Marsupials, also known as metatherian mammals, are an ancient and diverse mammal group. They are distinguished from other mammals by a number of cranial and skeletal characteristics, including larger numbers of teeth. Marsupials also share a unique pattern of reproduction and development of the young. Marsupial young are born at an early stage of development after a gestation period that can be as short as twelve days. After birth, they crawl over the mother's fur and skin and attach themselves to a nipple. Many, but not all, marsupials develop a pouch that protects the nursing young, and most development occurs within the pouch.
The marsupial lineage is thought to be the sister group to the lineage of placental mammals. The two groups are believed to have diverged 140 million years ago by the mid-Cretaceous, but are first known from the late Cretaceous fossil record. Marsupials have never evolved flying or marine forms, but they are morphologically diverse and occupy every other ecological niche .
Most marsupial diversity occurs in the Australasian region (about two hundred species) and in the tropical regions of Central and South America (about seventy species). Examples of marsupials are the red kangaroo ( Macropus rufus ), the koala ( Phascolarctos cinereus ), and the Virginia opossum ( Didelphis virginiana ), the only native marsupial found in the United States and Canada.
SEE ALSO Mammal
Nowak, Ronald M., ed. Walker's Mammals of the World. Baltimore, MD: Johns Hopkins University Press, 1991.
Vaughan, Terry A. Mammalogy. Philadelphia, PA: Saunders College Publishing, 1986.