Feels like there was a lot more megafauna in prehistoric earth than there are today
In: 20
There is a multitude of factors, some of these include:
1. Higher oxygen levels in certain times in earths history, i.e. the Cretaceous and Carboniferous, in particular arthropods have open circulatory systems so can get bigger because oxygen can travel to this system more effectively.
2. Warmer climates, reptiles require warm climates because they are cold blooded and rely on external heat for their internal temperature to increase. This is why we had large snakes like Titanoboa(in the Palaeocene) 60 mya for example.
3. There was also an abundance of CO2, this in turn could have assisted in plants being more abundant and the herbivores would then evolve larger bodies and in turn the carnivores would of evolved larger bodies to hunt these large herbivores.
4. For mammals a cooler climate assisted in the evolution of larger animals to remain warm and store enough food in these climates. It is known that colder climates select for larger bodies in mammals(think the polar bear). Also land area was another important factor.
5. Most of the land animals went extinct in the megafaunal extinction event, due to climate change/possible human activities. Africa is said to be the only place left with its megafaunal population.
6. In regards to Cenozoic sea animals, the predators like Megalodon went extinct due to habitat change, the disappearance of their prey and other predators hunting the new prey like the ancestors of great white sharks. Though Baleen whales are doing okay in the size department, we live with the largest of all animals today the gigantic blue whale!
OP, I saw a very nice article that explained what the specific advantage of dinosaurs was for bigger size:
1 – their breathing system was bird-like. More efficient.
2 – being oviparous means the problems of giving birth – which become much worse as you increase in size – are sort of sidestepped.
Made good sense.
Where are these large fauna supposed to live? Anywhere habitable has been overrun with us resilient two-legged beasties. Also, small bones are way harder to find.
ELI5 answer: we killed them so that more people can live on the resources they used to occupy.
Mass extinction around 11,000 years ago wiped out most of the big animals in Europe and North America. Africa wasn’t as affected, that’s why the “big” animals we have left are all found in Africa.
The ocean still has large animals, so I assume that your question refers to land animals. Two extinction events to consider are the one that wiped out large dinosaurs and the modern man-made extinction event. There is a theory that burrowing animals were able to hide from the effects of the asteroid impact while large animals had no place to hide. Small animals can hide from humans. We developed the ability to kill large animals.
Dinosaurs were huge.
Nothing before or after them were that large.
Some of the mechanism of why are understood, but ultimately we don’t really know.
Mammals were a lot bigger, but the world has been getting warmer which makes those sizes impractical for mammals (they get too hot).
Before the dinosaurs, animals were roughly equivalent to post-dinosaur sizes. Probably on the smaller size compared to mammals before humans came along.
Also, it should be noted that modern whales are massive.
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