DELTOID AND ORAL PLATE HOMOLOGY IN BLASTOIDS

Blastoids are one of the most successful clades of Paleozoic echinoderms, yet the homology of their thecal plating with respect to other echinoderm clades remains problematic. Of particular importance is the homology of the deltoid plates that form the peristomal border. Deltoids are interradially positioned and bear the food grooves after the adoral termination of the lancet plates. Recently, Sevastopulo (2005) presented a convincing argument that Passalocrinus is a larval blastoid 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. Waters and Marcus (2002a,b) have reinterpreted previous studies of blastoid development to propose a two stage larval history (cystidean and pentacrinoid) of blastoids similar to that seen in modern crinoids. We believe that comparison of the post-metamorphic history of blastoids to primitive Paleozoic crinoids and cystidean stage modern crinoid larvae helps unravel the homology of the oral plates and deltoid plates of blastoids. In Passalocrinus, the cystidean stage equivalent in blastoids, 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. Deltoids develop in the pentacrinoid stage, form the peristomal border and bear the most proximal ambulacra on their sutures. In adult blastoids, deltoids contribute to the external thecal wall to varying degrees in various blastoid clades. They are interpreted as homologous to the peristomal bordering plates of primitive crinoids and oral plates on numerous blastozoan clades.