Theria

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Therians
Temporal range: Late JurassicHolocene, 160–0 Ma
Cohunu koala, 2013(6).JPG
A human (placental) holding a koala (marsupial), 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 theríon, 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 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 ones such as the marsupials. Genetic studies have enlighted the viral origin of syncytins through the endogenization process.[3]

The marsupials and the placental mammals evolved from a common therian ancestor that gave live-birth by suppressing the mother's immune system. While the marsupials continued to give birth to an underdeveloped fetus after a short pregnancy, the ancestors of placental mammals gradually evolved a prolonged pregnancy.[4]

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, though some therians, such as the earless seals, have lost them secondarily.[5]

Evolution[edit]

The earliest known therian mammal fossil is Juramaia, from the Late Jurassic (Oxfordian stage) of China. However, molecular data suggests that therians may have originated even earlier, during the Early Jurassic.[6]

Taxonomy[edit]

The rank of "Theria" may vary depending on the classification system used. The textbook classification system by Vaughan et al. (2000)[7] 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)[8] 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)[9] 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. ^ Ancient “genomic parasites” spurred evolution of pregnancy in mammals
  5. ^ ""Theria-Specific Homeodomain and cis-Regulatory Element Evolution of the Dlx3–4 Bigene Cluster in 12 Different Mammalian Species"". PMC 3651898Freely accessible. 
  6. ^ 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.
  7. ^ Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000. Mammalogy: Fourth Edition. Saunders College Publishing, 565 pp. ISBN 0-03-025034-X
  8. ^ 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
  9. ^ 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]