and the Marsupials - A Historical and Modern Perspective
| 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
A Tiger quoll (Dasyurus
maculatus), one of a number of marsupials belonging to the family Dasyuridae.
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.
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 succeeded in becoming 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
lupus dingo), came in with another wave of ancient immigrants
quite some time later.
How the marsupials established
themselves in early Australia without the placentals arriving simultaneously,
and competing with them for ecological niches and becoming the dominant
mammalian fauna, is a question of particular interest to zoologists.
In other parts of the world, when placentals and marsupials have come into
contact with one another, the marsupials have not fared particularly well
and extinctions have followed. A well known example occurred after
the Great American Interchange, a significant paleozoogeographic
event which took place in the Pliocene Epoch, peaking dramatically around
three million years ago. During this time, the continents of North
and South America became connected by a land bridge. Subsequently,
placental carnivores from North America moved into the domain of the ancient
marsupial carnivores of South America. A similar effect has occurred
more recently in Australia with the introduction of placental
of mammal migration during the Great American Interchange. North
American species with South American ancestors are indicated by green silhouettes,
and South American species of North American origin by blue ones.
Diagram courtesy: Wouldloper.
|mammals which compete with the native
marsupials. The thylacine certainly represents a tragic example of
this, where its decline in Tasmania was caused by persecution
from a placental (humans). At least some marsupials have shown however,
that they are able to thrive despite even long-term competition with placentals,
as many species of South American opossums are flourishing to this day.
Palaeoecological evidence from Australia also supports the suggestion that
marsupials can sometimes deal effectively with placental co-habitation.
An examination of the
fossil evidence from both Pleistocene times and earlier will do much toward
forming an understanding of what has transpired in Australia over the course
of the Cenozoic Era. Unfortunately, the very early history of the
mammals in Australia remains as of yet rather poorly known. The further
one follows the continent's geological record back into time, the scarcer
the remains of mammals become. In comparison to those from the Quaternary,
mammal fossils from the Tertiary (2-66 million years ago) are relatively
rare in Australia.