Theria ( or ; ) is a subclass of

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s amongst the

Theriiformes Theriiformes is a clade of mammals. The term was coined by Timothy B. Rowe in his doctoral dissertation, and is defined as the clade formed by the most recent common ancestor of multituberculates (which form part of the broader group Allotheria ...

. Theria includes the

eutheria Eutheria (from Greek , 'good, right' and , 'beast'; ), also called Pan-Placentalia, is the clade consisting of Placentalia, placental mammals and all therian mammals that are more closely related to placentals than to marsupials. Eutherians ...

ns (including the

placental mammal Placental mammals ( infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguish ...

s) and the

metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as wel ...

ns (including the

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

s) but excludes the egg-laying

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s and various extinct mammals evolving prior to the common ancestor of placentals and marsupials.

Characteristics

Therians give birth to live young without a shelled

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...

. This is possible thanks to key proteins called syncytins which allow exchanges between the mother and its offspring through a

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

, even rudimental ones such as in marsupials. Genetic studies have suggested a viral origin of syncytins through the endogenization process. The marsupials and the placentals evolved from a common therian ancestor that gave live birth by suppressing the mother's

immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...

. While the marsupials continued to give birth to an underdeveloped

fetus A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic development, embryonic stage, the fetal stage of development takes place. Pren ...

after a short

pregnancy Pregnancy is the time during which one or more offspring gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Conception (biology), Conception usually occurs ...

, the ancestors of placentals gradually evolved a prolonged pregnancy. The exit openings of the

urogenital system The genitourinary system, or urogenital system, are the sex organs of the reproductive system and the organ (biology), organs of the urinary system. These are grouped together because of their proximity to each other, Development of the urinary a ...

and the rectal opening (

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

) are separated. The

mammary gland A mammary gland is an exocrine gland that produces milk in humans and other mammals. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primates (for example, human ...

s lead to the

teat A teat is the projection from the mammary glands of mammals from which milk flows or is ejected for the purpose of feeding young. In many mammals, the teat projects from the udder. The number of teats varies by mammalian species and often corr ...

s. Therians no longer have the

coracoid A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...

bone, unlike their cousins,

s. Pinnae (external ears) are also a distinctive trait that is a therian exclusivity, though some therians, such as the

earless seal The earless seals, phocids, or true seals are one of the three main groups of mammals within the seal lineage, Pinnipedia. All true seals are members of the family Phocidae (). They are sometimes called crawling seals to distinguish them from th ...

s, have lost them secondarily. The flexible and protruding nose in therians is not found in any other vertebrates, and is the product of modified cells involved in the development of the upper jaw in other tetrapods. Almost all therians have whiskers. The SRY gene is a protein in therians that helps initiate male sex determination.

Evolution

The earliest known therian mammal fossil is ''

Juramaia ''Juramaia'' is an extinct genus of a therian mammal, possibly a very basal eutherian mammal, known from the Late Jurassic ( Oxfordian stage) or Early Cretaceous deposits of western Liaoning, China. It is a small shrew-like mammal weighing aro ...

'', from China's Late Jurassic ( Oxfordian stage). However, the age estimates of the site are disputed based on the geological complexity and the geographically widespread nature of the Tiaojishan Formations. Further, King and Beck in 2020 argue for an Early Cretaceous age for ''Juramaia sinensis'', in line with similar early mammaliaformes. A recent review of the Southern Hemisphere Mesozoic mammal fossil record has argued that tribosphenic 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. Molecular data suggests that therians may have originated even earlier, during the Early Jurassic. Therian mammals began to diversify 10–20 million years before the dinosaur extinction.

Taxonomy

The rank of "Theria" may vary depending on the classification system used. The textbook classification system by Vaughan et al. (2000) gives the following: In the above system, Theria is a subclass. Alternatively, in the system proposed by McKenna and Bell (1997) it is ranked as a supercohort under the subclass Theriiformes: Another classification proposed by Luo et al. (2002)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. does not assign any rank to the taxonomic levels, but uses a purely

cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

system instead.

References

External links

{{Taxonbar, from=Q130942 Mammal taxonomy Extant Middle Jurassic first appearances Taxa described in 1897 Taxa named by William Aitcheson Haswell

Characteristics

Evolution

Taxonomy

See also

References

External links