Cotylorhynchus is an extinct genus of herbivorous caseid synapsids that lived during the late Lower Permian (Kungurian) and possibly the early Middle Permian (Roadian) in what is now the states of Texas and Oklahoma in the United States. The large number of specimens found make it the best-known caseid.
Like all large herbivorous caseids, Cotylorhynchus had a short snout sloping forward and very large external nares. The head was very small compared to the size of the body, which was massive, barrel-shaped, and ended with a long tail. The barrel-shaped body must have housed large intestines, suggesting that the animal had to feed on a large quantity of plants of low nutritional value. The limbs were short and robust. The hands and feet had short, broad fingers with powerful claws.
Caseids are generally considered to be terrestrial, though a semi-aquatic lifestyle has been proposed by some authors. The genus Cotylorhynchus is represented by three species, the largest of which could reach more than in length. However, a study published in 2022 suggests that the genus may be paraphyletic, with two of the three species possibly belonging to separate genera.
Discovery
The genus name Cotylorhynchus comes from the Greek kotyle ("cup" or "hollow"), and rhynchos ("beak" or "snout"). The genus was named after the nasal opening, which is surrounded by a depressed, cup-shaped bony surface.
The genus Cotylorhynchus contains three species which differ in size and proportion: C. romeri (the type species), C. hancocki, and C. bransoni. There are two size groups in C. romeri, which presumably represents sexual dimorphism. There is no size overlap between adults of C. romeri and C. hancocki, but larger specimens of C. bransoni have roughly the same dimensions as smaller specimens of C. romeri. In 2022, Werneburg and colleagues suggested that the species C. hancocki and C. bransoni might not belong to the genus Cotylorhynchus. These authors consider that a detailed revision of these two taxa is necessary to clarify their status.
Description
The skull of Cotylorhynchus exhibits the typical caseid morphology, with a forward sloping snout, very large nasal opening, a skull roof with numerous small depressions, and a very large pineal foramen. The latter is more wide than long, as in Ennatosaurus, and thus differs from that of Euromycter, which is subcircular. The number of teeth in the upper and lower jaws ranges from 16 to 20. In the upper jaw, the anterior teeth are long and slender, while those behind decrease in size posteriorly and are slightly spatulate. All of the marginal teeth have their distal end slightly inclined towards the interior of the mouth, and the top of their crowns each have three small cuspules arranged longitudinally. These teeth also show an enlargement of the central part of the crown. In the lower jaw, the anterior teeth, not denticulate according to Olson, are shorter and tilt slightly forward. Other lower teeth are similar to those in the upper jaw.
The postcranial skeleton is massive. The ribs are very long, heavy and curved to form a bulbous body. Ribs are present on all the presacral vertebrae and the first caudal vertebrae. The five posterior presacral ribs are fused with the transverse processes of the vertebrae. The sacrum contains three vertebrae. The neural spines of larger specimens become proportionately taller, especially in the pelvic region. The limbs are short and strong. The femur is characterized by its proximal end having a broad shelf marked by a margin slightly overhanging the dorsal surface of the femur. The pes and manus are broad and short, and terminate in strong, sharp, and curved ungual phalanges which must have supported powerful claws. Muscle and tendon scars are very developed.
Cotylorhynchus romeri
The type species Cotylorhynchus romeri is the best known species of the genus. 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 in Logan County, Oklahoma. The name of the species honors the American paleontologist Alfred Sherwood Romer.
Shortly after the holotype's discovery, numerous specimens were found in some 20 sites surrounding the town of Norman in Cleveland County, Oklahoma, also from the Hennessey Formation. These finds include several fairly complete skeletons and many more fragmentary ones, totaling about 40 individuals. Specimens from the two regions are more or less contemporaneous and are only known within a thick stratigraphic interval. In Navina, the holotype comes from a level about above the base of the Hennessey Formation. The numerous specimens from the Norman area have been found in several layers located between above the base of the formation.
