Instead, they give birth very early and the young animal, essentially a helpless embryo, climbs from the mother's birth canal to the nipples. There it grabs on with its mouth and continues to develop, often for weeks or months depending on the species. The short gestation time is due to the lack of a placenta in the mother marsupial, which would otherwise nourish the developing embryo using the mother's blood supply. A few marsupials do have a sort of placenta, but these have evolved convergently in marsupials and are not exactly the same as those found in placental mammals. Like other mammals, the marsupials are covered with hair.
Mothers nurse their young, as shown in the picture of two kangaroos at right.
The only naturally occurring marsupial in the United States is the possum, Didelphis marsupialis . In the past, however, marsupials were quite common. In South America and Australia, however, marsupials continued to be an important group of land mammals. Many South American forms are similar to the North American possum. The marsupials of South America began to go extinct in the late Miocene and Early Pliocene when a land connection with North America formed, allowing placental mammals to cross into South America.