both trees gave different branching topologies for the order of emergence of asteroids and ophiuroids, it seems likely that these two groups emerged within a very short period of time. A rough estimate of the timing of the divergence of the five classes from the present molecular analysis coincided with that deduced from the fossil record.