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Huh! I was aware of an animal looking vaguely like this by the name of “quokka,” but I didn’t realize they were marsupial. What a weird biological niche.
I do not mean to be pedantic, but this is topic I love.
Marsupials do not fill a niche by virtue of their lack of placement. Instead, they have survived so long by virtue of their isolation.
It turns out that the adaptions required for marsupials to birth and raise young without a placenta make them inferior to placental mammals in almost every scenario. They get out competed and die off in almost every instance. South America had marsupials, not placentals, until it formed a land bridge with North America. What happened then? All the marsupials died off with the weird exception of the American possum. The placentals straight up out competed them across the board.
Australia has kept marsupials only because of its extreme isolation. When any type of placental mammal has been introduced to Australia, it has ruined the ecosystem and taken over the niche it fills.
Independent of humans, marsupials are a dying design. We just happen to live at a time when we can see that extinction in process. Yes, humans have sped it up by more rapidly introducing placental species, but we can see how it happened without human intervention as well.
Thanks for the detailed reply! So if I may make use of your knowledge to ask a follow up question:
What is it about American Possums that let them compete with the other placental mammals?
Oh, this is great. There may have been more results since I was working on a field project studying them, but to my knowledge we have absolutely no idea! They are not particularly well adapted to the cold, but their range keeps extending northward. This well predates the rapid climate change caused by humans, so we cannot use that as a reason. They are a bit of a mystery.
My guess would be that they are occupying a niche where limited brain and limb development (problems all marsupials face) are not limiting factors on success. Maybe their lack of a close genetic relation when surrounded by placental mammals gives them some pathogen resistance when scavenging? Those are just mildly educated guesses. When I was working with them we had no idea, and our field results were not at all enlightening.
They're too uglycute to die.
Various forms of the kangaroo build are spreading across the world, it's apparently a pretty efficient form factor. Wallabies I believe have spread to England, Vietnam?, and the US and are now invasive species in every continent.