ABSTRACT
It is easy to place almost any living echinoderm at the class level. Concentricycloids (Xyloplax) are the exception. Though assumed to stem from recent asteroids, they lack rays and other typical asteroid features. We examined this assumption using cladistic analyses of morphological characters at two levels. At the class level, echinoids and holothuroids formed a solid sister group, while concentricycloids clustered with asteroids and ophiuroids, a robust but unresolved trio. An analysis of asteroid orders, with ophiuroids as an outgroup, placed concentricycloids firmly outside asteroids. Comparison with tiny, deep-sea asteroids and ophiuroids underlined the power of convergence and the unique and ambivalent position of concentricycloids. Critical evaluation of the available evidence provides no support for submerging Xyloplax within the asteroids; concentricycloids should be retained as a separate clade.
