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.

Prozostrodon brasiliensis (originally Thrinaxodon brasiliensis, Barberena et al. 1987; Triassic; the size of a cat) was originally considered congeneric with Thrinaxodon, then closer to mammals. Here it nests between Thrinaxodon and the Chiniquodon clade. The foot was more symmetrical. The teeth were relatively smaller with molars no deeper than the incisors and with a larger canine.

A referred specimen, CAPPA/UFSM 0210 (above), is more complete and slightly different was described by Stefanello et al 2023. This specimen has deep holes in the maxilla for the dentary fangs. The skull is overall lower.

Borealestes serendipitus (Waldman and Savage 1972; Middle Jurassic) was considered a docodont. Here it nests with Late Triassic Prozostrodon. The interesting thing about this taxon is the parallel evolution of mammal-like teeth into the Middle Jurassic. There were more teeth than any mammal. Here the double-rooted canine is reinterpreted as the first of six premolars. The single-rooted canine was probably no larger than the incisors, based on its diameter.

Haldanodon exspectatus (Kühne & Krusat, 1972, Martin 2005; Late Jurassic. 145 mya; 7.6 cm jaw, 38 cm overal length) was an aquatic stem-mammal that was the first in the lineage to lose the postorbital bar to make the orbit and lateral temporal fenestra confluent. Here it nests between Probainognathus and Liaconodon.

Bolotridon frerensis (Broili and Schröder 1934; Pusch, Kammerer and Fröbisch 2021; BSPG 1934-VIII-7) was originally considered a non-eucyndont cynodont, then a sister to Eucynodontia (= all cynodonts more derived than Thrinaxodon). This is where Bolotridon nests in the TST. Note the laterally facing orbits and relatively narrow skull. The posterior skull is not known. Oddly, Chiniquodon is not a taxon in the Pusch et al. 2021 cladogram.

The teeth of Bolotridon are distinct from those of Thrinaxodon and Chiniquodon, less suited to tearing flesh and more suited to mashing pulpy vegetation.

Cynognathus crateronotus (Seeley 1895, Wynd et al 2018, Middle Triassic, 1.2m snout-vent length) extended across South Pangaea. No complete skeletons are known, so a composite (chimaera) is created above based on Thrinaxodon proportions. The secondary palate is complete. Note the robust lumbar ribs.

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.

Broili F and Schröder J 1934. Beobachtungen an Wirbeltieren der Karrooformation. II. Über den Cynodontier Tribolodon frerensis Seeley. Sitzungsberichte der Bayerischen Akademie der Wissenschaften, Sonderdruck aus dem Jahrgang 1934 163–177.

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.

Jenkins FA Jr 1967. The postcranial skeleton of African cynodonts. Peabody Museum of Natural History Yale University Bulletin 36.

Kühne and Krusat 1972. Legalisierung des taxon Haldanodon (Mammalia, Docodonta). Neues Jahrbuch für Geologie, Paläontologie and Mineralogie, Monatshefte 5: 300-302.

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.

Pancirolli E, Benson R and Luo Z-X 2016. The mandible and dentition of Borealestes serendipitus (Docodonta) from the Middle Jurassic of Skye, Scotland. Journal of Vertebrate Paleontology 39(3):Article: e1621884

Pusch LC, Kammerer CF and Fröbisch J 2021. Cranial anatomy of Bolotridon frerensis, an enigmatic cynodont from the Middle Triassic of South Africa, and its phylogenetic significance. PeerJ 9:e11542 DOI 10.7717/peerj.11542

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.

Seeley HG 1895. Researches on the Structure, Organization, and Classification of the Fossil Reptilia. Part IX, Section 5. On the Skeleton in New Cynodontia from the Karroo Rocks. Philosophical Transactions of the Royal Society of London B 186: 59-148.

Stefanello M et al (4 co-authors) 2023. A complete skull of a stem mammal from the Late Triassic of Brazil illuminates the early evolution of prozostrodontian cynodonts. Journal of Mammalian Evolution.    DOI: 10.1007/s10914-022-09648-y

Waldman M and Savage RJG 1972. The first Jurassic mammal from Scotland". Journal of the Geological Society. 128 (2): 119–125.

Wynd BM, Peecock BR, Whitney MR and Sidor CA 2018. The first occurrence of Cynognathus crateronotus (Cynodontia: Cynognathia) in Tanzania and Zambia, with implications for the age and biostratigraphic correlation of Triassic strata in southern Pangea. pp. 228–239 in: C.A. Sidor and S.J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Paleontology Memoir 17. Journal of Vertebrate Paleontology 37(6, Supplement)

wiki/Thrinaxodon
digimorph.org//Thrinaxodon
wiki/Prozostrodon
wiki/Bolotridon
wiki/Cynognathus
wiki/Borealestes
wiki/Haldanodon