Abstract

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. Emended descriptions of the genus Stephanocrinus and the two common species S. angulatus and S.(?) gemmiformis, incorporate new data on the structure and homologies of the ventral plates, brachiolar appendages, hydropore orifice, coronal canals, column and holdfast. 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.--Modified journal abstract.

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