Trirachodontidae

Trirachodontidae is an extinct, possibly paraphyletic^[1] family of cynognathian cynodonts from the Triassic of China and southern Africa. Trirachodontids appeared during the Early Triassic soon after the Permian-Triassic extinction event and quickly spread over a wide geographic area in a comparatively brief amount of time from 250 to 237 million years ago.

Trirachodontids Temporal range: Early - Middle Triassic, 251–237 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Life restoration of Trirachodon berryi in a burrow
Skull of Langbergia in dorsal view
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Clade:	Cynodontia
Clade:	†Neogomphodontia
Family:	†Trirachodontidae Crompton, 1955
Genera
†Beishanodon †Cricodon †Guttigomphus †Impidens †Langbergia †Sinognathus †Trirachodon

Trirachodontids have wide skulls and short, narrow snouts. Two large holes called temporal fenestrae run along the back of the head and have a uniformly large width. Trirachodontids also have two large canine teeth and smaller cusped postcanines. Most of the features that distinguish trirachodonts from other cynodonts are found in their dentition.^[2]

Trirachodontids lived in semi-arid environments with seasonal rainfall. The bone structure of trirachodontids suggests that they grew quickly in seasons with high rainfall and slowly in less favorable seasons.^[3] One trirachodontid, Trirachodon, has been found in association with complex burrow systems. These burrows were probably used by many individuals to hide from predators, raise young, or stay warm.

Two subfamilies of trirachodontids are recognized: Trirachodontinae from Africa and Sinognathinae from China. Below is a cladogram from Gao et al. (2010) showing the phylogenetic relationships of trirachodontids:^[2]

Cynognathia	Cynognathus  Gomphodontia  Diademodon  Trirachodontidae   Trirachodontinae  Trirachodon Langbergia Cricodon  Sinognathinae  Beishanodon Sinognathus Traversodontidae

The cladogram above shows a monophyletic Trirachodontidae, but multiple studies have found the family to be paraphyletic with respect to the family Traversodontidae.^[1][4] Below is a cladogram from Hendrickx et al. (2020), who also recovered the members of the subfamily Trirachodontinae as a basal polytomy within the clade Neogomphodontia:^[1]

Cynognathia	Cynognathus  Gomphodontia   Diademodontidae  Diademodon Titanogomphodon  Neogomphodontia  Langbergia Trirachodon  Sinognathinae  Beishanodon Cricodon Sinognathus Traversodontidae Trirachodontidae

References

1. Hendrickx, C.; Gaetano, L. C.; Choiniere, J. N.; Mocke, H.; Abdala, F. (2020). "A new traversodontid cynodont with a peculiar postcanine dentition from the Middle/Late Triassic of Namibia and dental evolution in basal gomphodonts". Journal of Systematic Palaeontology. 18 (20): 1669–1706. doi:10.1080/14772019.2020.1804470. S2CID 221838726.
2. Gao, K.-Q.; Fox, R.C.; Zhou, C.-F.; Li, D.-Q. (2010). "A new nonmammalian eucynodont (Synapsida: Therapsida) from the Triassic of Northern Gansu Province, China, and its biostratigraphic and biogeographic implications" (PDF). American Museum Novitates (3685): 1–25. doi:10.1206/649.1. hdl:2246/6069. S2CID 85789838.
3. Botha, J.; Chinsamy, A. (2004). "Growth and life habits of the Triassic cynodont Trirachodon, inferred from bone histology" (PDF). Acta Palaeontologica Polonica. 49 (9): 619–627.
4. Liu, J.; Olsen, P. (2010). "The Phylogenetic Relationships of Eucynodontia (Amniota: Synapsida)". Journal of Mammalian Evolution. 17 (3): 151. doi:10.1007/s10914-010-9136-8. S2CID 40871206.