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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Small post apocalypse dinosaurs did fill niches. They are the birds ~~and reptiles~~ of today. They just didn’t return to being the dominant life form. (Edit: because reptiles had already split evolutionarily from dinosaurs. Reptiles, birds, crocodiles, and the extinct dinosaurs were together in a bigger group called the sauropsids)
After the Asteroid Hit, earth was changed Forever. Les Oxygen in the Atmosphere, an ice age caused by the Tons of dust and smoke in the Atmosphere. Most Things just died, like the surviving big dinosaurs That just couldnt Find enough to eat. And now 65 Million years later we have Humans…
>Why didn’t the small post apocalypse survivor dinosaurs just refill the niches
They did. That’s what modern birds are. Modern birds are the descendants of the dinosaurs that survived the extinction 66 million years ago. If you’re asking why didn’t those surviving dinosaurs evolve into the dinosaurs that had just gone extinct, that’s not how evolution works.
Because anything that wasn’t protected in some way burned. The impact shot particles into friggin space. Those particles, when they came back into the atmosphere, burned on reentry. There were so many that the atmosphere literally cooked just about anything exposed. It wasn’t just a big boom – it turned the whole planet into an oven.
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.
The world wide layer of the rare (on earth) metal Iridium that marks the KT boundary is accompanied by another layer world wide of fossilized fern spores. Ferns are excellent early colonizers of broken ground.
Entire forest ecosystems were globally replaced by barren naked soil. All of the large dinosaurs that were well adapted to such ecosystems died out with those ecosystems.
A similar question to above, is why are there no land critters bigger than the elephant today?
It seems to me that evolution has selected smaller animals after the apocalypse. We do have a number of largish mammals, and there are some large crocodiles/alligators, and certainly the large ocean mammals, sharks, and fish but nothing on the scale of a T-Rex or Sauropod.
When you need literally tons of plant matter to survive per week and basically everything green died off, you simply don’t have the time to “weather it out”. Same for big carnivores once the big hervivores died off.
Remember that big thing wasn’t the impact per se, was the amount of material that drastically reduced sunlight for years
Simply put, there were environmental changes, and some creatures were not-as-well-adapted as some others. Those that weren’t died off, those that were able to adapt thrived.
The thriving, adapted organisms out-competed the ill-adapted ones.
If we’re talking about biodiversity, the dinosaurs did recover. There are ~11,000 species of birds, and only ~5400 species for mammals.
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