THE INTERRELATIONSHIPS OF THE ECHINODERM CLASSES - MORPHOLOGICAL AND MOLECULAR EVIDENCE

The relationship between the five echinoderm classes has perplexed phy logeneticists for some time. Although each of the crinoids (C), astero ids (A), ophiuroids (O), echinoids (E) and holothuroids (H) are morpho logically distinct, evidence from larval and adult morphology, molecul ar biology, and stratigraphy have failed to provide a single consensus solution. We have reviewed all available morphological and molecular data, added new data and reanalysed independent data sets individually and in combination, in order to resolve echinoderm class relationship s. In total, we present 21 larval and 50 adult morphological character s, partial 28S-like large subunit rRNA gene data for 39 taxa and compl ete IBS-like small subunit rRNA gene data for 37 taxa. For a 5 taxon p roblem there are 105 possible rooted tree topologies, and yet we were consistently presented with three competing hypotheses when data sets were analysed both individually and in combination. The total evidence solution favoured (outgroup(C(A(O(E,H))))) although the alternative t ree topology, (outgroup(C(O(A(E, H))))) was only one step longer and ( outgroup(C((A,O),(E,H))))) was two steps longer. Only these three tree s are serious contenders and the distribution of morphological charact ers suggests we should discount the solution placing ophiuroids as sis ter group to an asteroid + echinoid + holothurian clade. Thus we are l eft with (outgroup(C(A(O(E,H))))) and (outgroup(C((A,O),(E,H)))) as th e two most plausible phylogenetic hypotheses. Our data showed high lev els of phylogenetic signal and these trees best fit the available data . (C) 1997 The Linnean Society of London.