Why didn’t the small post apocalypse survivor dinosaurs just refill the niches? Did the asteroid alter the planet in some way fundamentally different than the prior 200M years?
Edit: (collaborative clarification with my 5yo, with paraphrasing): Birds are boring dinosaurs. Why didn’t the big dinosaurs (non bird) come back? Why are there no mammal-sized (non bird) dinosaurs? (e.g., mouse through elephant-sized or even whale size)
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Another comment that hasn’t been mentioned is that the oxygen levels increased. During the Triassic, Jurassic, and Cretaceous, oxygen levels were much lower, like 10-15%. Dinosaur lungs, as well as crocodilians and some monitor lizards, were better adapted to low oxygen tensions. Birds have conserved this adaptation, which is why you can put a bird at the top of Everest, and they won’t have any problems, while most mammals will struggle and die.
The mammalian lung has slight advantages in settings of higher oxygen tensions, but absolutely sucks in low oxygen. Which is one reason why the largest mammals during the age of the dinosaurs were the size of a dog.
Mammals additionally were more likely to be tunnel dwellers, scavengers, and warm blooded, so they were better suited to survive the immediate years following the KT extinction. If there are a million mammals for every one dinosaur, guess which one has more chances to fill the empty niches?
So one way to look at it is that mammals struggled during the time of the dinosaurs. There was an immediate event that knocked out all the megafauna, and the surviving animals had a slight advantage in this new environment. They filled all the niches because they had a head start.
Additionally, birds still filled some of those niches. The South American terror bird was a large predator before it went extinct.
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