The Three Groups of Mammals
Placental mammals
- placenta nourishes embryo throughout development; long internal development prior to birth
- comprise 17 of 19 mammal orders
- Early placental mammals were already present at the end of the dinosaur era; they survived dinosaur extinction (it is speculated that their fur allowed them to survive the cold climate that caused the dinosaur extinction)
- primates are NOT a young group of placentals