What is the fundamental reproductive difference that classifies the springbok as a placental mammal rather than a marsupial?

Answer

It relies on placental nourishment during a relatively long gestation period

Marsupials are defined by extremely short gestation leading to embryonic young that complete development externally in a pouch. The springbok has a gestation period of four to six months, nourishing the fetus via a placenta within the uterus.

What is the fundamental reproductive difference that classifies the springbok as a placental mammal rather than a marsupial?
mammalclassificationmarsupialSpringbok