Mammal | Placental

The defining feature of this group is the placenta, a vascular organ that develops in the mother’s uterus during pregnancy.

: It allows for the exchange of oxygen, nutrients, and water from the mother to the fetus, while whisking away waste like carbon dioxide. placental mammal

While the exact "first" placental is debated, fossil evidence suggests a massive explosion in their numbers shortly after the extinction of the dinosaurs about 65 million years ago. They quickly filled the ecological niches left behind, eventually spreading to almost every habitat on Earth. Quick Comparison: Mammal Reproductive Types Marsupial Monotreme Birth Style Live birth (Mature) Live birth (Underdeveloped) Early Care Internal (Placenta) External (Pouch) Egg Incubation Examples Kangaroos , Opossums Platypus , Echidna The defining feature of this group is the