Cotylorhynchus
Cotylorhynchus is an extinct genus of very large synapsids that lived in the southern part of what is now North America during the Early Permian period.[1]: 161 It is the best known member of the synapsid clade Caseidae, usually considered the largest terrestrial vertebrates of the Early Permian,[2] though they were possibly aquatic.[3]
Cotylorhynchus was a heavily built animal (and one of the largest synapsids) with a disproportionately small head and a huge barrel-shaped body. While the smaller species, C. romeri, only grew to lengths of 4.5 - 4.8m (14.7 - 15.7ft), the larger species, C. hancocki, stretched 6m (20ft) long.[4]
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. They also featured a large parietal eye and a snout or upper jaw that overhangs the row of teeth to form a projecting rostrum. Rounded deep pits and possibly large depressions were present on the outer surface of the skull. Their teeth were very similar to those of iguanas with posterior marginal teeth that bore a longitudinal row of cusps.
Their skeletal features included a massive scapulocoracoid, humeri with large flared ends, stout forearm bones and broad, robust hands that had large claws. Certain features of their hands indicate that they were paddle-like in shape and structure, being used to swim in a manner much similar to that of modern turtles.[5]
Their digits were believed to have a considerable range of motion and large retractor processes on the ventral surfaces of the unguals allowed them to flex their claws with powerful motions. Also, the articulatory surfaces of their 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.
Cotylorhynchus shows evidence indicative of a diaphragm. Unlike that of modern mammals it was probably weak and necessitating support from other muscle groups, a deficiency exacerbated by its aquatic habits.[5]
