Marsupials or Metatheria
These mammals, comprise 9 orders, 23 families and around 330 species( more may be found when people chart out the whole of Papua and New Guinea). Most of these species are concentrated in Australasia but some 100 odd species are also found in the Americas. These animals have usually been in competition with Eutherians and have given ground in areas where both had to fight for the same ecological niche.
In Australia, they have evolved to occupy all sorts of niches that the tradional moles,shrews,primates, carnivores and other Eutherians occupy in other parts of the world. However, lest you think I am putting these elegant beast down as also-rans, let me hasten to add that these elegant beasts have also created some wonderful types like the kangaroo; a dazzling array of species specifically evolved and adapted for a land with little water and seas of grass.
In times of yore, when the world was less cluttered with pushy primates, filled with self-love, the marsupials had come up with incredible types like large flesh-eating kangaroos, rhino sized 'roos and even a sabre-tooth marsupial cat.
Geographic Range
Most marsupials are found today in Australasia(200 species) and the rest are usually found in South and Central America. There is a species, quite innocuous really, that is a marsupial and maybe digging into your trash as you read this. It is the Virginia Opposum.
Metatherians are highly adapted creatures and today most of them are to be found in Australasia while some species hold out against the onslaught of their eutherian cousins in Central and South America.
Origins
Metatheres diverged from the mammalian lineage leading to eutherian (placental) mammals by the Middle Cretaceous. Early diversification of metatheres is thought to have taken place in North America although, by the middle Miocene, the lineage became extinct on that continent, only reappearing around the time that North and South America regained contact in the Pliocene. The earliest marsupials are believed to have resembled North American opossums and other members of the family Didelphidae. There is some evidence in fossil record of marsupials being present in Europe, Africa and Asia but the group never really got a toe-hold in these continents.
According to the latest information on classification, the metatherians are classified into two super orders, Ameridelphia and Australidelphia. The former contains mainly American species while the latter is spread widely around Australia. This classification relies on molecular biology and our ever increasing fossil record.
Behavior
Most marsupials are solitary animals. However, kangaroos and wallabies may be found in groups numbering from two to about ten members, depending on the size of the animals, their habitat, and other factors. The whiptail wallaby gathers into large mobs of up to 50 individuals. To mark their territory or communicate their desire to mate, marsupials primarily use scent from their skin glands, urine, or feces. Some arboreal (tree-dwelling) species also communicate vocally. Marsupials do not form male-female bonds that extend beyond the time necessary for mating, and the female cares exclusively for the young.
Communication
As with mammals in general, vision, hearing, hearing and touch are all important to varying degrees among species of Metatheria. Communication can take many forms as well. Some marsupials communicate with sound signals, particularly during mating or territorial encounters. Many species have conspicuous color patterns that may convey information about sex or species identity.
Predation
Marsupials are rarely the top predators in the areas that they roam. Most of them are herbivores and are therefore open to the depredations of humans as well as alien species introduced into their environments.
Conservation
Many marsupials are threatened or endangered. The International Union for the Conservation of Nature and Natural Resources (IUCN) currently lists over 200 species (i.e., more than 2/3 of all marsupials) as being of some level of concern. Habitat destruction, overexploitation, and competition with exotic species and livestock have greatly reduced many populations. A number of species have gone extinct within the past two centuries as a direct result of human activity. The most famous among them is the Tasmanian tiger or thylacine.
Orders within Metatheria
Superorder Ameridelphia
Order Didelphimorphia (95 species)
Order Paucituberculata (6 species)
Superorder Australidelphia
Order Yalkaparidontia EXTINCT
Order Microbiotheria (1 species)
Order Dasyuromorphia (71 species)
Order Peramelemorphia (24 species)
Order Notoryctemorphia (2 species)