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
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.)