Theria

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"Therians" redirects here. For the contemporary subculture, see Therianthropy § Psychiatric aspects.
Therians
Temporal range: Middle JurassicHolocene, 160–0 Ma
Cohunu koala, 2013(6).JPG
A human (placental) holding Koalas (marsupials), representing the two surviving groups of therian mammals.
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Tribosphenida
Subclass: Theria
Parker & Haswell, 1897[1]
Infraclasses

Theria (/ˈθɪəriə/; Greek: θηρίον, wild beast) is a subclass of mammals[2] amongst the Theriiformes (the sister taxa to Yinotheria). Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials).

Characteristics[edit]

Therian mammals give birth to live young without using a shelled egg. It is possible thanks to key proteins called syncytins, which allow exchanges between the mother and its offspring through a placenta even rudimental such as the marsupial ones. Genetic studies have enlighted the viral origin of syncytins through endogenization process.[3]

Therian mammals no longer have the coracoid bone, contrary to their cousins monotremes.

Pinnae (external ears) are also a distinctive trait that is a therian exclusivity.[4]

Evolution[edit]

The earliest known therian mammal fossil is Juramaia, from the Middle Jurassic of China. However, molecular data suggests that therians may have originated even earlier, during the Early Jurassic.[5]

Taxonomy[edit]

Main article: Mammal classification

The rank of "Theria" may vary depending on the classification system used. The textbook classification system by Vaughan et al. (2000)[6] gives the following:

Class Mammalia

  • Subclass Theria: live-bearing mammals

In the above system Theria is a subclass. Alternatively, in the system proposed by McKenna and Bell (1997)[7] it is ranked as a supercohort under the subclass Theriiformes:

Class Mammalia

  • Subclass Theriiformes: live-bearing mammals and their prehistoric relatives

Another classification proposed by Luo et al. (2002)[8] does not assign any rank to the taxonomic levels, but uses a purely cladistic system instead.

See also[edit]

References[edit]

  1. ^ ITIS Standard Report Page: Theria
  2. ^ Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. "Subclass Theria". Animal Diversity Web. 
  3. ^ ""Retroviral envelope gene captures and syncytin exaptation for placentation in marsupials"". PMC 4321253Freely accessible. 
  4. ^ ""Theria-Specific Homeodomain and cis-Regulatory Element Evolution of the Dlx3–4 Bigene Cluster in 12 Different Mammalian Species"". PMC 3651898Freely accessible. 
  5. ^ Hugall, A.F. et al. (2007) Calibration choice, rate smoothing, and the pattern of tetrapod diversification according to the long nuclear gene RAG-1. Syst Biol. 56(4):543-63.
  6. ^ Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000. Mammalogy: Fourth Edition. Saunders College Publishing, 565 pp. ISBN 0-03-025034-X
  7. ^ McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
  8. ^ Luo, Z.-X., Z. Kielan-Jaworowska, and R. L. Cifelli. 2002. In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica, 47:1-78.

External links[edit]