Abstract

A relatively large new specimen of the callocystitid rhombiferan Novacystis hawkesi Paul and Bolton, 1991, is described from the Wenlock (Middle Silurian) Thornloe Formation of east-central Ontario. This new specimen adds to the known morphology of Novacystis hawkesi, including the plating of the thecal side opposite the periproct, the presence of four pectinirhombs in standard callocystitid placements, and brachiole morphology. It also confirms the unusual placement of the periproct previously described for this species. A phylogenetic analysis of Callocystitidae indicates that, as presently defined, Callocystitinae sensu Paul and Bolton is polyphyletic and Apiocystitinae is paraphyletic. Novacystis forms part of a small, relatively primitive echinoderm fauna, probably of middle Wenlock age from the Timiskaming area, suggesting that shallow cratonic areas of the Canadian shield area may have served as refugium for archaic echinoderm taxa.