Cotylorhynchus was a very large synapsid that lived in the southern part of what is now North America during the Early Permian period. It was the largest member of the clade Caseidae  and was the largest terrestrial vertebrate of its time. It was massively built animal with a disproportionately small head and a huge barrel-shaped body. It could grow up to 6 metres (20 feet) long and could weigh up to 2 tons. Its skeletal features included a massive scapulocoracoid, humeri with large flared ends, stout forearm bones and broad, robust hands with large claws. The digits were thought to have a considerable range of motion and large retractor processes on the ventral surfaces of the unguals let them flex their claws with powerful motions. The articulatory surfaces of the phalanges were oblique to the bone's long axis rather than perpendicular to it. This allowed for much more surface area for the flexor muscles. The skull had large temporal openings and very large nostril openings, which could have been used for better breathing or may have housed a sensory or moisture conserving organ. It had large pineal openings and a snout or upper jaw overhanging the row of teeth to form a projecting rostrum. There were rounded deep pits and possibly large depressions on the outer surface of the skull. The teeth were very similar to those of iguanas, with posterior marginal teeth bearing a longitudinal row of cusps.
Cotylorhynchus was a herbivore, and due to its enormous size, probably had no predators. Features of the hands indicate that it had to dig considerably to obtain their food supply and may have used these features to dig burrows for shelter or safety.
Cotylorhynchus was considered part of the first wave of amniote diversity. The three species are C. hancocki (northern Texas in Hardeman and Knox counties), C. romeri (central Oklahoma in parts of Cleveland County) and C. bransoni (Kingfisher and Blaine Counties of central-northwest Oklahoma).
C. hancocki is thought to be a descendent of the slightly smaller C. romeri.