Orders (all listed are various forms of marsupials):
What makes a marsupial a marsupial?
The most obvious characteristic is, of course, the pouch. However, you may ask yourself, what is the significance of a pouch? Marsupials are aplacental. The baby is "born" in an embryonic stage, and further develops outside of the mother's body, as opposed to eutherian mammals such as humans, dogs, cats, etc., who complete their embryonic development inside the mother's uterus. When the babies (or "joeys," as they are called) are born, they resemble a tiny, pink lima bean! In order to get bigger and stronger, this amazingly tiny, newborn creature must climb from the cloaca up the furry exterior of the mother kangaroo, and into the mother's pouch, where it attaches to a nipple. The joey then leisurely drinks milk for many months, as it physically develops into a recognizable kangaroo. Eight to ten months after birth it will be time for the joey to leave the security of the pouch, and explore a new world.
Why are most marsupials from Australia and New Zealand?
Marsupials have been out competed (or basically pushed out) by the placental mammals. South America actually used to be home of many marsupials during the Tertiary period (about 65 million years ago). This was a time when South America was isolated. When Central America and South America joined (creating an isthmus) bringing more advanced mammals to South America, thus out competing them. Can you name the two orders of Marsupials from the America's?
Most of the information provided can be found in The Vertebrate Body, sixth edition by Romer and Parsons published by Harcourt Brace Jovanovich College Publishers, 1990
