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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Your question is basically “why didn’t convergent evolution bring the giant dinosaurs back from birds?” Short answer: because the same vegetation never came back.
Trees with flowers took over, the whole food network changed and growing to the same size didn’t make any sense or just wasn’t possible anymore. Birds did reached 3 m tall (10 ft) eventually, they were called Dinornis, and some carnivorous birds did grew up to 2 m, but they couldn’t compete with felines or other mammals.
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