The holotype of C. romeri has 20 teeth in upper jaws (three on the premaxilla and 17 on the maxilla) and 19 teeth in lower jaws. C. romeri from the Norman region show a lower number of teeth. Four skulls where tooth counting was possible have 15 or 16 teeth in upper jaws. Some authors have thus considered that the holotype of C. romeri and the referred specimens from Norman represent two different species. However, the lack of specimens in the type locality (the holotype of C. romeri being the only known fossil there) and the number of teeth being the only difference with the Cotylorhynchus from Norman, it was decided to keep all these specimens in the same species.
C. romeri is a large species that can exceed in length and in weight according to Romer and Price, or in length according to Stovall. Robert Reisz and colleagues have identified several cranial autapomorphies in this species. 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.
As the other two species of Cotylorhynchus, the dentition consists of tricuspid teeth (except for the most anterior teeth). However, C. romeri is the species where the cuspules are least developed. According to Olson, the premaxillary teeth had no cuspules. The latter, however, have been reported on the premaxillary teeth by Reisz and colleagues. All marginal teeth have their distal ends curved lingually. Numerous teeth are also present on several bones of the palate (palatal dentition). A short row of three large, slightly recurved teeth are present on each vomer. They are taller than all other teeth on the palate. 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. The latter, triangular in shape, has many teeth divided into four distinct groups: a medial row bordering the interpterygoid vacuity, a group of smaller teeth which contributes to the pterygo-palatine tooth cluster, a posterolateral cluster of very small teeth on the transverse flange of the pterygoid, and behind this cluster a row of large teeth that borders the posterior margin of the transverse flange and extends medially to the basicranial region. In summary, the pterygoid bears more, smaller and slender teeth than those present on the pterygoid of C. bransoni. A few teeth are also present on the parasphenoid. Several palatal teeth have well-preserved tips showing the same distal morphology as marginal teeth with three small cuspules. In lower jaws, the dentary has between 16 and 19 teeth, which have the same morphology as the teeth of the upper jaws. In C. romeri, the dental row does not show spaces for replacement teeth which could be related to reduced rates of tooth replacement and increased longevity of functional teeth.
The vertebral column consists of 25 or 26 presacral vertebrae, three sacral vertebrae, and approximately 55 caudal vertebrae. C. romeri is distinguished by its widely spaced postzygapophyses on the dorsal vertebrae, while in C. hancocki and C. bransoni they usually contact along the midline. The relatively short limbs were more robust than those of C. bransoni but less massive than those of C. hancoki. The manus and the pes show a phalangeal formula of 2-2-3-3-2. The skeletons from the Norman region show two different size groups within adult specimens. One of these groups is composed of individuals about 20% smaller than those in the other group. This size difference was interpreted as possible specific differentiation or more likely as the expression of sexual dimorphism.
Cotylorhynchus hancocki 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. The species is named after J. Hancock, who made it possible to explore much of the locality of Pease River. 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. This site has yielded the most complete specimens of the species such as FMNH UR 581, an almost complete skeleton missing only the skull, some cervical vertebrae, a scapulocoracoid and some limb bones; FMNH UR 622, a partial skeleton including part of the skull and palate, various vertebrae, ribs, limb bones, clavicle, and bones of the foot; and FMNH UR 703, part of the skeleton of a very large individual including dorsal, lumbar, sacral, and caudal vertebrae, pelvis, femur, radius, ulna, and ribs. Other notable specimens include several isolated cranial bones. All of the skull bones known in this species come from the Kahn quarry.
With a size of up to in length and a weight of over , C. hancocki is by far the largest species of the genus, and is one of the largest known caseids along with the genus Alierasaurus. Its dimensions also make it one of the largest non-mammalian synapsids. No complete skull of C. hancocki is known. The various known elements (maxilla, dentaries, braincase, palate bones), indicate a skull similar to that of C. romeri but slightly larger. The upper teeth are not fully known. Several isolated mandibles show that the lower dentition had up to 18 slightly spatulate and tricuspid teeth. The cuspules of the upper teeth are weaker than those of the lower teeth. In addition, cuspules of C. hancocki are more pronounced than those of C. romeri, but less developed than those of C. bransoni.