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. |
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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. |
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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. |
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| 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 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 |
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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". |
