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INTRODUCING THYLACOLEO:
- AUSTRALIA AND THE MARSUPIALS -
(page 7)
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Australia and the Marsupials - A Historical and Modern Perspective
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    Along with the discovery of marsupial fossils on Seymour Island, the continued development of the understanding of continental drift makes the old theory of an Asian origin for the marsupials of Australia even less likely than before.  In the early Tertiary, the continent of Australia lay much further south of its present-day position, and had a wide expanse of ocean separating it from Asia.
Only when Australia moved much closer to Asia during more recent geological times did island hopping again become possible, and Australia became home to placentals for the first time since the continent separated from Antarctica back in the Early Eocene.

    Current knowledge suggests that either Asia or North America was the most probable geographic center of origin of the early Marsupialia.  Most palaeontologists today believe that the Australian marsupials, specifically, have their origin (i.e. differentiation from more primitive marsupial stock) in the Cretaceous, either within or through Gondwana, although the details of such are still under debate.  In any case however, it is evident that the ancient ancestor (probably a microbiothere) from which the Australian marsupials decended was from the geographical region that is now the Americas (Rose 2006).

    Recently discovered fossil evidence indicates that placental mammals did in fact exist in Australia prior to its separation from Antarctica, but for some reason, they became extinct there soon afterward.  Why did the placentals die out in early Australia?  This is but one of many great mysteries in palaeontology.

Tasmanian devil (Sarcophilus harrisii)
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A Tasmanian devil (Sarcophilus harrisii) shows off its impressive set of teeth.  Unlike its arboreal relatives the quolls, it is a ground dweller.  An opportunistic scavenger, Sarcophilus will eat virtually anything edible, but is primarily carnivorous.
Perhaps marsupials are not so ill-prepared to cope with competition from placentals as has long been believed.  Possibly, the marsupials long ago won the evolutionary race in Australia and became the masters of their zoological realm.  Marsupials are by no means any less well-equipped to survive than are the placentals.  Being marsupial is not a more primitive mammalian state than is placental - it is simply another way of being a mammal.
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life reconstruction of Procoptodon goliah - Anne Musser
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Life reconstruction of the giant, short-faced kangaroo Procoptodon goliah of the Late Pleistocene Epoch.  A member of the once diverse subfamily Sthenurinae, it stood more than 2m (6.5ft) tall, weighed up to 200kg (440lbs), and is the largest species of macropodid known to have ever existed. Courtesy: Anne Musser.
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    Although the search for Cretaceous mammal fossils in Australia has continued in recent years with increasing enthusiasm, relatively little progress has been made.  The remains of Mesozoic mammals on the continent are extremely rare indeed.  However, the discovery in Australia of a fossilized flea of Early Cretaceous age (Jell & Duncan 1986), of a type found associated elsewhere in the world with mammals and not with birds, raises questions about the identity of its host and illustrates the incomplete nature of the fossil record.  Environmental conditions are unfortunately not always ideal for the preservation of fossils, and many of the millions of species that have inhabited the Earth down through the ages have probably left behind no evidence of their existence at all.

    Although there is still relatively little known about the early Tertiary history of Australia's mammalian fauna, it is quite clear that by late Tertiary times, the continent's marsupials had become quite

abundant and diversified.  By the arrival of the Pleistocene Epoch (10,000-1.6 million years ago), Australia had given rise to species such as the diprotodontid Diprotodon optatum and the kangaroo Procoptodon goliah, among the largest marsupials the world has ever known.  A wealth of Australian marsupial fossils have been unearthed which date from the Pleistocene, especially in ancient lake and river deposits, among the most notable of which have been found within the caves of South Australia and New South Wales.

    For a comprehensive overview of the evolution of Australia's mammalian fauna, the museum recommends the publications "Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution" and "Australia's Lost World: Prehistoric Animals of Riversleigh".

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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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