. A 1903 photo of a male thylacine which lived at the Berlin Zoo for approximately six years.

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 separated from Antarctica in the Early Cretaceous, some 135 million years ago; Australia from Antarctica between the Late Paleocene to Early Eocene epochs, 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, 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 1981 however, a discovery of marsupial remains of Late Eocene age (35-40 million years ago) was made on Seymour Island, near the Antarctic Peninsula by Michael Woodburne and Bill Daily (Zinsmeister 1986).  The first fossil mammal ever recovered from Antarctica, it was a find of enormous significance.  It belongs to a marsupial of the family Polydolopidae, a family which was previously known only from South America, and it adds considerable support to Harrison's hypothesis.  Along with the finding 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.
 

According to the most widely accepted reconstruction, in Cretaceous times the continent of Australia lay much further south of its present-day position, and had a wide expanse of ocean separating it from Asia.  Only 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 far back in the Eocene.

. Fossil mandible of a polydolopid marsupial found on Seymour Island by M. Woodburne & B. Daily.

    Most palaeontologists today believe that the Australian marsupials 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.  Numerous arguments have been brought forward in favor of South America, North America, Antarctica and Australia itself.

    Over the course of the last few decades, the possibility that the marsupials as a whole had their origin in Australia has received renewed attention.  Could it be that the modern centre of marsupial diversity is also the place of marsupial origin?  Recently discovered fossil evidence indicates that placental mammals did in fact exist in Australia prior to its separation from Antarctica, and for some reason, they became extinct there soon afterward.  Why did the placentals die out in Australia?  This is but one of many great mysteries in palaeontology.  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 merely another way of being a mammal.

    Though 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.
 

. A thylacine photograph taken at the London Zoo in 1902.

However, the discovery in Australia of a fossilized flea of Early Cretaceous age, 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 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 abundant and diversified.  By the arrival of the Pleistocene Epoch (10,000-1.6 million years ago), Australia had produced 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.  Some of the most noteworthy assemblages have been found within the caves of South Australia and New South Wales.  Along with the Pleistocene, the Holocene Epoch (our current time) makes up the Quaternary Period of the Cenozoic Era.