Phylogenetic relationships of extant echinoderm classes

Abstract

A well-supported phylogeny of echinoderm classes has eluded morphological, embryological, molecular, and combined analyses. From this body of work it is apparent that (i) echinoids (sea urchins) and holothuroids (sea cucumbers) are related, and (ii) crinoids (sea lilies) are the sister taxon to extant eleutherozoan classes (asteroids, ophiuroids, echinoids, and holothuroids). However, the relationships of asteroids and ophiuroids to other echinoderm classes have been difficult to recover. To address relationships between the asteroids and ophiuroids and other echinoderms, I have sequenced additional nuclear loci and taxa and used novel computational approaches for co-optimizing morphological with molecular evidence at the level of sequence alignment. Support for the monophyly of each class is strong. Support for a monophyletic Asteroidea + Xyloplax is as strong as for Asteroidea. Support for Asterozoa (Asteroidea + Ophiuroidea) is apparent, albeit not as strong as for other clades (e.g., Echinozoa, Eleutherozoa, and Echinodermata). I also present detailed sensitivity analyses to provide (i) a test of the monophyly of groups under a variety of evolutionary models and (ii) a statement of the evidential value of various character systems.