AGAINST HOMOLOGY OF CRINOID AND BLASTOZOAN ORAL PLATES

Cambrian echinoderms include the earliest pentaradiate forms, blastozoans and edrioasteroids sensu lato, but no living classes. The earliest unequivocal crinoids appear early in the Ordovician. One side in the ongoing debate over crinoid origins recently promoted similar oral region morphology as evidence of blastozoan ancestry, the two historically assigned pelmatozoans (stemmed echinoderms). Instead we find evidence of ancestry within edrioasteroid-like taxa, recently found to also include stemmed forms.

Does the oral region, and specifically oral plating surrounding the peristome (mouth) provide synapomorphies indicating blastozoan-crinoid monophyly? Proponents document morphologic similarities including: hydropore position, moveable peristomial and ambulacral cover plates, 2-1-2 ambulacral symmetry, arrangement of oral plates, and rigid attachment of the oral surface to the underlying calyx. These five traits are supposed to represent synapomorphies of some subset of blastozoans with crinoids. The early echinoderm record shows the first four of these traits actually comprise symplesiomorphies for all pentaradiate echinoderms, including edrioasteroids. Therefore they are uninformative for any blastozoan-crinoid linkage. The 2-1-2 symmetry and oral arrangement are closely related, the former constraining the latter. Lacking support from these similarities, blastozoan-crinoid oral homology becomes conjectural. Lastly, thecal rigidity represents an iterative theme in pentaradiate echinoderm evolution across the critical stratigraphic interval (Cambro-Ordovician), underscoring potential for homoplasy. Considering evidence from all other skeletal regions, we conclude that symplesiomorphy and homoplasy, not phylogenetic relationship, explain blastozoan and crinoid similarity, ruling against a pelmatozoan clade.