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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.

Earth in the Late Jurassic Epoch
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.
    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,
fossil mandible of Antarctodolops dailyi
Fossil mandible of Antarctodolops dailyi, a polydolopid marsupial found on Seymour Island by M. Woodburne & B. Daily.
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
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.
Earth in the Early Eocene Epoch
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.
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.
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