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About Thylacosmilus:
The "saber-toothed" mammal plan has been favored by evolution more than once: Killer fangs didn't develop only in the large placental mammals of the Miocene and Pliocene epochs, but in marsupials as well. Exhibit A is the South American Thylacosmilus, the huge canines of which apparently kept growing throughout its life and were kept tucked in pouches of skin on its lower jaw. Like modern kangaroos, Thylacosmilus raised its young in pouches, and its parental skills may have been more developed than those of its saber-toothed relatives to the north. This genus went extinct when South America was colonized by the "true" mammalian saber-toothed cats, exemplified by Smilodon, starting about two million years ago.
By this point you may be wondering: how is it that the marsupial Thylacosmilus lived in South America rather than Australia, where the vast majority of all modern marsupials reside? The fact is, marsupials evolved tens of millions of years ago in Asia (one of the earliest known genera being Sinodelphys), and spread to various continents, including South America, before making Australia their favored habitat. In fact, Australia had its own version of large, catlike carnivore, the similar-sounding Thylacoleo, which was only distantly related to the line of pseudo-saber-toothed cats occupied by Thylacosmilus.
