Rhombifera Barrande, 1867, and the origin of the Blastoidea (Echinodermata, Blastozoa)

Abstract

Rhombifera bohemica Barrande is redescribed from Barrande’s original material and new specimens from Spain and the Czech Republic. Rhombifera had a heteromorphic proximal stem, a theca with three basals, three infralaterals and five laterals, both greatly elongated, but a reduced oral area in which five, facet-bearing radials alternate with five interradial orals which form the oral frame. The CD oral plus two others share the hydropore and gonopore. Rhombifera is the only glyptocystitoid rhombiferan with a periproct surrounded by two plates, L4 and L5, and ambulacral facets on radial not oral plates. Rhombifera was derived from echinoencrinitid glyptocystitoids, both sharing reduced oral areas with five, not six, radial plates and pectinirhombs with short oval slits. The oral areas of Rhombifera and Lysocystites are similar. In Lysocystites five facet-bearing ‘ambulacral plates’ alternate with five interradial orals forming the oral frame. Extra orals in the CD interambulacrum share the hydropore and gonopore. Lysocystites, coronates and blastoids all possess three basals, five ‘radials’, five deltoids and five homologues of blastoid lancet plates. We suggest that basals, laterals, radials and orals of glyptocystitoids are homologous with basals, radials, lancet plates and deltoids of blastoids, respectively. Rhombifera is a link between glyptocystitoid rhombiferans and blastoids.