Crinoid Systematics

While the crinoids were the dominant echinoderm of the Paleozoic with more than 6,000 described species, there are only 600 or so species living today. This small number of living representatives complicates the task of reconstructing a comprehensive phylogeny for the crinoids. The taxon Crinoidea was established in 1821 by J. S. Miller pulling the stalked crinoids out of the starfish group Stellarides. Subsequent attempts to resolve the crinoid history were all based on the arrangement of thecal plates relative to the position of the arms. Traditionally, the thecal plates were decribed as either monocyclic or dicyclic with the first row containing the arms as the "orals", the row just below the orals as the "radials", the next row as the "basals", and for the dicyclic groups, the last row as the "infrabasals". This long established precedence of thecal plate organization has forced the assumption that each row of plates bearing the same name were in fact homologous. However, a reinterpretation of thecal plate homology by Michael J. Simms (1993) provides new insight into the relationships among crinoids. Thecal plate homology suggested by Simms is based on the position of the plates relative to the stem rather than the arms, this is supported by ontogeny and the position of other plates in the theca.

The cladogram to the left is based on Simms cladistic analysis of his revised homology for crinoids. (Simms, Michael J. 1993: Reinterpretation of thecal plate homology and phylogeny in the Class Crinoidea. Lethaia, Vol. 26, pp. 303-312.)