Cynodonts have nearly all the characteristics of mammals. Their teeth were fully differentiated, the braincase bulged at the back of the head, and many of them walked in an upright manner unlike reptiles. Cynodonts still laid eggs, as all Mesozoic proto-mammals probably did. Their temporal fenestrae was much larger than its ancestors, and the widening of the zygomatic arch allowed for more robust jaw musculature supporting the evidence of a more mammal-like skull. They also have the secondary palate that other primitive therapsids lacked, except the therocephalians, who were the closest relatives of cynodonts. Their dentary was the largest bone in their lower jaw, as other smaller bones moved into the ears. They were probably warm-blooded, and covered in hair.
The cynodonts themselves are part of a group of therapsids called theriodonts, together with the extinct gorgonopsians and the therocephalians. Cynodonts' evolutionary track began late in the Permian, as a small, Gorgonopsid-like theriodont. Among the most basal of the cynodonts were the Procynosuchids, a family that includes Procynosuchus and Dvinia. They were among the groups which survived the Permian mass extinction.
The most derived cynodonts are found within Eucynodontia clade, which also contains the members of Mammalia. Representative genera include the large carnivorous cynognathids, equally large herbivorous traversodonts, and small and mammal-like tritylodontids and ictidosaurs. It is likely that Cynodonts were at least partially if not completely warm-blooded, covered with hair, which would have insulated them and helped to maintain a high body temperature. The mammal-like structure of Cynodonts hints that all mammals have descended from a single group of eucynodonts.
During their evolution, cynodonts changed their teeth from being designed for catching and holding prey and then swallowing whole, to adding specialized teeth, including molars, designed for better mastication of food allowing for quicker digestion. Additionally, the jaw of the cynodonts reduced the number of jaw bones. This freed up the superfluous bones to evolve into an entirely new function, becoming parts of the mammal's inner ear.
Improved hearing gave these creatures a better awareness of their environment and, in turn, this increasing sensitivity called for a greater capacity for processing the auditory information in the brain. Cynodonts also developed a secondary palate in the roof of the mouth. This allowed air to flow to the lungs through the back of the mouth, allowing cynodonts to chew and breathe at the same time. This characteristic is present in all mammals.
Taxonomy and Phylogeny
- Order Therapsida
- Suborder CYNODONTIA
- Family Dviniidae
- Family Procynosuchidae
- Suborder CYNODONTIA
- Hopson, J.A. & J.W. Kitching (2001). "A probainognathian cynodont from South Africa and the phylogeny of non-mammalian cynodonts." Bull. Mus. Comp. Zool. 156: 5-35.
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