This is where the mammal story stops being merely “mammal-side” and starts becoming recognizably mammal-bound.

The first cynodonts evolved in the Late Permian, roughly 260 million years ago, probably first known from southern African ecosystems, with related early forms spreading more widely before the end-Permian crisis. Strictly speaking, cynodonts were therapsids — a more derived branch within Therapsida — so the distinction is not “cynodonts instead of therapsids,” but cynodonts as a new kind of therapsid. What made them different was the tightening of the mammal-side package: more complex teeth, stronger jaw muscles, a larger dentary bone in the lower jaw, and a developing secondary palate that helped separate breathing from chewing. This is where the mammal story starts to feel less like a broad synapsid experiment and more like a focused line moving toward mammals.

Procynosuchus is one of the best early examples of this transition. It lived near the end of the Permian and still looked low, long-bodied, and not very mammal-like to modern eyes. But inside the skull, the mammal-bound direction was already underway. Its teeth and jaws show the early cynodont shift toward more precise food processing, while its palate and skull structure point toward better breathing and a more active lifestyle. Procynosuchus was not a mammal, and not even close in appearance, but it is a strong marker for the branch that would eventually lead through later cynodonts to mammaliaforms and, much later, true mammals.