Say the word "Australia", and one of the first things to come to your mind would be the marsupials.
Of the 334 species of pouched mammals whose earliest ancestors witnessed the fall of the dinosaur empire 66 million years ago, close to 70% of them live in Australia, the rest living in the New World. Practically everyone is familiar with a kangaroo, a koala and a Tasmanian devil, and sugar gliders have been exploited as pets.
But that was only our Earth.
Here is the map of an alternate Earth that I've been working on for years:
If you look closely at Australia, you'll notice that it's much closer to Antarctica. This will, by itself, affect the evolution of the continent's marsupials. But the focus to the question are two major points of departure: 55.5 ma and 14 ma.
55.5 ma, the global temperature spiked up into the Paleocene-Eocene Thermal Maximum, just like back home. But whereas our PETM lasted 200,000 years, theirs lasted three to four times that long. That alone would butterfly out of existence the major, most iconic of the marsupials because they debuted anywhere between the Oligocene and Miocene epochs.
14 million years ago, the global temperature dropped so dramatically and so quickly that it drove half of all plant an animal species into extinction. To clarify on that last statement, the temperature dropped from four to five degrees Celsius higher than today to five to ten degrees Celsius lower within a period of ten to forty millennia.
Would this ice age extinction be enough to drive all of the marsupials into extinction?