Thrinaxodon liorhinus (Seeley 1894, Estes 1961) total length: 50 cm. ~245 mya earliest Triassic (Induan) represents a derived cynodont, between Procynosuchus and Chiniquodon.
Thrinaxodon somehow survived the Permian/Triassic extinction event that wiped out ninety percent of the animals living in the Permian. Slightly smaller than Procynosuchus, Thrinaxodon was much more advanced.
The snout has small foramina (holes) in the snout, indicative of mobile skin and perhaps vibrissae (whiskers). The tooth-bearing jawbone (dentary) was much larger and so was the coronoid process. The posterior jawbones continued becoming smaller. The reflected lamina that first appeared on Haptodus, developed a distinct curve. The secondary palate was complete. The first indication of nasal turbinals (small projections inside the nasal passage), suggest the first signs of endothermy (Hillenius 1994).
The ribs developed broad areas, overlapping costal plates that served to stiffen the back. This reduced lateral undulation. The reduction of the tail to mammal-like proportions also indicates less of a reliance on caudofemoral adductors and more on the gluteal muscles. A fifth sacral vertebrae is added.
The scapula was much larger than the coracoid.
The pelvis developed a small anterior process. This is further developed in mammals.
Hair probably started growing between the scales of Thrinaxodon. It appears to be a burrowing animal. Juveniles have been found with adults (Brink 1955).
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.