Biarmosuchus tener (Tchudinov 1960) 258 mya Late Permian is the most primitive therapsid. Derived from Haptodus, the teeth and limbs were emphasized in this specialized predator.
Huge canine teeth dominated the skull. These were used for killing large prey. The pineal opening (third eye) was raised like a little volcano. The reflected lamina became a thin sheet of bone reinforced by a cross of thicker bone.
The neck was slightly longer and included more vertebrae. The posterior ribs began to shrink.
The shoulder blades were less bulky and rode more loosely on the ribcage. The hands and feet were more similar in size. The lateral fingers and toes were shorter, brought about by the reductions of m3.2, m4.2 and m4.3. This meant the hands and feet were swinging more under each limb, rather than out to the sides, during the recovery stroke of each stride. This change helped reduce the amount of torso undulation and enabled Biarmosuchus to breathe more easily while walking.
The lower pelvis was much longer and the ilium developed a small anterior process.
Tetraceratops (Matthew 1908) was considered te most primitive therapsid (Laurin and Reisz 1996) but cladistic analysis indicates it shares more characters with Tseajaia, with which it was not previously tested.