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PALAEONTOLOGY:
- AUSTRALIA AND THE MARSUPIALS -
(page 2)
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Australia and the Marsupials - A Historical and Modern Perspective
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  Marsupials are essentially similar to the placentals in many respects, though they do possess a number of structural differences, primarily in respect to the reproductive organs, skull and skeleton (which includes the presence of two additional bones, the epipubics, located in the pelvic girdle).  Unlike placentals, marsupials are born in a semi-embryonic state, and in most species they are then protected within their mother's abdominal pouch where they are milk-fed until until their early development is complete.  Two thirds of the
Virginia opossum (Didelphis virginiana)
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A young Virginia opossum (Didelphis virginiana).  In the Americas, the Virginia opossum is the only marsupial species found north of Mexico.
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approximately 250 living marsupial species are found in Australia and its neighboring islands including New Guinea.  Most of the remaining species live in South America.  Currently, the only marsupial that naturally occurs in North America is the common and widespread Virginia opossum (Didelphis virginiana).
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    The third and most ancient group of living mammals are the monotremes.  They feed their young milk as do other mammals, but they also possess a number of reptilian characteristics, particularly in
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skulls of Ornithorhynchus anatinus and Obdurodon dicksoni
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The fossil skull of the Late Oligocene platypus Obdurodon dicksoni, as compared to that of the modern platypus (Ornithorhynchus anatinus).  Apart from being physically larger, unlike the living species, Obdurodon possessed teeth.
respect to their skeletal structure.  Also, like reptiles, they lay eggs.  The monotremes are represented today by only three genera: Zaglossus (Long-nosed echidna), Tachyglossus (Short-nosed echidna), and Ornithorhynchus (platypus).  The echidnas have shallow pouches in which they hold their single egg and resultant young, but the platypus does not.  Instead, the platypus keeps its eggs (most often two) in a leaf and stick nest built within a chamber which it excavates in the bank of a river or creek.  The young are raised there until old enough to venture outside, usually by 17 weeks of age.

    The precise ancestral history of the three living mammal groups is still not completely understood, and numerous theories exist about the evolutionary relationships between them.  As compared to the monotremes, placentals and marsupials share a closer common ancestry with each other.  Extremely early marsupials, or mammals very similar to them in form (proto-marsupials), are considered by some to be ancestral to the placentals, having split into two groups perhaps 125+ million years ago.  How the monotremes relate to the marsupials and placentals is less certain, and has been the subject of considerable debate.

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    Whatever the actual details of these evolutionary relationships may be, the three extant groups of mammals have been distinct from one another for a considerable length of time.  However, time itself is the only thing that separates them, since all mammal groups are descended from a common Mesozoic ancestor which probably appeared about 195 million years ago.
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    When zoologists began to classify the mammals of Australia, representatives of all three living groups were found among them, although marsupials were by far the dominant group.

    The great variety of Australian marsupials include such families as the kangaroos (Macropodidae), koala (Phascolarctidae), the possums (Phalangeridae, Burramyidae, Petaluridae), wombats (Vombatidae), quolls, Tasmanian devil, and marsupial "mice" (Dasyuridae), bandicoots (Peramelidae), the marsupial "mole" (Notoryctidae), the Numbat (Myrmecobiidae), and the thylacine (Thylacinidae).  Many Australian marsupials are analogues to the placentals of other continents in that they occupy the same types of ecological niches. Through evolutionary convergence, a number of them have even come to resemble certain placentals in general physical form.

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koala (Phascolarctos cinereus)
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The koala (Phascolarctos cinereus), a phascolarctid marsupial.
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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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