Relative Complexity

Phylogenetic relationships among the five extant classes have been the subject of considerable debate. Renewed interest in resolving this problem during the past decade has greatly narrowed the number of likely topologies (Littlewood et al. 1997). It is generally agreed that the lineage leading to the Crinoidea branched most basally, and that the Echinoidea and Holothuroidea are extant sister clades. Debate now centers on whether the Ophiuroidea and Asteroidea form a clade. The two best-supported hypotheses are: