Early phylogeny and subclass division of the Crinoidea (Phylum Echinodermata)

A stepwise approach is employed to determine the phylogeny of the initial crinoid radiation during the Arenig and Llanvirn series of the Ordovician. Parsimony-based character analysis is completed first on Arenig crinoids and then for Arenig and Llanvirn crinoids combined. The topology from well-resolved trees of this early crinoid radiation indicates that the Crinoidea should be subdivided into six subclasses. A new subclass and new order, Aethocrinea and Aethocrinida, respectively, are proposed for crinoids with four circlets of plates in the aboral cup: lintels, infrabasals, basals, and radials. This aboral cup construction is best displayed by Aethocrinus , one of the oldest known crinoids (?Tremadoc-Arenig). However, this primitive aboral cup construction is also present in two Llanvirn crinoids, Perittocrinus and Tetracionocrinus. 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 Aethocrinus , and the Perittocrinidae, which includes Perittocrinus and Tetracionocrinus. In addition to the Aethocrinea, the following crinoid subclasses are recognized based on character analyses of these earliest crinoids: Cladida, Camerata, Disparida, Flexibilia, and Articulata.