ECHINODERM PHYLOGENY – THE PATH FORWARD

With the advent of the Assembling the Echinoderm Tree of Life project, our understanding of echinoderm phylogeny has progressed. Current datasets infer several clades that reflect deep structure within the echinoderm tree. The total clade Pan-Echinodermata (crown clade Echinodermata and all taxa closer to it than its closest extant sister taxon) is diagnosed by a skeleton composed of monocrystalline ossicles of Mg-calcite that bear a stereom microstructure. A rooting issue clouds our understanding of character polarity in the deepest portion of the pan-echinoderm tree. A unirayed plesiomorphic condition would infer an early-evolved pentaradiate clade nested within Pan-Echinodermata, whereas a plesiomorphic pentaradiate condition would infer evolution of early derived uniradiate clade(s) of homalozoans. Regardless, crown clade Echinodermata (including all taxa descended from the last common ancestor of all extant echinoderms) circumscribes the crown clade Crinoidea and crown clade Eleutherozoa including: Asteroidea, Ophiuroidea, Holothuroidea and Echinoidea. Relationships among modern eleutherozoan and extinct Somasteroidea and Ophiocistioidea remain problematic and are complicated by long branch attraction. The recent identification of both abradial and adradial floor plate series and associated peristomial bordering systems suggest an early split in the pentaradiate echinoderm lineage. Some taxa bear adradial floor plates and oral frame plates (Imbricata, Gogiida and related taxa) whereas others bear abradial floor plates and oral plates (oral plate-bearing blastozoans). Current analyses infer that crown clade Crinoidea is nested in the latter clade though alternate views have been recently proposed. Crown clade Crinoidea, related Paleozoic crinoids and protocrinoids form a clade and recent analyses show promise for resolving their internal relationships. Remaining major issues to be solved include: 1) an understanding of basal polarity in Pan-Echinodermata, 2) an understanding of branching position of the homalozoan clades, 3) a clarification of the tree structure within eleutherozoans, 4) a clarification of the branching position of crinoids. New data and taxa at critical intervals such as the Middle and Late Cambrian should help resolve these issues.