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It was once commonly believed that marsupials were a primitive forerunner of modern placental mammals, but fossil evidence, first presented by researcher M.J. Spechtt in 1982, conflicts with this assumption. Instead, both main branches of the mammal tree appear to have evolved concurrently toward the end of the Mesozoic era. In the absence of soft tissues, such as the pouch and reproductive system, fossil marsupials can be distinguished from placentals by the form of their teeth; primitive marsupials possess four pairs of molar teeth in each jaw, whereas placental mammals never have more than three pairs.
Using this criterion, the earliest known marsupial is Sinodelphys szalayi, which lived in China around 125 million years ago. This makes it almost contemporary to the earliest placental fossils, which have been found in the same area.
The discovery of Chinese marsupials appears to support the idea that marsupials reached Australia via Southeast Asia. There are a few species of marsupials still living in Asia, especially in the Sulawesi region of Indonesia. These marsupials coexist with primates, hooved mammals and other placentals. However, due to the fact that Australia and China were separated by the wide Tethys Sea in the early Cretaceous into the Northern continent of Laurasia and Southern continent of Gondwana, marsupials had to take a much longer route around. From their origin in East Laurasia (modern day China), they spread westwards into modern North America (still attached to Eurasia) and skipped across to South America, which was connected to North America up until around 65MYA. Here they radiated into Borhyaenids and Shrew Opossums, creating a unique fauna found in South America and Antarctica (which were connected until 35MYA). Marsupials reached Australia via Antarctica about 50MYA just after Australia had split off, suggesting a single dispersion event of several of just one species, related to South America's Monito del Monte (Microbiothere), rafted across the widening, but still narrow gap between Australia and Antarctica at that time. In Australia, being the only mammals present (except a few Austrosphenids like echidnas and platypuses) they radiated into the wide varieties we see today, even island hopping some way through the Indonesian archipelagos, almost completing a circumnavigation back to their homeland in China. 
On most continents, placental mammals were much more successful and no marsupials survived, though in South America the opossums retained a strong presence, and the Tertiary saw the genesis of marsupial predators such as the borhyaenids and the saber-toothed Thylacosmilus. In Australia, however, marsupials displaced placental mammals entirely, and have since dominated the Australian ecosystem. Marsupial success over placental mammals in Australia has been attributed to their comparatively low metabolic rate, a trait which would prove helpful in the hot Australian climate. As a result, native Australian placental mammals (such as hopping mice) are more recent immigrants.
- ^ http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0001858;jsessionid=A57F0FDB595AC49992E2B5A390FA104C
- ^ Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. p. 306. ISBN 0-412-73810-4.
- ^ Rincon, P., Oldest Marsupial Ancestor Found, BBC, Dec 2003
- ^ Pickrell, J., Oldest Marsupial Fossil Found in China, National Geographic, December 2003
- ^ Klinger, M.A., Sinodelphys szalayi, Carnegie Mellon Natural History, 2003
- ^ Ji, Q., et al., The Earliest Known Eutherian Mammal, Nature, 416, Pages 816-822, Apr 2002
- ^ Harrison, L., The Migration Route of the Australian Marsupial Fauna, Australian Zoologist, Volume 3, Pages 247-263, 1924
- ^ [2005. T.S. Kemp The origin and evolution of mammals.]