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, indicating 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 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.

Barberena MC, Bonaparte JF and Teixeira AMSA 1987. Thrinaxodon brasiliensis sp. nov., a primeira ocorrencia de cinodontes gales - sauros para o Triasico do Rio Grande do Sul. Anais do X Congresso Brasileiro de Paleontologia, Rio de Janeiro, Brazil, pp. 67-76.

Bonaparte JF and Barbierena MC 2001. On two advanced carnivorous cynodonts from the Late Triassic of Southern Brazil. Bulletin of the Museum of Comparative Zoology 156(1):59–80.

Bonaparte JF, Martinelli AG, Schultz CL and Rubert R 2003. The sister group of mammals: small cynodonts from the Late Triassic of Southern Brazil. Revista Brasileira de Paleontologia 5:5-27.

Bonaparte JF and Crompton AW 2017. Origin and relationships of the Ictidosauria to non-mammalian cynodonts and mammals. Historical Biology. https://doi.

Brink AS 1955. Note on a very tiny specimen of Thrinaxodon liorhinus. Palaeontol. Africana, vol. 3, pp. 73-76,

Estes R 196. Cranial anatomy of the Cynodont Reptile, Thrinaxodon liorhinus. Bulletin, Museum of Comparative Zoology, Harvard University, 1961,125: 165-180. online paper

Damiani R, Modesto S, Yates A and Neveling J August 2003. Earliest evidence of cynodont burrowing. Proc. Biol. Sci. 270 (1525): 1747–51. doi:10.1098/rspb.2003.2427. PMID 12965004.

Hillenius WJ 1994. Turbinates in therapsids: evidence for Late Permian origins of mammalian endothermy. Evolution 48: 207-229

Olson EC 1944. Origin of mammals based upon cranial morphology of the therapsid suborders. Special Papers of the Geological Society of America 55: 136 pp.

Seeley HG 1894. Researches on the structure, organisation, and classification of the fossil reptilia.-Part IX, Section 1. On the Therosuchia: Philosophical Transactions of the Royal Society of London, series B, v. 185, p. 987-1018.

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