Coronoidea: A new class of blastozoan echinoderms based on taxonomic reevaluation of Stephanocrinus

The new Class Coronoidea is proposed to include five genera of Ordovician-Silurian blastozoan echinoderms-Stephanocrinus, Mespilocystites, Paracystis, Stephanoblastus and Tormoblastus-formerly referred to the Order Coronata of the inadunate crinoids. Reassignment of these genera is based largely upon features observed in well-preserved specimens of Stephanocrinus angulatus Conrad and S.(?) gemmiformis Hall from the Silurian of New York and Indiana. Criteria include: 1) the coiled arm-like appendages are exothecal, homologous with brachioles ofblastozoans rather than with crinoid arms; 2) plates in the oral region can be homologized with deltoids of blastoids; 3) growth lines are external; and, 4) the coronal crests bear unique U-shaped coelomic canals. Coronoids differ from eocrinoids in having regular, blastoid-like arrangement of thecal plates and in having coronal canals rather than epispires. Coronoids are most similar to blastoids but lack hydrospires and lancet plates and possess specialized, erect, brachiolar trunks rather than recumbent ambulacra. Emended descriptions of the genus Stephanocrinus and the two common species S. angulatus and S.(?) gemmiformis, presented herein, incorporate new data on the structure and homologies of the ventral plates, brachiolar appendages, hydropore orifice, coronal canals, column and holdfast. Review of the functional morphology of Stephanocrinus suggests that these coronoid echinoderms were permanently attached by a column of moderate length and a cemented discoidal holdfast. Coronal canals probably served a respiratory function, gas exchange between sea water and coelomic fluids taking place through thin stereom of the coronal processes. Additionally, coronal processes may have served to protect ambulacral appendages, to provide hydrodynamic stability, and/or to funnel currents through the brachioles. Abnormalities occur in a low proportion of Stephanocrinus specimens (0.5%), comparable to the frequencies of aberrant specimens of blastoids and smallcrinoids. Similarities of thecal structure suggest that coronoids and blastoids are phylogenetically related. The two lineages may be related through a common Cambrian-Early Ordovician ancestor. Alternatively, blastoids may have evolved from coronoids by paedomorphosis.