Here's why:
* Placental mammals nourish their young inside the mother's womb through a placenta. This allows for longer gestation periods and more developed offspring at birth.
* Marsupials, on the other hand, give birth to relatively undeveloped young that complete their development in a pouch on the mother's abdomen.
Squirrels are born relatively developed and do not have a pouch. Therefore, they are placental mammals.