>>5063773
I live in the marsupial country "Australia" and Marsupials are extremely stupid in general.
You are definitely onto something.
Synapsids were definitely extremely stupid in comparison to modern mammals.
>Also, what are the chances of keratin spines having evolved on mammals during the Mesozoic?

That's an easy question to answer -

"The mammal in question is a new species, named Spinolestes xenarthosus. Like many mammals in the Mesozoic, it was a small creature weighing only around two to three ounces, growing up to around nine inches in length. It’s assumed that it would have been mostly nocturnal, as mammals generally needed to stay out of the way of the larger dinosaurs who dominated the ecosystem. S. xenarthosus‘ feet indicated that it was a burrower more than a climber, and its reinforced spine probably helped it pry and shred rotting plants to find insects to eat, like a modern armadillo or shrew. Rounding out its outward appearance were some well preserved hairs, some growing into thicker spines like a hedgehog. These delicate structures also showed signs of a fungal infection, rotting some hairs away in a discolored patch of skin."
https://2newthings.com/fossilized-fur-spines-and-organs-found-in-cretaceous-mammal/

https://en.wikipedia.org/wiki/Spinolestes