Dinocephalian

Dinocephalia is a clade of large-bodied early therapsids that flourished for a brief time in the Middle Permian between 270 and 260 million years ago (Ma), but became extinct, leaving no descendants. Dinocephalians included herbivorous, carnivorous, and omnivorous
 forms. Many species had thickened skulls with many knobs and bony
projections. Dinocephalian fossils are known from Russia, China, Brazil,
 South Africa, Zimbabwe, and Tanzania.

Apart from the Biarmosuchians,
 the dinocephalians are the least advanced therapsids, although still
uniquely specialised in their own way. They retain a number of primitive
 characteristics (e.g. no secondary palate, small dentary) shared with their pelycosaur ancestors, although they are also more advanced in possessing therapsid adaptations like the expansion of the ilium and more erect limbs. They include carnivorous, herbivorous, and omnivorous forms. Some were semiaquatic, others were fully terrestrial. They were among the largest animals of the Permian period; only the biggest Caseidae and Pareiasauridea rivaling or exceeding them in size.
Size
Dinocephalians were generally large. The biggest herbivores (Tapinocephalus) and omnivores (Titanosuchus) may have massed up to 2 tonnes (4,400 lb), and were some 4.5 metres (15 ft) long, while the largest carnivores (such as Titanophoneus and Anteosaurus) were at least as long, with heavy skulls 80 centimetres (31 in) long, and overall masses of around a half tonne.
Skull
All
 dinocephalians are distinguished by the interlocking incisor (front)
teeth. Correlated features are the distinctly downturned facial region, a
 deep temporal region, and forwardly rotated suspensorium. Shearing
contact between the upper and lower teeth (allowing food to be more
easily sliced into small bits for digestion) is achieved through keeping
 a fixed quadrate and a hinge-like movement at the jaw articulation. The
 lower teeth are inclined forward, and occlusion is achieved by the
interlocking of the incisors. The later dinocephalians improved on this
system by developing heels on the lingual sides of the incisor teeth
that met against one another to form a crushing surface when the jaws
were shut.

The dinocephalians are an ancient group and their ancestry is not clear.
 It is assumed that they must have evolved during the earlier part of
the Roadian, or possibly even the Kungurian epoch, but no trace has been found. These animals radiated at the expense of the dying pelycosaurs, who dominated during the early part of the Permian. Even the earliest members, the estemmenosuchids and early brithopodids of the Russian Ocher fauna, were already a diverse group of herbivores and carnivores.
During the Wordian and early Capitanian, advanced dinocephalians radiated into a large number of herbivorous forms, representing a diverse megafauna. This is well known from the Tapinocephalus Assemblage Zone of the Southern African Karoo.
At the height of their diversity (middle or late Capitanian
 age), all the dinocephalians suddenly died out. The reason for their
extinction is not clear; although disease, sudden climatic change, or
other factors of environmental stress may have brought about their end.
They were replaced by much smaller therapsids; herbivorous Dicynodontia and carnivorous Biarmosuchians, Gorgonopsians and Therocephalians.