Marsupials are a group of mammals commonly called as pouched mammals. Marsupials are common during the Mesozoic Era (251-65.5 million years ago). Marsupials used to be very common in North America.

Marsupials are believed back then to be the primitive forerunners of the modern day placental mammals. Because marsupial mammals and placental mammals are always being compared to one another, because of several similarities. But the comparison and conclusion has stopped, since there are concrete evidence that proved, that there is no truth to this belief. 

The fossil evidence was presented by a researcher named M.J. Spechtt in 1982. The reason why there's no truth to this is that by the end of Mesozoic Era, both marsupial mammals and placental mammals have evolved.

The main difference between a marsupial and a placental mammal can be distinguished by their teeth form: Marsupials have 4 pairs of molars in each jaw and placental mammals don't have more than 3 pairs of molars.

The Sinodelphys szalayi or the Chinese Opossum is the earliest known marsupial. This marsupial lived in China, 125 million years ago. This discovery supported the idea on how marsupials found their way to Australia. It is researched that marsupials reached Australia by using the Southeast Asia route. 

Because Australia is known for being the main habitat of marsupials. Australia may have the largest  marsupial population and marsupial species, but there are also other marsupials in other parts of the world like in Asia, specifically  in the region of Sulawesi, Indonesia.

Additional findings have also proved that marsupials did not take the Southeast Asia route. Because China and Australia were separated by the wide Tethys Ocean. This ocean existed during the Mesozoic Era. It is located before the opening of the Indian Ocean.

That's why it is now believed that marsupials reached Australia ad South America by taking the Antartica route. On most continents, where marsupials travelled, placental mammals were able to survived successfully than them (marsupials) like in Asia. In South America, opossums successfully survived and have retained a healthy number for a strong population. 

In Australia, marsupials have took over placental mammals, population-wise. Marsupials have dominated the Australian animal kingdom. The reason for the marsupial's success in Australia can be attributed to the marsupial's slow metabolism. This trait is very helpful to any animal that lives in Australia. Since the climate in Australia is quite dry & arid and hot during the summer time.

Because of the marsupial's low metabolic rate, it is able to successfully survive in Australia and continue to dominate. Marsupials outside Australia, like the ones in South America are only limited to the opossum specie.