 "Cotylorhynchus" was a heavily built animal with a disproportionately small head and a huge barrel-shaped body.Adults of the species "C. romeri" were about  while those of the younger "C. hancocki" were around 20-25% larger in linear measurements, making it one of the largest synapsids of the early Permian.Their skulls are distinctive in the presence of large temporal openings and very large nostril openings, which could have been utilized for better breathing or may have housed some sort of sensory or moisture conserving organ.