Thrinaxodon liorhinus - A Mammal-Like Reptile
 

Thrinaxodon liorhinus was a mammal-like reptile of the Triassic. Fossils of it have been found in South Africa and Antarctica. They date to the Induan subperiod (248.2 - 243.4 million years ago). Thrinaxodon was a small creature; it was only about 50 cm. (1.3 feet) long. The teeth of Thrinaxodon were varied; they included canines, molars, and incisors. Fossil skulls of Thrinaxodon have many tiny holes, which suggest that hair may have grown on the head. The holes could have been where the roots of the hair were located.
Richard Estes discusses fossils of Thrinaxodon (ones found in the Lystrosaurus zone of South Africa) in an article titled CRANIAL ANATOMY OF THE CYNODONT REPTILE THRINAXODON LIORHINUS. In this excerpt he talks about skulls of two juveniles specimens::

The frontals and parietals are relatively broader than those of the adults, and the parietals lack a saittal crest. The temporal crests remain discrete throughout their length; between them the flattened cranial roof is narrowest just posterior to the nearly circular parietal foramen.

He also says:

The prefrontals are small and just touch the postorbitals. The latter are fragmentary, but indicate that the posterior extent of the postorbital was at the fronto-parietal suture or a little posterior to it. The postorbital bars are broken away. The jugals and squamosals closely resemble those of adults.

In another portion of the article, Estes discusses the snout of an adult skull:

The maxilla bears a row of four to six large superior labial foramina along its ventral exterior surface. Other more dorsally placed foramina are more numerous, smaller, and open anteriorly, though some of the posterior ones are directed backwards.

Estes states a point of comparisons between the juvenile specimens and the adult in this excerpt:

The relatively very large frontal of the juvenile is separated from the orbital margin by a very narrow meeting of the postorbital and postfrontal. The posterior margin of the secondary palate is at the level of the fifth tooth in the juvenile rather than the third as in the adult. The latter may be explained by the greater relative growth of the snout in the adult and the greater number of anterior teeth in the juveniles.

The temperature was cool in the region that Thrinaxodon inhabited. Australia and South Africa were part of the southern continent called Gondwana.

Neal Robbins

P.S. The taxonomy of Thrinaxodon liorhinus is:

Kingdom: Animalia
Phylum: Chordata
Class: Synapsida
Order: Therapsida
Suborder: Cynodontia
Family: Galesauridae
Genus: Thrinaxodon
Species: Thrinaxodon liorhinus