Dinocephalia

Dinocephalians were a group of generally large-bodied therapsids that thrived during the Middle Permian, from about 275 to 260 million years ago, before dying out in the Capitanian mass extinction. This clade included both carnivorous forms (the Anteosauria) and herbivorous or omnivorous ones (the Tapinocephalia), making them the dominant megaherbivores and apex predators of their time. Many species had thickened skulls covered in knobs and bony projections. They were the first non-mammalian therapsids to be scientifically described, and their fossils have been found in Russia, China, Brazil, South Africa, Zimbabwe, and Tanzania.

Dinocephalians were generally large, with some ranking among the biggest animals of the Permian and being the first synapsids to reach a body mass of around one tonne. The herbivorous or omnivorous *Jonkeria* has been estimated at a certain weight, while the large carnivore *Anteosaurus* has been estimated based on scaled 3D models.

Their skulls and lower jaws share several distinctive features. The vomerine process of the premaxilla is lost, so the vomer bone borders the main part of the premaxilla. The transverse flange of the pterygoid bone lies forward of the eye socket. There are no depressions or ridges on the outer surface of the reflected lamina of the angular bone, and a foramen is visible on the inner surface between the prearticular and angular bones. The temporal fenestra at the back of the skull is relatively enlarged, as are the quadrate and quadratojugal bones. Unusually among therapsids, the premaxillae have a long, backward-pointing spur that extends between the paired nasal bones, sometimes preventing the nasals from touching and reaching all the way to the frontal bones. The septomaxilla usually has a small surface exposure. There is no preparietal bone. The prefrontal and lacrimal bones are generally large and help form the eye socket border. The tabular bone at the back of the skull is also large. The zygomatic arch is made up of the squamosal bone. The single temporal fenestra varies in size and shape, from small and slit-like to large. The quadratojugal is elongated and supports the quadrate, which is large and extends forward and downward. The stapes are short and thick. Most dinocephalians also developed a thickened, ornamented skull roof and braincase, sometimes with horn-like projections, making their skulls highly resistant to fracture.

All dinocephalians have interlocking incisor teeth, a trait once thought unique to them but also found in biarmosuchians. In tapinocephalids, all teeth interlock, though less closely in the back. Ancestrally, dinocephalians have a pair of enlarged canine teeth in both jaws, though these are absent in many tapinocephalids. Teeth generally have a pointed "talon" and a "heel" for cutting or crushing food, with this morphology best developed in tapinocephalines, especially tapinocephalids, which show precise occlusion for shearing. Some tapinocephalines, like titanosuchids and estemmenosuchids, have leaf-shaped postcanine teeth. In anteosaurs and titanosuchids, which have differentiated incisors and canines, there are usually 4–5 pairs of incisors and a pair of canines in each jaw, with postcanine numbers ranging from 8–11 in anteosaurs to 19 in the upper jaw and 18–21 in the lower jaw of titanosuchids. In tapinocephalids, teeth show little differentiation, often lacking distinct canines, with 3–5 teeth in the premaxilla, 8–17 in the maxilla, and 14–19 in the lower jaw. Ancestrally, dinocephalians had palatal teeth on the pterygoid and palatine bones, with *Estemmenosuchus* also having them on the vomer, uniquely among therapsids. Advanced tapinocephalids lost all palatal teeth. The coronoid bones and coronoid process of the lower jaw were lost.

In the postcranial skeleton, a proatlas bone between the atlas vertebra and skull was probably present. The vertebrae are concave on both faces. The neck typically has seven or eight cervical vertebrae, with about 33 vertebrae total before the sacrum. Tail length and number of caudal vertebrae vary widely, from seven in the tapinocephalid *Tapinocaninus* to over 60 in some anteosaurs, with the caudal vertebrae getting shorter toward the tail tip. Intercentra are associated with the caudal, atlas, and axis vertebrae but not with the remaining cervical or dorsal vertebrae. The cleithrum is reduced, and the dorsal process of the coracoid is lost. Dinocephalian limb bones are generally built for muscle strength, and the animals likely had a sprawling posture.