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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Because being big is not a really good trait for evolution to select. It has clear advantages, but requires a lot of energy to maintain. So in a post apocalyptic scenario, it would be extremly hard to thrive and reproduce while being big.
I suspect that part of reason why so many species die out was that the world had been a very stable place for a very long time before that impact.
Evolution and natural selection tend to make a species more attuned to its environment, better adapted and more efficient than the species around it competing for the same resources. As a species gets more and more attuned to that specific environment, it becomes less and less able to accommodate changes. Because things had been so stable (unchanging) for so long, species were VERY reliant on that stability. When something interrupted it, the most reliant died off.
Wow. I’ve read most of these answers and almost no one has read the question.
What you’re asking is why didn’t small bird sized dinosaurs that weren’t birds survive. Small dinosaurs that weren’t birds did exist. Some even burrowed because we have fossils in burrows.
One answer provided was that maybe birds could fly to unaffected areas. In that case, why did mammals survive but flying reptiles like pterosaurs didn’t?
Part of the answer is probably luck or what we call stochasticity in biology. Once populations are small an entire species can go extinct through ‘drift’. Bad luck in terms of negative genes in the pool, sex ratio imbalances, disease and whatever else. Some lineages probably didn’t have a ‘better’ reason to go extinct than others… they just happened to be knocked off by some random population effect.
If you’re looking for a more deterministic explanation, the most interesting I’ve heard is the possibility that non-avian dinosaurs may have lacked the ability to torpor. Torpor is a physiological state much like hibernation but occurs in response to events in the environment rather than for an extended period over winter. A huge number of small birds and mammals can torpor. Most torpor overnight in response to cold, but torpor in response to food shortages or after bush fires in Australia is known. Basically: there’s no food, everything is ash, so I’m going to hibernate in a hole or tree hollow for awhile.
Using the size of arterial holes entering bone you can ballpark non-avian dinosaur metabolism. It’s a bit of a guess but non-avian metabolisms appear to have been weirdly higher than you’d expect. Ok. But if you’re running a really high metabolism, that might limit you’re ability to torpor and survive a bad period after a catastrophe. It’s even possible that the seeming prevalence of torpor in modern small endotherms is no coincidence. Maybe everything today is descended from animals that were able to torpor.
But, this is still just a guess. Really interesting guess, but a guess.
Typed on phone so probably full of typos. Hope that explains things a bit for you and your 5 yo.
Edit. Typos, of course.
Even the “cool” dinosaurs may have been more bird-like than you think. There is a theory about the T-Rex, that all these years we’ve been assembling the arm bones backwards… meaning they’re not arms, but wings. Certainly not enough to fly or anything, but with large feathers could have been used for balance and other uses.
Picture a T-Rex, but covered with feathers. Basically a gigantic chicken.
I can’t find it, but I saw a theory that post Yucatan asteroid and resulting volcanic activity removed a significant amount of oxygen from the atmosphere – from around 30% to current 21% which made it hard for any large animals like the dinosaurs to take in enough in respiration to power their bodies.
Also, resulting dust in the atmosphere from volcanic activity and global cooling because of dust meant the evolutionary niche they filled non viable for life of their type.
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