Dinosaurs seemed to have generally been so much larger than animals today. Huge dragonflies that dwarf their modern counterparts, turtles 10ft long. What is the mechanism that allowed them to be so large, or conversely makes modern ones smaller? Is it about Oxygen levels, or efficiency, or something else?
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Well, to ELI5 this, as you can see from the discussion, we aren’t really sure why. Humongous insects apparently required higher oxygen levels to exist, but not all gigantic land life happened at the same time as giant insects, so that cannot be the only reason.
Some have argued that the one global continent played a role by allowing ready contact for all and thus favoring size as a defensive response (leading to increase in size of predators in a runaway arms race). Some argue that lush conditions (location of land in equatorial regions, difference CO2 levels, and so forth) led to vegetation running wild and allowing large herbivores (and thus large predators) to exist. Climate would be a key factor here. However, even polar regions at the time had some pretty large animals so not clear this idea works as a cause either.
It might even be something as relatively simple as random chance. Once things started to grow big (for no real reason at all), the process snowballed and loads of things became big. Then the asteroid/comet thing hit and we went down a different path that did not result in huge being favored.
What do we actually, truly know? Big animals need more food than small ones. Thus, to have big numbers of large animals requires lots of vegetation for a given range. This suggests that plant availability and size is a major factor. Why plants would be huge, fast growing, and in massive amounts so large animals could survive in large numbers? Climate seems a good candidate to me.
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