299-252 million years ago




Early cynodonts became more mammal-like in a number of their body features, especially the skull and vertebral column.

In early cynodont skull, the synapsid opening widened further and the postorbital bone was reduced. The prootic was developed to the point that it is called the periotic and contained the fenestra ovalis. The external auditory meatus was longer. Early cynodonts evolved a pair of occipital condyles, unlike the single structure in reptiles. Early cynodonts lost the tuber of the basisphenoid and the cultriform process of the parasphenoid was reduced. The vomers completely fused in cynodonts (although they were still unfused in the earliest cynodont Dvinia). Early cynodonts possessed a larger dentary, a flattened coronoid process, and an adductor fossa on the dentary.
Cynodonts evolved cusps on their cheek teeth as they evolved chewing mechanisms to better process food to support higher metabolisms. Although first cynodont, Dvinia possessed 6 incisors, there were 4 lower incisors in Procynosuchus and later cynodonts. Early cynodonts evolved cusps on post canine teeth and some contact between molars.
Early cynodonts developed a distinction between the thoracic and lumbar portions of the vertebral column not found in reptiles. In early cynodonts the two heads of thoracic ribs were modified to form a head and tuberculum and the ribs of the tail became shorter.