Placentalia

Placentalia, also known as Eutheria, is a major group of mammals characterized by the presence of a placenta. This vascular organ develops during gestation and facilitates the exchange of nutrients and waste between the mother and the fetus. This allows for a longer developmental period within the womb, contributing to the evolutionary success of placental mammals.1

Fossil evidence suggests that the first placental mammals emerged during the Jurassic Period, between approximately 163 and 157 million years ago. Placental mammals include all living mammals except marsupials and monotremes. Some scientists classify placental mammals within the clade Placentalia, which encompasses all living placental mammals and their most recent common ancestor. This classification helps to avoid confusion with extinct nonplacental eutherians, which were the ancestors of modern placentals.1