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)
In: 77
because the larger species couldn’t find enough food to survive whatever happened to the dinosaurs, but the birds being small needed less food at any given time, and therefore had an easier time surviving.
Another comment that hasn’t been mentioned is that the oxygen levels increased. During the Triassic, Jurassic, and Cretaceous, oxygen levels were much lower, like 10-15%. Dinosaur lungs, as well as crocodilians and some monitor lizards, were better adapted to low oxygen tensions. Birds have conserved this adaptation, which is why you can put a bird at the top of Everest, and they won’t have any problems, while most mammals will struggle and die.
The mammalian lung has slight advantages in settings of higher oxygen tensions, but absolutely sucks in low oxygen. Which is one reason why the largest mammals during the age of the dinosaurs were the size of a dog.
Mammals additionally were more likely to be tunnel dwellers, scavengers, and warm blooded, so they were better suited to survive the immediate years following the KT extinction. If there are a million mammals for every one dinosaur, guess which one has more chances to fill the empty niches?
So one way to look at it is that mammals struggled during the time of the dinosaurs. There was an immediate event that knocked out all the megafauna, and the surviving animals had a slight advantage in this new environment. They filled all the niches because they had a head start.
Additionally, birds still filled some of those niches. The South American terror bird was a large predator before it went extinct.
Hey! there are some pretty cool dino birds
Have you heard of a Cassowary? Emu? Ostrich?
Unfortunately, Humans hunted Moa to extinction 🙁
The Moa, the terror birds and the Elephant bird would like a word.
The reason birds were limited is because by the time they grew to large sizes again, mammals had already filled most of the “big animal” niche.
That’s also why the biggest birds were in isolated landmasses with no placental mammals (South America, New Zealand, and random small islands)
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.
All the big dinosaurs died in the extinction. In fact, _everything_ living on land that was more than a few kilograms died. Almost all the birds died too, it’s just that a handful made it through to repopulate. The mass extinction was very, very bad. The sky turned baking hot from reentering debris, everything caught on fire, then the sun was blotted out and temps dropped for years and surviving plants died, then the ecology was a mess for millennia after that. There just weren’t any survivors left to repopulate.
One leading theory is that reptiles are poorly adapted to dealing with fungal infections. Fungi don’t handle high temperatures very well: warm-bloodedness comes with many advantages but one important one could be the difficulty it causes for fungi to infect our bodies. As some evidence, scientists analyzed the energy cost to maintaining a higher body temperature and calculated the best cost-to-benefit temperature and found it to be… 98.6°F!
Not all dinosaurs were *cold* blooded. Some of them may have been mesothermic, and some may have actually been warm-blooded. However, in general they weren’t adapted to have the kind of high body temperature necessary to prevent fungal infections.
Post-asteroid was the perfect environment for fungi to thrive: it was dark, cool but not cold, and *covered* in dead, decaying organic material. That greatly increased the risk of fungal infections. The warm-blooded mammals had the advantage, being less likely to get infected.
Likely there were many factors, but that **may** have been a significant reason for why mammals were able to replace the dinosaurs.
Our world today oxygen is so low compared to their time.
They would have died from really short life span.
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.
they made a full recovery. just starting from q different point with different spaces available in the eco system.
Latest Answers