Why are placental mammals more widespread than marsupials?

The marsupial fetus does not have the benefit of a complex placenta. A placenta is a developmental adaptation in some mammals that increases the surface area across which a fetus can receive nutrients from the mother. read more

Placental mammals are more widespread because most modern day marsupials evolved in isolation on the continent of Australia. Historical reconstruction of the evolutionary history of marsupials has found that Australia separated (via plate tectonics) from Antartica in the past. Antarctica was also connected to South America. read more

Essentially, marsupials spend far more time nurturing and nursing their young after they are born than placentals, mammals that invest more time and energy in pregnancy. The Marsupial Pouch If there's one thing marsupials are known for, it's the characteristic pouch in the female of the species. read more