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INTRODUCING THYLACOLEO:
- AUSTRALIA AND THE MARSUPIALS -
(page 3)
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Australia and the Marsupials - A Historical and Modern Perspective
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    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
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Tiger quoll (Dasyurus maculatus)
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A Tiger quoll (Dasyurus maculatus), one of a number of marsupials belonging to the family Dasyuridae.
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.

    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.

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    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

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mammal migration during the Great American Interchange
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Examples 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.
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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.

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Acknowledgement: This subsection of Thylacoleo Revealed has been referenced (in part) from: SUTCLIFFE, A. J. 1985. "On the Track of Ice Age Mammals". Harvard Univ. Press: Cambridge. pp. 186-99.
References
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