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Cynodonts Fossil range: Late Permian - mid Cretaceous (non-mammalian)
Cynognathus
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Synapsida Order: Therapsida (unranked) Theriodontia Suborder: Cynodontia Owen, 1861
Families
See text

Cynodonts, or 'dog teeth', are a taxon of Therapsids, traditionally called mammal-like reptiles. They were one of the most diverse groups of therapsids. They are named after their dog-like teeth.

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 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.

See also: Evolution of mammals

Thrinaxodon from the Early Triassic of South Africa

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 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.

• Order Therapsida
  • (unranked) Eutheriodontia
  • Suborder Therocephalia
  • Suborder CYNODONTIA
    • Charassognathus
    • Family Dviniidae
    • Family Procynosuchidae
      • Procynosuchus
    • Epicynodontia
      • Family Galesauridae
      • Family Thrinaxodontidae
        • Thrinaxodon
      • Infraorder Eucynodontia
        • (unranked) Cynognathia
          • Family Cynognathidae
          • Family Diademodontidae
          • Family Trirachodontidae
          • Family Traversodontidae
            • Santacruzodon
            • Scalenodon
            • Luangwa
            • Traversodon
            • Massetognathus
            • Exaeretodon
          • Family Tritylodontidae
            • Oligokyphus
            • Tritylodon
            • Kayentatherium
            • Bienotherium
            • Bienotheroides
        • (unranked) Probainognathia
          • Lumkuia
          • Ecteninion
          • Family Chiniquodontidae
          • Family Probainognathidae
          • (unranked) Ictidosauria
            • Family Tritheledontidae
          • (unranked) Mammaliaformes

• Tetrapod
• Triassic-Jurassic extinction event

• 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.

• Palaeos cynodonts
• Phylogeny of Cynodonts
• Bennett