DELTOID AND ORAL PLATE HOMOLOGY IN BLASTOIDS - INSIGHTS FROM PRIMITIVE CRINOIDS AND MODERN CRINOID DEVELOPMENT

Blastoids are one of the most successful clades of Paleozoic echinoderms, yet the homology of their thecal plating remains largely in doubt. Of particular importance is the homology of the deltoid plates that form the peristomal border. Recently, Sevastopulo (2005) presented a convincing argument that Passalocrinus is a larval blastoid, rather than a microcrinoid, based on its thecal plating and the inherent asymmetry in the blastoid basal circlet. He also concluded that the oral plates in larval blastoids were formed and later resorbed, and that blastoid deltoids and lancet plates develop later in ontogeny. We believe that comparison of Passalocrinus to primitive Paleozoic crinoids and cystidean stage modern crinoid larvae helps unravel the homology of the oral plates and deltoid plates of blastoids. Primitive Paleozoic crinoids have two sets of interradially placed plates that are both referred to as orals in different clades. One is a series of enlarged cover plates over the peristome that articulate at the bifurcation points of the ambulacra. The other is a series of plates that form the peristomal opening and bear the food grooves of the proximal ambulacra along their sutures. Importantly 1) because both plate types occur in a single taxon, these plates are not homologous, 2) the former covers the peristome whereas the later forms the peristome providing a functional basis for distinguishing these circlets. The cystidean stage of modern crinoids only has one of these circlets. Because 1) these plates form over the peristome and 2) do not form the frame of the peristomal opening, they are considered homologous to the cover plates over the peristome. Interestingly, except for very primitive crinoids that lack a tegmen, very few crinoid taxa retain the circlet that forms the peristomal border. In Passalocrinus, the oral plates that are lost ontogenetically form the cover of the peristome and are interpreted as homologous with the cover plates of primitive crinoids and the oral plates of cystidean stage modern crinoids. The later developing deltoids form the peristomal border and bear the most proximal ambulacra on their sutures. These plates are interpreted as homologous to the peristomal bordering plates of primitive crinoids and oral plates on numerous blastozoan clades.