I’ve always wondered why we never get anything as big as dinosaurs were, nowadays our biggest land animal is the african elephant with a average weight of 8500kg whilst back then we got creatures that were much bigger, what caused this
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Oxygen content in the air is lower now than it was at the time of dinosaurs. More oxygen means larger creatures are possible, since breathing can still feasibly support the whole body.
A few reasons.
First Dinosaurs had less size constraints compared to mammals. Their reptilian physiology favored a larger size. Hollow bones to lighten them while maintaining strength, avian respiratory system to maintain temperature, and egg-laying make it much easier to reach massive sizes.
Second, we would see more large animals if we weren’t at the tail end of a mass extinction event that hurt megafauna more than smaller animals. Humans are bio-engineers that can exploit nearly everything, large animals need plenty of resources and large, healthy environments to thrive. We are great about destroying those things.
Also, it has nothing to do with oxygen levels. Oxygen has no real effect on vertebrate size, only invertebrate size.
Mostly selection bias. Your comparing a group of animals that lived in different periods over a 200 million+ year times span to animals you can see outside today. Also only the biggest of those creatures are likely to leave behind easily discoverable fossils introducing another form of bias.
The other big issue is we did used to have bigger creatures than the Elephant but humans killed them all. If you’re an ancient human it made more sense to go for the big animals with tons of meat to feed your family vs say a crow.
I just want to point out that the blue whale is largest animal to ever live is still alive to today.
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