SEMANTICS AND PHYLOGENETIC ARGUMENTS IN THE DEBATE OVER THE ORIGINS OF CRINOIDEA

The debate over the origin of Crinoidea (one of the major echinoderm clades) is hampered by three basic issues: one semantic, two others related to the nature of data in clade definition. First, the two main research groups studying the origin of crinoids apply the name Crinoidea either at the node where four plate circlets presumably evolved or at a more inclusive group bearing distinctive crinoidal arms and stems. The crown definition used (implicitly or explicitly) by most biologists refers to the last common ancestor of all living forms and excludes all Paleozoic taxa from Crinoidea. Even if everyone agrees on the substantive issue (phylogeny of the taxa under consideration), a semantic issue (placement of the name Crinoidea) is not resolved.

Second, reliance on “defining characters” is misguided. Features such as the plating of the calyx and arms and meric stem construction are the evidence used to identify clades and clade membership, but the definition should be based on ancestry and descent. Characters diagnose, but do not define.

Third, characters including a meric stem, ambulacra bearing a large coelomic lumen, and one ambulacral cover plate per floor plate are used to place Crinoidea as a subclade of Edrioasteroidea. These characters are used to argue the exclusion of crinoids from Blastozoa because undisputed examples of these characters cannot be identified within blastozoans. Such arguments are dubious because synapomorphy informs only clade membership, not relationship outside the clade. Furthermore, these features can be found among blastozoans: Echinosphaeritids have meric stems, the diploporan Letenocrinus has epithecal ambulacra bearing a large coelomic lumen and uniserial brachioles, and the parablastoid Eurekablastus has one ambulacral cover plate per floor plate on the main food groove. Available data suggest that crinoids are a derived blastozoan clade.

All decisions to name clades are arbitrary. Phylogenetic definitions are less subjective than those based on “defining characters,” and their use would streamline arguments to be over the origin of the clade rather than its features.