Why don't marsupials (kangaroos, koalas etc.) have belly buttons?

Question

Why don't marsupials (kangaroos, koalas etc.) have belly buttons?

Answer 1

Marsupial embryos are not nourished by an umbilical cord - thus, no umbilical scar or "belly button." From the reference: Pregnant females develop a kind of yolk sac in their wombs, which delivers nutrients to the embryo. Marsupials give birth at a very early stage of development (about 4–5 weeks); after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a nipple, which is located inside the marsupium. There they remain for a number of weeks, attached to the nipple. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth and nourishment.

Answer 2

Which animal has?

Answer 3

Because female marsupials don't have placentas so have no umbilical cord and thus have, no "belly button."