Dinocephalia

Dinocephalia, whose name comes from the Ancient Greek for "terrible head," were a group of generally large-bodied therapsids that thrived during the Middle Permian, roughly between 275 and 260 million years ago. They died out during the Capitanian mass extinction. This clade included both meat-eaters (the Anteosauria) and plant-eaters or omnivores (the Tapinocephalia), making them the top predators and dominant large herbivores of their time. Many species had thickened skulls covered in knobs and bony bumps. Dinocephalians were the first non-mammalian therapsids to be formally described by science, and their fossils have been found in Russia, China, Brazil, South Africa, Zimbabwe, and Tanzania.

In terms of size, dinocephalians were often huge, with some being the largest animals of the Permian and the first synapsids to reach a body mass of around one tonne. The herbivorous or omnivorous *Jonkeria* may have weighed about 989 kilograms, while the big carnivore *Anteosaurus* is estimated at around 400 kilograms based on scaled 3D models.

Their skulls and lower jaws share several key 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 sits in front 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 present between the prearticular and angular bones on the inner side of the lower jaw. The temporal fenestra at the back of the skull is relatively large, and the quadrate and quadratojugal bones are also enlarged. Unusually for therapsids, the premaxillae have a long, backward-pointing spur that sits between the paired nasal bones; in some species, this spur reaches the frontal bones, preventing the nasals from touching. The septomaxilla usually has only a small exposure on the skull surface, and there is no preparietal bone. The prefrontal and lacrimal bones are generally large and help form the eye socket’s border, while the tabular bone at the back of the skull is also large. The zygomatic arch is made 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 itself is large and stretches 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 very strong and resistant to fracture.

All dinocephalians have interlocking incisor teeth, though this trait is also seen in biarmosuchians. In tapinocephalids, all teeth interlock, not just the front incisors, though the back teeth interlock less tightly. Ancestrally, dinocephalians had a pair of enlarged canine teeth in both the upper and lower jaws, but many tapinocephalids lack these. The teeth generally have a pointed "talon" and a "heel" that cuts or crushes food; this shape is most developed in tapinocephalines, especially tapinocephalids, where precise interlocking allows effective shearing. Some tapinocephalines, like titanosuchids and estemmenosuchids, have leaf-shaped postcanine teeth. In anteosaurs and titanosuchids, which have distinct incisors and canines, there are usually 4–5 pairs of incisors and one 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 (and *Estemmenosuchus* uniquely also had them on the vomer), but these were completely lost in advanced tapinocephalids. The coronoid bones and the corresponding coronoid process on the lower jaw were lost in dinocephalians.

In the postcranial skeleton, a proatlas bone between the atlas vertebra and the skull was probably present. The vertebrae are concave on both faces (amphicoelous). The neck usually has seven or eight cervical vertebrae, with about 33 vertebrae total in front of the sacrum. Tail length and the number of caudal vertebrae vary widely, from seven in the tapinocephalid *Tapinocaninus* to over 60 in some anteosaurs. The tail vertebrae get progressively shorter toward the tip. The caudal, atlas, and axis vertebrae had associated intercentra, but these were not present between the remaining cervical or dorsal vertebrae. The cleithrum is reduced in size, and the dorsal process of the coracoid is lost. Dinocephalian limb bones are generally built for strength.