A NEW MODEL FOR PLATE HOMOLOGY IN BLASTOZOAN ECHINODERMS

Over the last two centuries, echinoderm specialists have been naming ossicles (plates) and trying to infer their homology across Echinodermata. All too often the name of a plate or plate series is based on its position rather than on its homology, leading to difficulties in phylogeny reconstruction. Nowhere in the theca is this problem more pronounced than the plating around the peristome and ambulacral system. Advanced blastozoans are characterized by having oral plates defined as the plate series bordering the peristomal opening, that is interradially positioned bearing the proximal food grooves along their adjacent sutures and their descendant homologues. These plates include the oral plates of glyptocystitoids, paracrinoids, parablastoids, primitive crinoids, some of the “oral plates” of hemicosmitids, and the deltoid plates of blastoids and coronoids. Primary peristomal cover plates are defined as those plates that are interradially positioned, and form the peristomal cover and bifurcation points of the ambulacral segments and their descendant homologues. These plates include oral and bifurcation plates of edrioasteroids, the oral plates of microcrinoids, and the bifurcation plates of the ambulacral cover plate series of most other echinoderms. Oral cover plates are defined as those plates that cover the food grooves of the oral plates and their descendant homologues. These grade distally into the ambulacral cover plates that cover the food groove over the ambulacral floor plates. Floor plates are defined as the plate series distal to the oral plates that is radially positioned bearing the distal food grooves and brachiole facets and their descendant homologues. These include floor plates of glyptocystitoids, side plates of blastoids, and arm plates of hemicosmitids, coronoids, bockiids, paracrinoids, and trachelocrinids. Although a uniform morphological nomenclature across Echinodermata would be ideal, such a proposal would be unrealistic given the entrenchment of these names in the clades in which they are used. However, when larger scale phylogenies are being reconstructed it is imperative to code morphological data based on the homology of the skeletal elements rather than the names they are called.