MAH, C.L.; Smithsonian Institution: A new species of Xyloplax (Echinodermata; Asteroidea; Concentricycloidea) from the Northeast Pacific: Comparative Morphology and A Reassessment of Concentricycloid Phylogeny

Deep-sea environments are often home to unusual organisms which display novel morphological innovations with highly derived body plans. Among the most enigmatic of these taxa are the rarely encountered Concentricycloidea or sea-daisies, which were first discovered in 1986 from New Zeland, living on wood occurring in the deep-sea (>1000 m). Concentricycloids are a group of tiny (~1-5 mm in diameter) discoidal echinoderms with such extreme morphological divergence that they were considered by some as the sixth extant class of Echinodermata. The group is monotypic with one genus, Xyloplax which contains two known species, Xyloplax medusiformis from New Zealand and X. turnerae from the Bahamas. A new third species of Xyloplax from the Northeast Pacific is described using primarily SEM images and compared with the two known species. Xyloplax n. sp. is distinguished on the basis of tube foot and spine variations as well as a distinct cygnoid spinelet shape. Cygnoid spinelets are thought to be associated with reproduction suggesting that spinelet morphology may reproductively isolate species. Few morphological differences are observed among Xyloplax species. Prior phylogenetic hypotheses have presented Xyloplax as derived from crown group asteroids. A new phylogenetic hypothesis based on morphological characters derived from the Extraxial-Axial Theory and fossil asteroid characters is presented. Xyloplax is placed within the Asteroidea as the sister group to the Ambuloasteroidea, which is the primary lineage of crown-group asteroids displaying modern ambulacral morphology. This hypothesis suggests early divergence for Xyloplax from crown-group asteroids.