Cotylorhynchus

Cotylorhynchus is a genus of extinct herbivorous synapsids from the caseid family, known from numerous fossils found in Texas and Oklahoma. These animals lived during the late Lower Permian (Kungurian) and possibly into the early Middle Permian (Roadian). Because so many specimens have been recovered, it is the best-understood caseid. The genus includes three species, with the largest reaching over 6 meters (20 feet) in length.

Like other large herbivorous caseids, Cotylorhynchus had a short snout that sloped forward and very large nasal openings. Its head was tiny compared to its massive, barrel-shaped body, which ended in a long tail. This barrel shape likely housed large intestines, suggesting the animal ate large amounts of low-nutrient plants. Its limbs were short and sturdy, with broad hands and feet ending in short, wide fingers and powerful claws. Most researchers consider caseids terrestrial, though some have proposed a semi-aquatic lifestyle.

The name Cotylorhynchus comes from the Greek words *kotyle* ("cup" or "hollow") and *rhynchos* ("beak" or "snout"), referring to the cup-shaped depression around the nasal opening. The three species—*C. romeri* (the type species), *C. hancocki*, and *C. bransoni*—differ in size and proportions. In *C. romeri*, two size groups exist, likely due to sexual dimorphism. Adults of *C. romeri* and *C. hancocki* do not overlap in size, but larger *C. bransoni* specimens are similar in size to smaller *C. romeri*. A 2022 study suggested that *C. hancocki* and *C. bransoni* might not belong to the genus Cotylorhynchus, calling for a detailed revision.

The skull shows typical caseid features: a forward-sloping snout, very large nasal openings, a skull roof with many small depressions, and a large pineal foramen that is wider than long (like *Ennatosaurus* but unlike the subcircular one in *Euromycter*). Tooth counts range from 16 to 20 in both upper and lower jaws. Upper front teeth are long and slender, while those behind are shorter and slightly spatulate. All marginal teeth tilt inward at the tip, with crowns bearing three small cuspules in a row and an enlarged center. Lower front teeth are shorter and tilt forward, with the rest similar to upper teeth.

The postcranial skeleton is massive. Ribs are long, heavy, and curved, forming a bulbous body. Ribs attach to all presacral vertebrae and the first caudal vertebrae; the five rearmost presacral ribs fuse to the vertebrae. The sacrum has three vertebrae. In larger specimens, neural spines become proportionally taller, especially in the pelvic region. Limbs are short and strong. The femur has a broad shelf at its proximal end, with a margin slightly overhanging the dorsal surface. The feet and hands are broad and short, ending in strong, sharp, curved claws. Muscle and tendon scars are well-developed.

The type species, *C. romeri*, is the best known. It was named in 1937 by J. Willis Stovall after paleontologist Alfred Sherwood Romer, based on a holotype (OMNH 00637) from the Hennessey Formation near Navina, Oklahoma. This specimen includes the right side of a skull, an incomplete interclavicle, and both hands. Soon after, about 40 individuals—some fairly complete skeletons, others fragmentary—were found at roughly 20 sites near Norman, Oklahoma, also in the Hennessey Formation. Specimens from both areas are contemporaneous and occur within a thin stratigraphic interval. The Navina holotype comes from a level about 20 meters above the formation’s base; the Norman specimens come from layers between 30 and 40 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 Norman have fewer teeth: four skulls show 15 or 16 upper teeth. Some researchers have suggested the holotype and the Norman specimens might represent different species, but the lack of additional specimens from the type locality—the holotype is the only one found there—makes this uncertain.