Marsupials are generally distinguished by their pouch, although this is not an obvious pouch in all species, the carnivores, for example, have a fold of skin which swells to protect young rather than a fully developed pouch. In many respects, a marsupial is more like the egg laying Monotreme, (i.e. platypus or echidna), than other mammals, except that it’s young are born underdeveloped, rather than out of an egg.  Embryonic marsupials, like the developing embryo in an egg, are connected to a yolk sac, rather than to a placenta as in the more advanced mammals. At birth, they crawl to the pouch and once inside the pouch, attach to a nipple. But as we said before, not only marsupials have a pouch (Echidna also show a marsupium).  

The marsupial brain is relatively small, compared with that of placental mammals. It lacks the connection between the two halves of the brain (hemispheres) that is developed in the Eutheria.  

It is believed that Marsupial ancestors moved from the Americas to Europe in the Northern Hemisphere and to Australia and Tasmania via Antarctica in the Southern Hemisphere, around 65 million years ago, when the three continents were joined.

There are 251 living marsupials organized in 7 orders.  170 of these are indigenous to Australia and nearby islands including New Guinea and Tasmania.  The remaining 51 are indigenous to the Americas.  Most American opossums are arboreal (live in trees), but one (Chironoectes minimus) the “Water Possum” is aquatic, and looks a little like a small otter.

There are around 250 species of Marsupial in total.  The taxonomic relationships comprise the following groups:

They generally have a pouch, although this may be underdeveloped in some species.  Their tail is prehensile, used for grasping branches when climbing.  Some species show periods of inactivity during the colder months of the year (torpor). This species accumulate fat at the base of their tails to sustain them while inactive. They can be found in a range of different habitats from desserts to tropical forests, being burrowers and semi-arboreal. Male opossums are generally solitary by nature, whilst the females can live in small groups.

The Water Opossum, Chironectes minimus is the only marsupial adapted to water habitats such as freshwater streams and lakes and has an aquatic diet. It has webbed feet, water-repellent fur and both males and females possess a pouch.  The pouch in the females is completely watertight.  Females can dive for longer periods as the young can survive without oxygen for several minutes at a time.  These animals are solitary and hunt at night feeding primarily on fish, crayfish, frogs, shrimp and some aquatic vegetation.

The Virginia Possum, Didelphis virginiana is the largest member of the Order and is found in North America.  It is nocturnal and is well-known for its ability to feign death when faced with an apparent threat.  This is believed to be an involuntary reaction which occurs when the Opossum is faced with an extreme threat. However, opossums can also react quite aggressively when threatened, by screeching and baring their teeth.

Family Myrmecobiidae. This family includes only one species, the Numbat (Myrmecobius fasciatus), which lives in arid woodlands in Western Australia. The species is characterised by 50-52 poorly developed teeth which are not used in eating except by the very young. It feeds almost solely on termites and has an extremely long tongue protruding from a pointed snout for this purpose. Numbats don’t have a pouch, their young cling to the mother’s fur while suckling from her teats. As opposed to most other marsupials, Numbats are diurnal, most active in the morning and afternoon, which matches the activity of their prey, the termites.  They usually shelter in logs, burrows or tree hollows.  The individuals are solitary in nature and occupy vast territories, with only male and female territories overlapping.  Females usually give birth to four young which cling to her teats for around the first 6 months of life.

Family Thylacinidae.  This family is comprised of one species, the now extinct Tasmanian Tiger or Thylacine (Thylacynus cynocephalus).  This animal was once distributed across mainland Australia and Papua New Guinea, but was confined to Tasmania thousands of years ago, possibly due to competition with the Australian Dingo.  Thylacines were believed to have become extinct in Tasmania due to hunting, disease, habitat modification and competition with wild dogs.  The last individual died in captivity in Tasmania in 1936.  The Thylacine was the largest of the modern carnvirous marsupials.  It was dog-like in size and appearance, aside from its stiff tail and rear opening pouch.  It was characterised by horizontal stripes running along its back and extremely powerful jaws.  The Thylacine preyed on other marsupials, small rodents and birds.

Family Thylacomyidae.  This family includes two species of Bilby in the Genus Macrotis.  One of which, the Lesser Bilby (Macrotis leucura) became extinct prior to European settlement.  The surviving Greater Bilby or Rabbit-eared Bandicoot, Macrotis lagotis lives in arid Australia, occupying hummock and tussock grasslands and acacia shrublands.   They are nocturnal, resting in burrows of up to 3 metres in length during the day.  Bilbies are omnivorous, feeding on a range of prey including insects, larvae, spiders, and various vegetative matter.  They are characterised by their very long ears and elongated nose.  They also have strong forelimbs and thick claws for digging.