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Cynodontia () is a clade of eutheriodont that first appeared in the Late Permian (approximately 260 Megaannum), and extensively diversified after the Permian–Triassic extinction event. are cynodonts, as are their extinct ancestors and close relatives (Mammaliaformes), having evolved from advanced cynodonts during the Late Triassic.
Non-mammalian cynodonts occupied a variety of ecological niches, both as carnivores and as herbivores. Following the emergence of mammals, most other cynodont lines went extinct, with the last known non-mammaliaform cynodont group, the Tritylodontidae, having its youngest records in the Early Cretaceous.
Derived cynodonts developed epipubic bones. These served to strengthen the torso and support abdominal and hindlimb musculature, aiding them in the development of an erect gait, but at the expense of prolonged pregnancy, forcing these animals to give birth to highly altricial young as in modern marsupials and monotremes. Only placentals, and perhaps Megazostrodon and Erythrotherium, would lose these.Michael L. Power, Jay Schulkin. The Evolution of the Human Placenta. pp. 68–.Jason A. Lillegraven, Zofia Kielan-Jaworowska, William A. Clemens, Mesozoic Mammals: The First Two-Thirds of Mammalian History, University of California Press, 17 December 1979 – 321 A specimen of Kayentatherium does indeed demonstrate that at least tritylodontids already had a fundamentally marsupial-like reproductive style, but produced much higher litters at around 38 or possibly eggs.
Cynodonts are the only known synapsid lineage to have produced aerial locomotors, with gliding being known in and various mammal groups, and placental mammals having developed flight.
The largest known non-mammalian cynodont is Scalenodontoides, a traversodontid, which has been estimated to have a maximum skull length of approximately based on a fragmentary specimen.
The earliest cynodonts are known early Lopingian (early Wuchiapingian) aged sediments of the Tropidostoma Assemblage Zone, in the Karoo Supergroup of South Africa, belonging to the basal family Charassognathidae. Fossils of Permian cynodonts are relatively rare outside of South Africa, with the most widely distributed genus being Procynosuchus, which is known from South Africa, Germany, Tanzania, Zambia, and possibly Russia.
Cynodonts expanded rapidly in diversity after the Permian–Triassic extinction event. Peak disparity in cynodonts occurred from the Induan to the Carnian and in the middle Norian. Post-Early Triassic cynodonts were dominated by members of the advanced clade Eucynodontia, which has two main subdivisions, the predominantly herbivorous Cynognathia and the predominantly carnivorous Probainognathia. During the Early and Middle Triassic, cynodont diversity was dominated by members of Cynognathia, and members of Probainognathia would not become prominent until the Late Triassic (early Norian). Almost all Middle Triassic cynodonts are known from Gondwana, with only one genus ( Nanogomphodon) having been found in the Northern Hemisphere. Among the most dominant groups of Middle and Late Triassic cynodonts is the herbivorous Traversodontidae, predominantly in Gondwana, which reached a peak diversity in the Late Triassic. Mammaliaformes originated from probainognathian cynodonts during the Late Triassic. Early Mammaliaformes were small bodied insectivores. Only two groups of non-mammaliaform cynodonts existed beyond the end of the Triassic, both belonging to Probainognathia. The first is the insectivorous Tritheledontidae, which briefly lasted into the Early Jurassic. The second is the herbivorous Tritylodontidae, which first appeared in the latest Triassic, which were abundant and diverse during the Jurassic, predominantly in the Northern Hemisphere, persisted into the Early Cretaceous (Barremian-Aptian) in Asia, at least until around 120 million years ago, as represented by Fossiomanus from China.
During their evolution, the number of cynodont jaw bones reduced. This move towards a single bone for the mandible paved the way for other bones in the jaw, the articular and Angular bone, 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.
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