ABSTRACT
A new interpretation of crinoid thecal plate homology, using the orientation of the stem rather than the position of the arms as a reference point for the theca, indicates that two-circlet (monocyclic) crinoids may have been derived from three-circlet (dicyclic) taxa by the loss of any one of the three plate circlets in the theca rather than just the lowest circlet as has been assumed previously. Cladistic analysis based on this suggests that the early Ordovician Aethocrinus is sister group to all other crinoids, while cladids are primitive sister group to both disparids and camerates. Disparids, hybocrinids and perittocrinids together represent a monophyletic clade, as do the camerates. However, the camerate orders Monobathrida and Diplobathrida can no longer be considered to represent natural, monophyletic, taxa since two-circlet camerates evolved more than once from three-circlet forms.
