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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The extinction of the larger dinosaurs (I know, I know, ‘dinosaur’ is an ambiguous term in this case) coincided with the death of the majority of plant life on the planet.
Even way back then, bigger bodies burned energy very quickly. With the loss of plant life, larger life-forms such as dinosaurs couldn’t survive — they starved or froze in the impact winter that was caused by the meteor.
Smaller creatures that don’t require a lot of food and/or energy survived (relatively) unscathed, but even then it took ~300,000 years for marine ecosystems to fully recover on a global scale.
In the meantime, mammalian and avian species (birds) began to appear and occupy the evolutionary niches left by the lost dinosaurs, because they were generally more mobile (which meant that they weren’t constantly stripping away all the new plant growth in a particular area), and their smaller size and low metabolism meant that they didn’t need to eat as often.
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