I was introduced yesterday to the FIMS hypothesis by PBS Eons.
The Fungal-Infection-Mammalian-Selection (hey that ryhmes!) hypothesis asks the question of why reptiles didn't bounce back as much as mammals did after the asteroid K/Pg extinction event.
After all, they need less energy than mammals as cold-blooded creatures, and they produce way way more offspring than mammals.
One theory is fungi: there was an explosion in fungal activity after the asteroid due to the now dark and dingy hellhole the Earth became, and a ton of fungal spores were floating around at the time, as seen in geological record.
Apparently fungal infections are not that deadly to mammals (it just irritates us), but were disastrous for reptiles. Plus us mammals had a new food source in the absence of plants and meat.
There's no conclusive proof, still, it's an interesting theory as to why the dinosaurs didn't bounce back and why us mammals took over.
