Unlike placentals, marsupials are born in a semi-embryonic state, and in most species they are then protected within an abdominal pouch where they are milk-fed until much more developed.  Two thirds of the approximately 250 living marsupial species are found in Australia and its neighboring islands including New Guinea.  Most of the remaining species live in South America.  Currently, the only marsupial that naturally occurs in North America is the common and widespread Virginia opossum (Didelphis virginiana).

. An image of a thylacine from a film taken in 1928 at the Hobart Zoo (Domain site).

    The third and most ancient group of living mammals are the monotremes.  They feed their young milk as do other mammals, but they also posses a number of reptilian characteristics, particularly in respect to their skeletal structure.  Also, like reptiles, they lay eggs.  The monotremes are represented today by only three genera: Zaglossus (long nosed echidna), Tachyglossus (short nosed echidna), and Ornithorhynchus (platypus).  The echidnas have shallow pouches in which they hold their single egg and resultant young, but the platypus does not.  Instead, the platypus keeps its egg in a leaf and stick nest built within a chamber which it excavates in the bank of a river or creek.  The young are raised there until old enough to venture outside, usually by 17 weeks of age.

    The precise ancestral history of the three living mammal groups is still not completely understood, and numerous theories exist about the evolutionary relationships between them.
 

. A circa 1926 photograph of a thylacine at the London Zoo.

As compared to the monotremes, placentals and marsupials share a closer common ancestry with each other.  Extremely early marsupials, or mammals very similar to them in form (proto-marsupials), are considered by some to be ancestral to the placentals, having split into two groups around 120 million years ago.  How the monotremes relate to the marsupials and placentals is less certain, and has been the subject of considerable debate.

    Whatever the actual details of these evolutionary relationships may be, the three extant groups of mammals have been distinct from one another for a considerable length of time.  However, time itself is the only thing that separates them, since all mammal groups are descended from a common Mesozoic ancestor which probably appeared about 195 million years ago.  Indeed, one must keep in mind that all life on Earth is of a common ancestral origin which extends back at least 3.5 billion years.

    When zoologists began to classify the mammals of Australia, representatives of all three living groups were found among them, although marsupials were by far the dominant group.

    The great variety of Australian marsupials include such families as the kangaroos (Macropodidae), koala (Phascolarctidae), the possums (Phalangeridae, Burramyidae, Petaluridae), wombats (Vombatidae), quolls, Tasmanian devil, and marsupial "mice" (Dasyuridae), bandicoots (Peramelidae), the marsupial "mole" (Notoryctidae), the Numbat (Myrmecobiidae), and the thylacine (Thylacinidae).  Many Australian marsupials are analogues to the placentals of other continents in that they occupy the same types of ecological niches.  Through evolutionary convergence, a number of them have even come to resemble certain placentals in general physical form.
 

Today, the kangaroos are the dominant Australian marsupial group.  There are over 50 living species, which vary greatly in size.  Adapted to a variety of habitats, they are Australia's primary native herbivores, and thus are an analogue to the antelope and deer of other continents.  Of course, they are quite different from these animals in body structure, as well as reproductive method.  The niches of cats, dogs, and smaller carnivores are occupied in Australia by marsupial equivalents of such, including quolls, the thylacine, Tasmanian devil, and a number of marsupial "mice" and "rats".  The wombat holds the niche of a large, burrowing rodent such as a marmot.  Small, gliding marsupials called phalangers live in much the same way as do the "flying" squirrels of the New World.  There is even a marsupial version of the mole.  No marsupial analogues have evolved to fill the niches of aquatic placentals such as dolphins, whales and seals, possibly because the marsupial manner of reproduction would make such anatomical forms highly impractical.

. Koala (Phascolarctos cinereus).

    The monotremes, represented today by only three living species, are of particular interest in paleogeographical studies since they exist solely within the Australasian (Australia and New Guinea) zoogeographical realm.

    Prior to the time when Europeans began to colonize Australia, bringing their domesticated mammals with them, very few placental mammals had ever managed to become established on the continent.  A number of bats (which could fly there), a few rodents (which apparently washed in on floating logs carried from further north), and of course marine mammals such as dolphins, whales and seals were among the only placental inhabitants.  Stone Age humans migrated to Australia from Asia by perhaps 40,000 or more years ago.  Australia's wild dog, the dingo (Canis familiaris dingo), came in with another wave of ancient immigrants quite some time later.