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.