The Three Groups of Mammals
Marsupials: pouched mammals
- fertilized egg surrounded by chorion and amniotic membranes, no shell; internal development; embryo nutrition through yolk
- short-lived placenta forms briefly before birth
- birth sometimes within as little as 8 days after fertilization
- young crawls into pouch and latches onto nipple
- only in Australia, New Guinea, South America (early separation of these continents from Pangaea)
- Australia: marsupials in all ecological niches that placental mammals hold on other continents: ? convergent evolution
- Only marsupial in North America: virginia opossum