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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Now OP if you are asking why didn’t any non-avian dinosaurs survive the K-Pg extinction, as far as I know the answer is we don’t know. It could be that avian dinosaurs were able to fly to islands that weren’t as badly affected and develop niches there pre-event. If I remember correctly New Zealand was relatively unscathed. Also I think all modern day birds are from something like only 5 lineages coming out of the K-Pg event. That’s just off the top of my head so I could be wrong. Most terrestrial animals that survived could likely burrow and had food caches. It’s possible no small non-avian dinosaurs filled this niche and so couldn’t survive the dust atmosphere winter. But as far as I know this is still somewhat of a mystery.
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