EARLY PHYLOGENY AND SUBCLASS DIVISION OF THE CRINOIDEA (PHYLUM ECHINODERMATA)

A stepwise approach is employed to determine the phylogeny of the init ial crinoid radiation during the Arenig and Llanvirn series of the Ord ovician. Parsimony-based character analysis is completed first on Aren ig crinoids and then for Arenig and Llanvirn crinoids combined. The to pology from well-resolved trees of this early crinoid radiation indica tes that the Crinoidea should be subdivided into six subclasses. A new subclass and new order, Aethocrinea and Aethocrinida, respectively, a re proposed for crinoids with four circlets of plates in the aboral cu p: lintels, infrabasals, basals, and radials. This aboral cup construc tion is best displayed by Aethocrinus, one of the oldest known crinoid s (?Tremadoc-Arenig). However, this primitive aboral cup construction is also present in two Llanvirn crinoids, Perittocrinus and Tetraciono crinus. The Aethocrinea was a small, short-lived radiation of crinoids with this design that was different from those of other crinoids. Two families in the Aethocrinida are the Aethocrinidae, which includes Ae thocrinus, and the Perittocrinidae, which includes Perittocrinus and T etracionocrinus. In addition to the Aethocrinea, the following crinoid subclasses are recognized based on character analyses of these earlie st crinoids: Cladida, Camerata, Disparida, Flexibilia, and Articulata.