the encyclopedic entry of Cynodont

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Cynodont

Cynodonts, or 'dog teeth', are a taxon of Therapsids which includes modern mammals and their extinct close relatives. They were one of the most diverse groups of therapsids. They are named after their dog-like teeth.

Characteristics

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. Cynodonts still laid eggs, as all Mesozoic proto-mammals probably did. Their temporal fenestrae were much larger than in 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.

Evolutionary history

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. The oldest and the most basal cynodont yet found is Charassognathus. Other basal cynodonts were the Procynosuchids, a family that includes Procynosuchus and Dvinia. Cynodonts were among the groups which survived the Permian-Triassic extinction event and had a slow recovery after the extinction.

The most derived cynodonts are found within the clade Eucynodontia, 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' teeth changed 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 to perform an entirely new function, becoming parts of the mammal's middle 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 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.

Taxonomy and Phylogeny

Order Therapsida

Procynosuchus

Epicynodontia

Family Galesauridae
Family Thrinaxodontidae

Thrinaxodon

Infraorder Eucynodontia

(unranked) Cynognathia

Family Tritylodontidae

(unranked) Probainognathia

Family Tritheledontidae

(unranked) Mammaliaformes

References

Hopson, J.A. & Kitching, J.W. (2001). "A probainognathian cynodont from South Africa and the phylogeny of non-mammalian cynodonts." Bull. Mus. Comp. Zool. 156: 5-35.