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Australia
and the Marsupials - A Historical and Modern Perspective
By the mid 1920s, Launcelot Harrison (1924) had already put forward the
idea that the Australian marsupials had reached the continent by crossing
over land connections through Antarctica. However, at the time, continental
drift was such a new and controversial concept that his idea was not readily
accepted by the scientific community. As an understanding of forces
such as sea-floor spreading and lateral movements of the continents developed,
the new science of plate tectonics revolutionized the geological field,
and with it came a better understanding of zoogeography throughout Earth's
history. Until Early Jurassic times (approx. 200 million years ago),
the continents that we know today were unified as a single, enormous land
mass called Pangea. Soon afterward, it divided into a northern half
- Laurasia (which included what would become North America, Greenland,
Europe and Asia; and a southern half - Gondwana (which included the land
that is now South America, Africa, India, Antarctica and Australia. |
| An
approximation
of the Earth as it was some 150 million years ago, during the Late Jurassic
Epoch, as Pangaea was in the process of rifting apart. The new continents
shown here are Laurasia to the north and Gondwana to the south. Light
blue indicates areas of shallow water, where land bridges reconnecting
the continents would have formed from time to time as a result of geological
changes. Image courtesy: Dr. Ron Blakey. |
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After this great division took place, the two newly formed continents began
breaking into still smaller parts. The Americas drifted to the west,
Antarctica to the south, and India and Australia northwards. It is
believed that Africa drifted away from Antarctica in the Early Cretaceous,
some 135 million years ago; Australia from Antarctica between the Late
Paleocene to Early Eocene epochs, |
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| Fossil
mandible of Antarctodolops dailyi, a polydolopid marsupial found
on Seymour Island by M. Woodburne & B. Daily. |
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about
55-60 million years ago; and Antarctica from South America sometime during
the Eocene Epoch, 34-55 million years ago. Direct land connections
existed prior to these dates, and exchanges of mammals via island-hopping
would have been possible for a further period of time. Back then,
Earth's climate was much warmer; Antarctica was not the ice-covered land
that it is today, and so would not have acted as a barrier preventing faunal
exchange. A lack of any fossil marsupial remains from Antarctica
made testing |
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| Harrison's theory
difficult for quite some time. In 1982 however, a discovery of marsupial
remains of Middle Eocene age (45 million years ago) was made on Seymour
Island, near the Antarctic Peninsula by Michael Woodburne and Bill Daily
(Zinsmeister 1986). Named Antarctodolops dailyi, it was the
first fossil mammal known from Antarctica, and a find of enormous significance.
It is a member of the family Polydolopidae, a group of marsupials which
was previously known only from South America, and it adds considerable
support to Harrison's hypothesis. |
| A
approximate
paleogeographic reconstruction of the Earth as it was during the Early
Eocene Epoch, some 50 million years ago. Australia, now slowly drifting
northward, had separated from Antarctica between 5-10 million years earlier,
and the rest of the world's continents are starting to take on their modern
form. Image courtesy: Dr. Ron Blakey. |
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