It was erected in 1937 by J. Willis Stovall from the holotype OMNH 00637, consisting of the right side of a skull, an incomplete interclavicle, and the right and left manus, found in the red mudstones of the lower part of the Hennessey Formation, near the locality of Navina, Logan County, Oklahoma.

[1] Shortly after the holotype's discovery, numerous specimens were found in some 20 sites surrounding the town of Norman, Cleveland County, also from the Hennessey Formation.

Cotylorhynchus romeri is distinguished by transversely broad postparietals that contact the supratemporals laterally, a large supratemporal that restricts contact between the parietal and postorbital, a stapes that has a short massive distal shaft and a ventral process that is braced against the quadrate ramus of the pterygoid, both vomers bearing three large teeth along the medial edge of the bone, the presence of teeth on the parasphenoid, and a surangular overlapping the posterodorsal tip of the dentary and excluding it from the coronoid eminence.

However, Reisz and colleagues emphasize the fact that these autapomorphies are ambiguous because they are identified, with a few exceptions (a few bones of the palate), on parts of the skull still unknown in other species of the genus, thus limiting comparisons.

The palatines bear 10 subconical teeth located on a slightly thickened region of bone adjacent to the middle part of the suture shared with the pterygoid.

[6][2] Cotylorhynchus hancocki was named in 1953 by Everett Claire Olson and James R. Beerbower, from a right humerus and a proximal end of a tibia (constituting the holotype FMNH UR 154) found in the upper part of the San Angelo Formation, near the Pease River, in Hardeman County, Texas.

[9] Subsequently, more than sixty specimens, ranging from isolated bone to nearly complete skeleton, were discovered in several localities in Knox County, the majority however coming from the Kahn quarry.

[10][15] Cotylorhynchus bransoni was named in 1962 by Everett C. Olson and Herbert Barghusen from numerous bones found in the Omega Quarry in Kingfisher County, Oklahoma.

According to these authors, the species Cotylorhynchus hancocki and C. bransoni would not belong to this genus and would require a detailed revision to clarify their status, these taxa not having been studied since the 1960s.

The hyoid apparatus preserved in some caseids (Euromycter and Ennatosaurus), indicates the existence of a relatively mobile massive tongue which must have worked in concert with the palatal teeth during swallowing.

[2] However, the very short neck implied a low amplitude of vertical movements of the head which precluded the large species from feeding at ground level.

[20] This hypothesis is however disputed by Kenneth Angielczyk and Christian Kammerer as well as by Robert Reisz and colleagues based on paleontological and taphonomic data combined with the absence in these large caseids of morphological adaptations to an aquatic lifestyle.

According to Angielczyk and Kammerer, the low bone density of caseids identified by Lambertz et al. does not resemble that of semiaquatic animals, which tend to have a more strongly ossified skeleton to provide passive buoyancy control and increased stability against current and wave action.

Cotylorhynchus bone microstructure is more similar to what is seen in animals living in the open ocean, such as cetaceans and pinnipeds, which emphasize high maneuverability, rapid acceleration and hydrodynamic control of buoyancy.

Thus, due to these unusual data, Angielczyk and Kammerer consider that the available evidence is still insufficient to question the more widely assumed terrestrial lifestyle of caseids.

[21] According to Reisz and colleagues the presence of numerous skeletons of the amphibian Brachydectes preserved in estivation and of the lungfish Gnathorhiza, another well-known aestivator, combined with the absence of obligate aquatic vertebrates strongly suggests that the Hennessey fauna lived in a dry habitat periodically punctuated by monsoons.

Combined with the fact that Cotylorhynchus shows no morphological adaptations to an aquatic lifestyle, these authors consider it as a terrestrial animal that had to endure monsoon rains, with some individuals occasionally succumbing to major floods.

[4] In 2022, Werneburg and colleagues proposed a somewhat different semiaquatic lifestyle, in which large caseids like Lalieudorhynchus (whose bone texture is even more osteoporotic than that of Cotylorhynchus) would be ecological equivalents of modern hippos, passing part of their time in the water (being underwater walkers rather than swimming animals) but coming on land for food.

[22] Ammonoid faunas found in marine strata present at the base and top of the Clear Fork Group indicate that the three formations that compose it (Arroyo, Vale, and Choza) are entirely included in the Kungurian.

[23][26][24] However, Michel Laurin and Robert W. Hook argued that the fusuline marine intercalation cited above does not belong to the San Angelo Formation in which it was mistakenly included, and cannot be used to date the latter.

[25] A Roadian age was also suggested based on the presence in the Chickasha fauna of the nycteroleterid parareptile Macroleter, a genus that was only known from the Middle Permian of European Russia.

[31] The Hennessey, San Angelo, and Chickasha formations correspond mainly to fluvial and aeolian sediments deposited in a vast deltaic plain dotted with lakes and lagoons.

The rivers ending in the delta came from modest reliefs located further east and corresponding to the ancestral uplifts of the Ouachita, Arbuckle and Wichita mountains.

The continental facies is mostly composed of red mudstones, locally accompanied by lenses and beds of sandstones and siltstones interpreted as fluvial and floodplain deposits.

[22] However, detailed facies analyses later revealed that these rocks were more likely of aeolian origin, corresponding to silts, clays, and sands deposited as loess and sometimes trapped in mud flat, shallow salt lakes or wadi-type ephemeral streams.

[20] Apart from C. romeri, other known vertebrates in the Hennessey Formation are the Captorhinidae Captorhinikos chozaensis[35] and Rhodotheratus parvus,[36] the lungfish Gnathorhiza,[22][4] and the amphibians Diplocaulus, Brachydectes,[37][38] Rhynchonkos, Aletrimyti, and Dvellacanus.

The central part of the formation consists mainly of red mudstones corresponding to clayey and silty mud deposited in coastal plains during periodic flooding episodes.

[10] The fauna of the upper San Angelo Formation includes, among others,[nb 4] the caseid Caseopsis agilis and Angelosaurus greeni, the sphenacodontid Dimetrodon angelensis, the captorhinids Rothianiscus multidonta, and Kahneria seltina, and the tupilakosaurid dvinosaur Slaugenhopia.

[9][10] A few tetrapod tracks also indicate the presence of a nycteroleterid pareiasauromorpha (ichnotaxon Pachypes ollieri), a partial skeleton of which is known from slightly younger deposits of the Chickasha Formation.

The sediments that compose it are varied and include red shales, sandstones, mudstones, conglomerates, and evaporites, deposited in floodplains and channels bordering the sea and coastal lagoons.