THE SYSTEMATICS OF CORONOID ECHINODERMS

The clade Coronoidea is a small group of blastozoan echinoderms, ranging from the Middle Ordovician to the Late Silurian of North America and Eurasia. The systematic position of this clade as the sister group to the clade including Lysocystites and true blastoids had been generally agreed upon in recent years. However, evolution within Coronoidea has been largely ignored.

An hypothesis of species-level relationships within Coronoidea is presented here, based upon the results of a cladistic analysis of 15 morphological characters scored for the 14 described coronoid species and two new species using Caryocrinites ornatus and Lysocystites sculptus as the outgroup. Cladistic analysis yields two most parsimonious trees, differing only in the positions of Mespilocystites lemenni and M. tregarvanicus. The clade Cupulocorona includes two species: “Stephanocrinus” gemmiformis and “S”. osgoodensis, formerly placed within Stephanocrinus and appears to be the most primitive group within Coronoidea , possessing small globular thecae and simple ornamentation. All other coronoid species form the sister group to this clade. The clade Mespilocystites contains the oldest known coronoid species M. bohemicus, which because of its age (Lower Caradoc) has been assumed to represent the most primitive coronoid. However, cladistic analysis does not support this hypothesis. M. bohemicus possesses a number of derived characters such as a conical theca, flat interambulacra, geniculate radial furrows, and flared coronal processes. The clade Stephanocrinus is the sister group to the clade including both Mespilocystites and Stephanoblastus. Synapomorphies of this clade are distinct thecal ridges, thick, sinuous ornamentation, and near vertical coronal processes. The oldest and most primitive member of Stephanocrinus is S. ramsbottomi from the Ashgill of Great Britain. All other species of Stephanocrinus are from the Wenlock of North America and possess abaxial thickening of the radial limbs and a flat platform formed by the adoral surface of the deltoids. The systematic relationships presented here differ somewhat from non-cladistic interpretations of coronoid evolution, but are well supported by morphological evidence and are fairly congruent with stratigraphic data.