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.