ORAL REGION PLESIOMORPHY AND HOMOPLASY IN STEMMED ECHINODERMS

Much effort has been given to the oral region of early pentaradiate echinoderms to address the question of crinoid ancestry. It is argued here that the case for “oral” homology of crinoids to any other echinoderm remains ambiguous and that differences in stems and feeding appendages argue for plesiomorphy and homoplasy as the cause of similarity between blastozoans (eocrinoids, “cystoids”) and crinoids. The template for both crinoid and blastozoan oral regions derives from basal pentaradiate echinoderms. Much similarity between crinoid and blastozoan oral regions is demonstrably deep seated within pentaradiate echinoderms, this clade also including early edrioasteroids. Expressions such as 2-1-2 ambulacral symmetry, moveable cover plates over the peristiome (mouth) and beyond, and hydropore position, are examples. Ordovician blastozoans and crinoids were diverse, rapidly radiating into an amazing array of forms, many of which share similar designs and life modes. Stemmed forms with ovoid thecae became standardized, with reduced plating often trending toward true pentamery, this pattern alone providing fertile ground for convergence. Traits such as oral plate development, differentiated peristomial cover plates, and shift of 2-1-2 pattern to near pentamery, are potentially independently derived. Greatest similarity among potential sister groups is found in earlier presumably basal taxa, but greatest resemblance of crinoids to blastozoans is found in later, derived taxa. Except for a few later Paleozoic taxa, “oral” bearing crinoids are all cyathocrinines or hybocrinids, mostly from the Late Ordovician. Further, “oral” similarity is incomplete. Crinoid “orals” express coelomic notches leading into feeding appendages from the theca and a madrepore, traits unknown among blastozoans. Consistent stalk, feeding appendage, and oral region differences lead to the conclusion of crinoid derivation from an unknown pentaradiate ancestry. Basal crinoid traits not found in blastozoans include thin edrioasteroid-like floor plates bearing podial pores, platelet webs at arm branches, regular interlocking stem and cup, and pentameric stalk. The early crinoid record now includes over 20 Floian, Early Ordovician, taxa awaiting description, hard evidence for better resolution of the current controversy.