The cynodonts ("dog teeth"), in the clade Cynodontia, are therapsids that first appeared in the Late Permian (approximately 260 Ma). The group includes modern mammals as well as their extinct ancestors and close relatives. Nonmammalian cynodonts spread throughout southern Gondwana and are represented by fossils from South America, Africa, India, and Antarctica. In the northern continents, fossils have been found in eastern North America as well as in Belgium and northwestern France. Cynodontia is one of the most diverse groups of therapsids.
Richard Owen named Cynodontia in 1861, which he assigned to Anomodontia as a family.Robert Broom (1913) reranked Cynodonia as an infraorder, since retained by others including Colbert and Kitching (1977), Carroll (1988), Gauthier et al. (1989), and Rubidge and Cristian Sidor (2001). Olson (1966) assigned Cynodontia to Theriodonta, Colbert and Kitching (1977) to Theriodontia, and Rubridge and Sidor (2001) to Eutheriodontia. William King Gregory (1910), Broom (1913), Carroll (1988), Gauthier et al. (1989), Hopson and Kitching (2001) and Botha et al. (2007) all considered Cynodontia as belonging to Therapsida. Botha et al. (2007) seems to have followed Owen (1861), but without specifying taxonomic rank.
During their evolution, the jaw of the cynodonts reduced the number of jaw bones. This move towards a single bone for the mandible paved the way for other bones in the jaw, the articular and angular, to migrate to the cranium where they function as parts of the mammalian hearing system.
Cynodonts also developed a secondary palate in the roof of the mouth. This caused air flow from the nostrils to travel to a position in the back of the mouth instead of directly through it, allowing cynodonts to chew and breathe at the same time. This characteristic is present in all mammals.
Early cynodonts have many of the characteristics of mammals. The teeth were fully differentiated and the braincase bulged at the back of the head. Outside of some crown-group mammals (notably the therians) all cynodonts probably laid eggs. The temporal fenestrae were much larger than those of their ancestors, and the widening of the zygomatic arch in a more mammal-like skull would have allowed for more robust jaw musculature. They also have the secondary palate that other primitive therapsids lacked, except the therocephalians, who were the closest relatives of cynodonts. The dentary was the largest bone in their lower jaw. They were probably warm-blooded.
^R. Broom. 1913. A revision of the reptiles of the Karroo. Annals of the South African Museum 7(6):361-366
^S. H. Haughton and A. S. Brink. 1954. A bibliographical list of Reptilia from the Karroo Beds of South Africa. Palaeontologia Africana 2:1-187
^Ruta, M.; Botha-Brink, J.; Mitchell, S. A.; Benton, M. J. (2013). "The radiation of cynodonts and the ground plan of mammalian morphological diversity". Proceedings of the Royal Society B: Biological Sciences280 (1769): 20131865. doi:10.1098/rspb.2013.1865.edit