Cynodont

Source: Wikipedia, the free encyclopedia.
Not to be confused with Conodont, Cynodon, or Cynodontidae.

Cynodonts
Temporal range:
Ma
Examples of cynodonts.

1st row:

Trirachodon berryi
;
2nd row:
Megazostrodon rudnerae
;
3rd row:
Loxodonta africana (African bush elephant
).

Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: Eutheriodontia
Clade: Cynodontia
Owen, 1861
Clades

Cynodonts (clade Cynodontia lit. 'dog-teeth') are

mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extinct ancestors and close relatives (Mammaliaformes), having evolved from advanced probainognathian
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
.

Description

Skeleton of Procynosuchus

Early cynodonts have many of the skeletal characteristics of

dentary
was the largest bone in their lower jaw.

Skull of Thrinaxodon

The cynodonts probably had some form of

primitive mammal Morganucodon and onwards.[1] Nonetheless, recent studies on Permian synapsid coprolites show that more basal therapsids may have had fur,[2] and at any rate fur was already present in Mammaliaformes such as Castorocauda and Megaconus
.

Early cynodonts had numerous small

prozostrodontian cynodonts, the group that includes mammals, the foramina are replaced by a single large infraorbital foramen, which indicates that the face had become muscular and that whiskers would have been present.[6]

Derived cynodonts developed

placentals, and perhaps Megazostrodon and Erythrotherium, would lose these.[7][8] 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 perinates or possibly eggs.[9]

Cynodonts are the only known synapsid lineage to have produced aerial locomotors, with gliding and flying being known in haramiyidans[10] and various mammal groups.

The largest known non-mammalian cynodont is

traversodontid, which has been estimated to have a maximum skull length of approximately 617 millimetres (24.3 in) based on a fragmentary specimen.[11]

Evolutionary history

See also: Evolution of mammals
Thrinaxodon from the Early Triassic of South Africa

The closest relatives of cynodonts are therocephalians, with which they form the clade Eutheriodontia.[12]

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.[13]

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.[14] 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).[15] 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.[16] Early Mammaliaformes were small bodied insectivores.[17] 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.[16][18]

During their

articular and angular
, 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.

Taxonomy

Cristian Sidor (2001).[20] Olson (1966) assigned Cynodontia to Theriodontia, Colbert and Kitching (1977) to Theriodontia, and Rubridge and Sidor (2001) to Eutheriodontia. William King Gregory (1910), Broom (1913), Carroll (1988), Gauthier et al. (1989), Hopson and Kitching (2001) and Botha et al. (2007) all considered Cynodontia as belonging to Therapsida. Botha et al. (2007) seems to have followed Owen (1861), but without specifying taxonomic rank.[21][22]

Phylogeny

Procynosuchus
Exaeretodon
Diademodon
Tritylodon
Oligokyphus

Below is a cladogram from Ruta, Botha-Brink, Mitchell and Benton (2013) showing one hypothesis of cynodont relationships:[15]

Cynodontia
 

†Charassognathus

 Epicynodontia 

†Cynosaurus

†Platycraniellus

 Eucynodontia 
†Cynognathia
Probainognathia

Distribution

Non-mammalian cynodonts have been found in South America, India, Africa, Antarctica,[23] Asia,[24] Europe[25] and North America.[26]

See also

References

  1. doi:10.1093/icb/40.4.585
    .
  2. doi:10.1111/let.12156
    .
  3. ^ Estes, Richard (1961). "Cranial anatomy of the cynodont reptile Thrinaxodon liorhinus". Bulletin of the Museum of Comparative Zoology. 125: 165–180.
  4. PMID 27157809
    .
  5. S2CID 254702274
    .
  6. S2CID 4318400
    .
  7. ^ Michael L. Power, Jay Schulkin. The Evolution of the Human Placenta. pp. 68–.
  8. ^ 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
  9. S2CID 205570021
    .
  10. S2CID 205259206
    .
  11. S2CID 237517965
    .
  12. S2CID 92370138
    .
  13. S2CID 228883951
    .
  14. PMID 23986112
    .
  15. ^
    PMID 23986112
    .
  16. ^
    ISBN 978-3-319-68008-8
    , retrieved 24 May 2021
  17. S2CID 195327500
    .
  18. S2CID 233183060
    .
  19. ^ Classification of R. Owen 1861.
  20. ^ Classification of B. S. Rubidge and C. A. Sidor 2001
  21. ^ R. Broom. 1913. A revision of the reptiles of the Karroo. Annals of the South African Museum 7(6):361–366
  22. ^ S. H. Haughton and A. S. Brink. 1954. A bibliographical list of Reptilia from the Karroo Beds of South Africa. Palaeontologia Africana 2:1–187
  23. doi:10.1016/j.palaeo.2010.01.011
    .
  24. ISBN 0-231-08482-X
    . Retrieved 30 November 2019.
  25. ^ Fraser, Nicholas C.; Sues, Hans-Dieter (1997). In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods. Cambridge University Press.
  26. doi:10.1080/02724634.1999.10011146.[permanent dead link
    ]

Further reading

  • 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
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