Aelurognathus tigriceps (Haughton 1924, von Huene 1950, SAM 2343) skull length 30 cm, total length: 1.5 m. 250 mya Late Permian represents a subset of the therapsids, a gorgonopsian, a group derived from Biarmosuchus and leading toward Procynosuchus.
Reptiles have two skulls. One to protect their brain and another to house their jaw muscles. Mammals essentially lose the outer skull. Only the cheekbone and the back of the skull remain. The transition from one to another occurred with gorgonopsians such as Aelurognathus in which the primitively small synapsid opening (lateral temporal fenestra) became progressively larger and larger.
The dentary (tooth-bearing jawbone) was larger and developed a posterior elevated area (the coronoid process) for muscle attachment.
Aelurognathus had a shorter tail. Caudofemoral muscles that had once pulled the femora while walking were moving forward, becoming rump muscles.
If you want to read the book, "From the Beginning - The Story of Human Evolution" by David Peters (Little, Brown 1991), which is where the above images were first published, click here for the PDF.