Ever since Australia began its long northward journey, the marsupials have been its dominant mammal group.     Early European cartographers (map makers) had long speculated the possible existence of a mysterious, unknown southern land, which they called "Terra Australis Incognita".  During the Age of Exploration, when the first European voyagers reached the shores of the legendary continent, they were amazed to find that the mammals living there were quite different from any that they had seen previously.  These newly discovered creatures were marsupials.  The existence of this group of mammals was known to the Europeans long before their encounter with Australia however, as opossums had already been found in North and South America.

. A map of Terra Australis produced in 1531 by the French mathematician Oronce Fine.

    However, Captain James Cook's kangaroo was the first marsupial to become widely known.  Cook's own description reads:

    "In form, it is most like the Jerboa, which it also resembles in its motion, ...; but it greatly differs in size, the Jerboa not being larger than a common rat, and this animal, when full grown, being as big as a sheep: ...  The head, neck and shoulders are very small in proportion to the other parts of the body; the tail is nearly as long as the body, thick near the rump, and tapering towards the end: the fore-legs of this individual were only eight inches long, and the hind legs two-and-twenty: its progress is by successive leaps or hops, of a great length, in an erect posture;  the fore-legs are kept bent close to the breast, and seemed to be of use only for digging..."

    Cook also reported bats, a 'kind of pole-cat', 'wolves' and

    "an animal of the Opossum tribe: it was a female, and with it he' (Joseph Banks) 'took two young ones:  it was found to resemble the remarkable animal which Mons. de Buffon has described in his Natural History by the name of Phalanger, but it was not the same.  Mons. Buffon supposes this tribe to be peculiar to America, but in this he is certainly mistaken."
 

. Eastern Grey kangaroos (Macropus giganteus).

Through such records, we can see that although the marsupials were not officially classified by zoologists as a unique group separate from the placental mammals until the nineteenth century, the similarities between the American and Australian marsupials were already starting to be noticed before the end of the 1700s.    Being an enormous island surrounded by deep water, Australia has been isolated from the rest of the world's faunal realms for many millions of years.  Thus, mammalian exchanges to and from other land masses (other than by flying or swimming species of placentals) have been quite rare, except within comparatively recent times with the assistance of man.

    The discovery of the mammals of Australia had a strong impact on scientific ideas about the magnitude of the animal kingdom.  Back in Europe, explorers' reports of the new animals were at first ridiculed by a number of zoologists of the time.  Preserved specimens of creatures as bizarre as the platypus (Ornithorhynchus anatinus), which seemed to be a taxidermist's handiwork of piecing together parts of completely different animals, were often thought of as hoaxes until scientific examination proved otherwise.
 

To begin forming an understanding of Australia's mammalian fauna, let us first consider the taxonomic relationships of the groups which exist there.  All living species of mammals are classified into three main sub-divisions; the placentals, marsupials, and monotremes.  The main characteristic of the placentals is that the young are retained in the uterus, where they are nourished for an extended length of time by a placenta.  Placentals do not have a marsupium (pouch).

. A reclining thylacine at the Hobart Zoo (Beaumaris site).  This photo was taken sometime between 1917 and 1921.

    The young are fed with milk, as in the other mammal groups.  Placentals also possess certain structural features of the skull which are unique.  This group is worldwide in distribution and is the division to which the vast majority of living mammal orders belong (e.g. carnivores, cetaceans, artiodactyls, bats, rodents, primates, etc.).  Marsupials are essentially similar to the placentals in most respects, though they do have a few structural differences, primarily in respect to the reproductive organs, skull and skeleton (which includes the presence of two additional bones, the epipubics, located in the pelvic girdle).