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Say the word "Australia", and one of the first things to come to your mind would be the marsupials.

enter image description here

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:

enter image description here

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?

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  • $\begingroup$ All marsupials? Don't oppossums live up north to the shores of the Great Lakes? $\endgroup$ – AlexP Apr 12 '20 at 22:04
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    $\begingroup$ Just FYI we are currently still in an ice age. We are just in an inter-glacial period. $\endgroup$ – John Apr 12 '20 at 22:05
  • $\begingroup$ @John Not relevant to the question here. $\endgroup$ – JohnWDailey Apr 12 '20 at 22:48
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    $\begingroup$ @AlexP Opossums only spread into North America about 10 to 20 thousand years ago. Before that they were restricted to Central and South America. It's not clear when opossums got to Central America but there are no opossum fossils north of the Rio Grande that certainly predate the late Pleistocene, and it's thought their expansion as far north as the Great Lakes region is due to piggybacking off of Native American and later European inhabitation of the area providing ample food and shelter. $\endgroup$ – user2352714 Apr 13 '20 at 1:44
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No. Equatorial marsupials in Central America and New Guinea would survive.

From the OP.

/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./

1: I would not expect an ice age to have much impact on equatorial regions.

  1. I assume the species going extinct would be in temperate colder regions.

  2. There is a lot of species diversity in the tropics. More there than points north or south. https://en.wikipedia.org/wiki/Latitudinal_gradients_in_species_diversity

    For half of all plant and animals species to go extinct, but spare those (or most of those) in the tropics I would expect that all the plants and animals in temperate regions will go extinct. I am concerned that some in the tropics might need to go extinct also to add up to half but I don't know what the mechanism for that would be.

  3. The center of Australia in JohnWDailey earth is roughly the latitude that Tasmania is now. The entirety of Australia is thus fair and square temperate. So all of the Australian and New Zealand marsupials go extinct. The Wollemi pines too, if you were wondering.

  4. Equatorial species will be spared. That means the various opossums in Central America. Also spared will be the marsupials of New Guinea and other points east of Wallace's line. New Guinea is close to the equator and has many marsupial species and there are some on other islands in the region.

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It depends on how bad the glaciation is, but probably not

Marsupials do really bad in the cold. It's thought to be the reason why most of the South American groups died out and studies of marsupial diversity have found that marsupial diversification tracks global temperatures (being positively correlated with warmer climates). However 14 million years ago is the same age as the famous fossil site of La Venta in Colombia, which has one of the most diverse marsupial faunas in South America, including opossums, paucituberculatans, sparassodonts, and microbiotheres. The only way to wipe out all of the marsupials would be if glaciation advanced far enough to disturb habitats in the tropics, and that would be getting dangerously close to a Snowball Earth scenario where pretty much everything would die.

Even if glaciation proceeded to the point that it severely disrupted the environments it wouldn't get anything, opossums survived and flourished even as sparassodonts, paucituberculatans, and microbiotheres declined in our timeline. In a mass extinction like you propose small generalized species tend to be the ones to survive, and marsupials have produced a lot of small, generalized species.

Another thing to consider is that Australia being further south is likely to be beneficial for marsupials in the long run. The broader reason for Australia's generally species-poor ecosystems relative to other continents is due to a general drying out of the continent, which is due to a more northerly position of Australia in the horse latitudes. Keep Australia further south and it ends up with a higher diversity of marsupials, and hence more species that could potentially survive any extinction. The only way to get every single marsupial at once would be glaciation at least to the top of Australia (and that's not counting New Guinea, which would just repopulate Australia after the glaciers had gone), a degree of glaciation not only more severe than any seen in the Cenozoic but that also gets very close to the kind of tipping point that produced the runaway icehouse effect of the Cryogenian Snowball Earth.

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  • $\begingroup$ I asked this question because I've been struggling with creating a marsupial family tree without any of the familiar kangaroos, koalas, wombats, possums or opossums, to name a few, because those guys evolved on the Oligocene/Miocene line, long after my first POD. $\endgroup$ – JohnWDailey Apr 13 '20 at 2:01
  • $\begingroup$ Vombatidae, Phascolarctidae, Macropodidae, Potoroidae, Hypsiprymnodontidae, and numerous possum families are first known from the late Oligocene. Genetic data has some idea as to how old they might be. There is exactly one Australian Cenozoic site that predates the late Oligocene (Tingamarra) so beyond that no one has any idea what was happening on the continent between 28 and 66 million years ago. A prolonged PETM would probably help marsupial diversity, as maximum marsupial diversity in our timeline is correlated with the PETM. $\endgroup$ – user2352714 Apr 13 '20 at 8:54

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