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, some 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 mammals which
compete with the native marsupials.
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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 proven 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. Mammal fossils from the Tertiary (2-66
million years ago) are very rare in Australia. |
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A Tiger quoll (Dasyurus
maculatus), one of a number of marsupials belonging to the family Dasyuridae.
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The oldest known Australian
marsupial fossils are from a 55 million year old, Early Eocene site at
Murgon in southern Queensland. This site has produced a range of marsupial
fossils, many with strong South American connections. The oldest
monotreme species yet known is Teinolophos trusleri, whose fossils
are dated at 123 million years old (Early Cretaceous), from Flat Rocks,
Victoria. Another very old monotreme from Australia is Steropodon
galmani, a somewhat platypus-shaped animal known from a
small section of a 110 million year old mandible. In 1991 and 1992,
several fossil teeth from a 61 million year old platypus were found at
Punta Peligro in southern Argentina. Named Monotrematum sudamericanum,
the teeth are thought to actually belong to a member of the genus Obdurodon,
the remains of which have also been found in Australia. Placentals
are known from the Tertiary of Australia as well, including some whales
and a bat from the Middle Miocene (15 million years ago), and rodents of
the family Muridae from the Early Pliocene (4-5 million years ago).
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| A
rare image of a thylacine taken at the Melbourne Zoo sometime prior to
1909. |
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The aforementioned 55 million year old Murgon site also produced an isolated
condylarth tooth in 1987, which has been named Tingamarra
porterorum. Condylarths are an ancient group of placental
mammals known from various other regions of the world, and the presence
of Tingamarra in Australia indicates that placentals were indeed
present there at the time when the continent broke away from Antarctica.
The world's oldest fossil bat was also found at the Murgon site. |
Very old marsupial fossils
have been found on other continents, most notably, North America and Asia.
Perhaps some knowledge can be gleaned about the possible origin of the
Australian marsupials from the geographical and chronological distribution
of these non-Australian fossils. Until recently, the oldest marsupial
fossils were some isolated teeth found in 110 million year old Cretaceous
sediments of North America, and so it was thought that the marsupials of
Australia most likely descended from ancestors which evolved in the New
World. In the year 2000 however, a new fossil (proto)marsupial which
has been named Sinodelphys
szalayi was found in China's Liaoning Province. Dating back
to 125 million years ago, it is the oldest marsupial yet known, and may
indeed be the great-grandparent of all marsupials.
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Another fossil mammal from Asia that has much increased our understanding
of marsupial evolution is 80 million year old Deltatheridium, from
the Late Cretaceous. At first, the species was known only from very
fragmentary remains, and there was considerable debate about whether it
was a marsupial or a placental. However, some well preserved fossils
of Deltatheridium were recently
discovered at Ukhaa Tolgod, Mongolia which have made it possible to
firmly classify it as a marsupial. |
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| Some
recently discovered mandibles of Deltatheridium, a Late Cretaceous
marsupial of Mongolia. |
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