What is the defining feature of placental mammals?

The defining feature of placental mammals is that they develop their young inside the mother. This characteristic is a hallmark of placental mammals, distinguishing them from other groups such as monotremes, which lay eggs, and marsupials, which give birth to relatively undeveloped young that continue to grow in a pouch. In placental mammals, the young are nourished through a complex organ called the placenta, which facilitates the exchange of nutrients and waste between the mother and developing fetus. This internal gestation allows for a longer developmental period and increases the chances of survival for the offspring, as they are born at a more advanced stage of maturity. This feature is crucial in the evolutionary success of placental mammals across diverse environments, making it a key element in their classification.