Theria is the lineage of mammals that give birth to live young. This includes the marsupials (Metatheria) and placental mammals (Eutheria), but not the egg-laying monotremes (echidnas and platypus are the only surviving monotremes).In addition to reproductive strategy and adaptations, more technical characters that define the Theria include (Killian et al. 2001):
•Olfactory bulb cell organization
•Braincase architecture
•Mammary glands with teats
•Cranial nerve distribution
•Tooth enamel
•Molar dentition: tribosphenic molars
•Inner ear architecture
•Ankle anatomy (crurotarsal ankle joint)
•pectoral girdle morphology
The traditional, and once again prevailing view of mammalian evolution is the therian hypothesis: that the monotreme lineage and therian lineage diverged from one another in the Triassic period (about 200 million years ago) from ancestors in different geographical areas, with the monotremes endemic to Gondwanaland and the therian mammals arising in Laurasia.Therian mammals are thought to have originated 170-150 million years ago (Hugall et al. 2007; Killian et al. 2001; Luo et al. 2001). For a time, morphological (especially dental) analysis of fossils and early molecular work disputed a sister relationship between marsupials and placental mammals, positing an alternative theory of relationship between the three mammals lineages (the marsupionta hypothesis): that monotremes are specialized members of the marsupial mammals (Gregory 1947; Janke et al. 1987; Kirsch and Mayer 1988).
Theria (/ˈθɪəriə/; Greek: θηρίον theríon, wild beast) is a subclass of mammals[2] amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes and various extinct mammals evolving prior to the common ancestor of placentals and marsupials.
Therian mammals give birth (see viviparity) to live young without a shelled egg. This 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 in marsupials. Genetic studies have suggested a 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]
The earliest known therian mammal fossil is Juramaia, from the Late Jurassic (Oxfordian stage) of China. However, the age estimates of the site are disputed based on the geological complexity and the geographically widespread nature of the Tiaojishan Formations.[6][7] Further, King and Beck in 2020 argue for an Early Cretaceous age for Juramaia sinensis, in line with similar early mammaliaformes.[8]
A recent review of the Southern Hemisphere Meosozoic mammal fossil record has argued that triosphenic mammals arose in the Southern Hemisphere during the Early Jurassic, around 50 million years prior to the clade's earliest undisputed appearance in the Northern Hemisphere.[9]
Molecular data suggests that therians may have originated even earlier, during the Early Jurassic.[10] Therian mammals began to diversify 10-20 million years before dinosaur extinction.[11]
The rank of "Theria" may vary depending on the classification system used. The textbook classification system by Vaughan et al. (2000)[12] gives the following:
Class Mammalia
In the above system Theria is a subclass. Alternatively, in the system proposed by McKenna and Bell (1997)[13] it is ranked as a supercohort under the subclass Theriiformes:
Class Mammalia
Another classification proposed by Luo et al. (2002)[14] does not assign any rank to the taxonomic levels, but uses a purely cladistic system instead.
Theria (/ˈθɪəriə/; Greek: θηρίον theríon, wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes and various extinct mammals evolving prior to the common ancestor of placentals and marsupials.
