Cynodont

Cynodonts Fossil range: Late Permian–Late Triassic Descendant taxon Mammalia survives to present. PreЄ Є O S D C P T J K Pg N
Cynognathus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Synapsida
Order:	Therapsida
Suborder:	Cynodontia Owen, 1861
Families
See text

Cynodontia or cynodonts ("dog teeth") are a taxon of therapsids endemic to all seven continents beginning during the Early Triassic 256 Ma.^[1] This taxon 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.

Taxonomy

Cynodontia was named by Owen (1861). It was considered monophyletic by Botha et al. (2007). It was reranked as the infraorder Cynodontia by Broom (1913), affirmed by Haughton and Brink (1954), affirmed by Olson (1966), affirmed by Colbert and Kitching (1977), by Carroll (1988), by Gauthier et al. (1989), by Hopson and Kitching (2001) and affirmed by Rubidge and Sidor (2001). It was reranked as the unranked clade Cynodontia by Gregory (1910) and Botha et al. (2007). It was assigned to Anomodontia by Owen (1861); to Theriodonta by Olson (1966); to Theriodontia by Haughton and Brink (1954) and Colbert and Kitching (1977); to Eutheriodontia by Rubidge and Sidor (2001); and to Therapsida by Gregory (1910), Broom (1913), Carroll (1988), Gauthier et al. (1989), Hopson and Kitching (2001) and Botha et al. (2007).^[2][3]

Evolutionary history

Thrinaxodon from the Early Triassic of South Africa

The cynodonts themselves are part of a group of therapsids called theriodonts with the earliest fossil record being that of Thrinaxodon (T. liorhinus).^[4] Cyodonts together with the extinct gorgonopsians and the therocephalians. Cynodonts evolved shortly before the end of Permian. 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 few groups of synapsids that 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 move towards a single bone for the mandible increased its structural integrity when compared to the kinetic jaws of reptiles. Several of the bones which were jaw bones in reptiles evolved in this creature 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.

Characteristics

Bienotherium yunnanense

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.

Phylogeny

Procynosuchus

Exaeretodon

Tritylodon

Oligokyphus

Diademodon.

• 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 Trithelodontidae
          • (unranked) Mammaliaformes

See also

• Permian–Triassic extinction event
• Prehistoric mammal
• Tetrapod
• Triassic-Jurassic extinction event

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.

• Davis, Dwight (1961). "Origin of the Mammalian Feeding Mechanism". Am. Zoologist, 1:229-234.

External links

• Palaeos cynodonts
• Phylogeny of Cynodonts
• Bennett

Paleontology portal