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PALAEONTOLOGY:
- AUSTRALIA AND THE MARSUPIALS -
(page 5)
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Australia and the Marsupials - A Historical and Modern Perspective

    Marsupials became extinct in North America by the end of the Oligocene, although they continued to do well in South America, which by that time had become separated from North America by an ocean channel (as a result of the ancient supercontinent of Pangea breaking up).  The mammalian carnivore niches in South America were filled entirely by marsupials of genera such as Borhyaena and Thylacosmilus, the latter being an excellent example of a marsupial saber tooth "cat".  North America's present-day opossum (Didelphis virginiana) is not descended from the continent's ancient marsupial stock, but is an immigrant from further south that arrived during the Pleistocene Epoch.  Even though the geographical and chronological relationship between the marsupials of Europe and the New World appears to now be fairly well understood, the relationship of the Australian marsupials, which are today isolated by wide ocean expanses from the other continents, was quite problematic to explain prior to the discovery that the Earth's continents had drifted over many millions of years.

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Thylacosmilus atrox skull - (Image - Claire Houck)
Thylacosmilus lentis skull - (image  C. Campbell)
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Two skulls (both cast replicas) of Thylacosmilus, a remarkable marsupial saber tooth predator from the Late Miocene and Early Pliocene of Argentina.  At left is Thylacosmilus atrox; at right, Thylacosmilus lentis.  It has been argued that lentis may in fact be the same species as atrox (Marshall, 1976).
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    Because the only (currently) convenient pathway to Australia is Asia, zoologists had long assumed that the marsupial fauna of Australia had come from North America via Asia and then "island-hopped" through Indonesia, probably during the Cretaceous Period.  For some mysterious reason the placentals failed to follow, and the great southern land mass became a sanctuary for the marsupials, which were free to evolve in isolation from them.  A theory such as this, however, has 
some problems.  There is a distinct faunal break (Wallace's Line) running along the deep channel between Java and Kalimantan (Borneo) to the north-west (the Oriental realm) and Sulawesi (Celebes), Irian (western New Guinea) and Australia (the Australasian realm) to the south-east.  Marsupials are present on many of the islands that lie between Wallace's line and Australia, but because there are far fewer species there than in Australia itself, it can be inferred that colonization had been from the Australian mainland to the islands, rather than vice versa.  Placental mammals, however, are present only on the western-most islands, with little overlap of the marsupial region.  This suggests that they spread from the Asian land mass. .
positions of the Wallace, Weber and Lydekker lines
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The Wallace line delineates the fauna of the Australian and Southeast Asian fauna.  It is named after naturalist Alfred Russel Wallace, who noticed this clear division during his travels through the East Indies in the 19th century.  The probable extent of exposed land at the time of the last glacial maximum, at which time the sea level was more than 110 meters lower than is the case today, is shown in grey.  The Lombok Strait between Bali and Lombok formed a deep water barrier even when lower sea levels linked the now-separated islands and landmasses on either side.  Diagram courtesy: Maximilian Dörrbecker.

    Studies of continental drift have provided further support to suggest that Asia was not the source of the Australian marsupial fauna.  In the mid-20th century, geological studies of plate tectonics showed that there is an Indo-Australian plate that has Wallace's Line as a boundary, resulting in a large drop in the sea floor at precisely the same point.  This means that it has never been possible for a land bridge to form in the region, hence the zoological distribution.

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Acknowledgement: This subsection of the Thylacine Museum 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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