Article: Coronate echinoderms from the Lower Palaeozoic of Britain
Stephen K. Donovan and Christopher R. C. Paul
Coronates are pelmatozoan echinoderms with a functional stem, a bud-shaped theca, and erect, biserial, pinnate arms. They evolved early in the Middle Ordovician, probably from the 'eocrinoid' Bockia, and gave rise to the blastoids sensu stricto. The subclass Coronata contains six genera. Mespilocystites (early Caradoc-Ashgill) had geniculate radial furrows, while all later genera had planar or gently convex radii. Of these Stephanoblastus (Caradoc-Wenlock) had a very narrow stem and triradiate keels at the base of the theca. All other genera had triangular thecal bases. Tormoblastus (Ashgill) had a conical theca with a protruding base bearing three flanges, Paracystis (Caradoc) had a bowl-shaped theca with a sunken base, Stephanocrinus (Ashgill-Ludlow) a tall, steeply conical theca, and Cupulocorona gen. nov. (Ashgill-Wenlock) a conical to cup-shaped theca with a protruding base. The British coronate fauna includes five new species: S. ramsbottomi (Hirnantian) characterized by a large, angular conical theca with low coronal processes, C. salopiae (early Wenlock) characterized by a pyriform theca with low coronal processes and low ridges at the plate sutures, C. rugosa (Cautleyan-Rawtheyan) with a conical theca bearing very coarse ornament, C. digitalis (Cautleyan-Rawtheyan) with a conical theca and very long coronal processes, and Stephanocrinus sensu lato sp. (Cautleyan) which is poorly known but had very fine ribbing.