299-252 million years ago
Dvinia is the most primitive known cynodont from Lower Permian. It possessed 6 upper and lower incisors, it still retained one precanine tooth (as do primitive therocephalians), the secondary palate is only partly fused and still exposed part of vomers, the vomers were still paired (but will be fused to a single bone in all other cynodonts), and the lower teeth bite against roof of mouth rather than occluding with (grinding against) the upper teeth.
Procynosuchus was close to the ancestral condition of later cynodonts.
It had all the primitive features of Dvinia except that the vomers are
fused. There was still no fusion of secondary palate and the palate didnt
continue behind the region of the teeth. There were 4 lower incisors.
The Upper Permian procynosuchids developed a secondary palate, complex
cheek teeth, and reorganized the jaw muscles (such as the masseter) (Kemp,
1982; Carroll, 1988).