Cotylorhynchus

Cotylorhynchus is a genus of extinct, plant-eating caseid synapsids known from the late Lower Permian (Kungurian) and possibly the early Middle Permian (Roadian) of Texas and Oklahoma. Because so many specimens have been uncovered, it is the most thoroughly understood caseid. The genus includes three species; the largest could exceed 6 meters (20 feet) in length.

Like other large herbivorous caseids, Cotylorhynchus had a short, forward-sloping snout and very large external nostrils. Its head was tiny compared to its massive, barrel-shaped body, which ended in a long tail. That barrel shape likely housed large intestines, suggesting the animal consumed great amounts of low-nutrient plants. Its limbs were short and sturdy, and its hands and feet bore short, broad fingers tipped with powerful claws. Most researchers consider caseids terrestrial, though some have proposed a semi-aquatic lifestyle. A 2022 study suggested that the genus may be paraphyletic, with two of its three species possibly belonging to separate genera.

The name *Cotylorhynchus* comes from Greek *kotyle* (“cup” or “hollow”) and *rhynchos* (“beak” or “snout”), referring to the depressed, cup-shaped bony surface around the nasal opening. The three species—*C. romeri* (the type), *C. hancocki*, and *C. bransoni*—differ in size and proportions. Within *C. romeri*, two size groups likely represent sexual dimorphism. Adults of *C. romeri* and *C. hancocki* do not overlap in size, but larger *C. bransoni* specimens are about the same size as smaller *C. romeri*. In 2022, Werneburg and colleagues argued that *C. hancocki* and *C. bransoni* might not belong to *Cotylorhynchus* and called for a detailed revision.

The skull shows the typical caseid shape: a forward-sloping snout, a very large nasal opening, a skull roof dotted with small depressions, and a very large pineal foramen that is wider than long (like *Ennatosaurus*, unlike the subcircular one of *Euromycter*). Upper and lower jaws each hold 16 to 20 teeth. In the upper jaw, front teeth are long and slender, while those behind are shorter and slightly spatulate. All marginal teeth tilt slightly inward at the tip, and their crowns each have three small cuspules arranged lengthwise, with an enlarged central part. In the lower jaw, the front teeth are shorter, tilt slightly forward, and are not denticulate (according to Olson); the rest resemble those in the upper jaw.

The postcranial skeleton is massive. The ribs are very long, heavy, and curved, forming a bulbous body. Ribs attach to all presacral vertebrae and the first caudal vertebrae; the five posterior presacral ribs are fused to the transverse processes. The sacrum has three vertebrae. In larger specimens, the neural spines become proportionately taller, especially over the pelvis. Limbs are short and strong. The femur has a broad shelf at its proximal end, with a margin that slightly overhangs the dorsal surface. The feet and hands are broad and short, ending in strong, sharp, curved ungual phalanges that supported powerful claws. Muscle and tendon scars are very pronounced.

*Cotylorhynchus romeri* is the best-known species. J. Willis Stovall named it in 1937 based on holotype OMNH 00637—a partial skull, an incomplete interclavicle, and both manus—found in red mudstones of the lower Hennessey Formation near Navina, Logan County, Oklahoma. The species honors paleontologist Alfred Sherwood Romer. Soon after, about 40 individuals (several fairly complete skeletons and many fragments) were discovered at roughly 20 sites around Norman, Cleveland County, Oklahoma, also in the Hennessey Formation. Specimens from both areas are roughly contemporaneous and come from a stratigraphic interval about 30 meters thick. The Navina holotype came from a level about 15 meters above the formation’s base; the Norman specimens came from layers between 25 and 30 meters above the base.

The holotype of *C. romeri* has 20 upper teeth (three on the premaxilla, 17 on the maxilla) and 19 lower teeth. Specimens from the Norman area have fewer teeth: four skulls with countable teeth show 15 or 16 in the upper jaw. Some authors have therefore suggested that the holotype and the Norman specimens represent two different species. However, the holotype is the only specimen from its locality, so the question remains unresolved.