Eutherians are a group of mammals characterized by the presence of placenta during fetal development. They are also known as placental mammals, belonging to the same subclass as marsupials and monotremes. Some common synonyms for eutherians include placental animals, placental vertebrates, placental mammals, placental synapsids, and viviparous animals. These terms are often used interchangeably to describe this group of mammals. Eutherians diverged from marsupials and monotremes during the early Mesozoic era, and today they make up the majority of mammalian species, including primates, rodents, carnivores, and ungulates. Despite their diversity, all eutherians share the same reproductive strategy, in which the embryo develops within the mother's uterus with the aid of a placenta.