Theria

Theria is a subclass of mammals that includes all living mammals except for monotremes (egg-laying mammals like the platypus and echidnas). This subclass encompasses two infraclasses: Metatheria (marsupials) and Eutheria (placental mammals).12

Key characteristics of Theria include:
* Live Birth: Unlike monotremes, therians give birth to live young.23
* Reproduction: Female therians have estrous cycles, a hormonal interaction that typically coincides with ovulation and receptivity to mating.4
* Marsupials (Metatheria): These mammals are characterized by premature birth, with newborns continuing development attached to nipples, often within a pouch (marsupium). Examples include kangaroos, koalas, and opossums.25
* Placental Mammals (Eutheria): This is the largest group, characterized by a placenta that nourishes the embryo during gestation. This group includes the vast majority of mammal species, such as rodents, bats, primates, and carnivores.42