Eli5. – How were prehistoric species able to be so much larger than current species?

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I feel that there must be some physiological stressors or environmental factors that inhibit modern organisms of all domains from reaching the size, of say, a megaladon or brontosaurus.

In: 18

The blue whale is a modern organism and is much bigger than a megaladon or brontosaurus. As far as we know, it’s the largest animal to ever exist.

If I remember correctly from science class there was a lot more oxygen back then because plants had been around for millions of years pumping O2 into the atmosphere.

So nature needed a way to use up all that O2 and make CO2. Hence really big dinosaurs with really big lungs sucking up lots and lots of Oxygen to make carbon dioxide for the plants.

The bird-like features most likely allow dinosaurs to get as big as they were.

They laid eggs instead of giving birth, which allow them to give birth more quickly and their offsprings were able to feed on their own very quickly, allowing the mothers to feed and produce more children etc.

They respiratory system allowed them to get big. Mammals cant get this big, because their weight would actually crush them. Whales need buoyancy of the water, but the bird-like air sacks allowed them to grew larger and larger. That probably also had evolutionary advantages, becuase the type of food available for them (some theories how most dinosaurs had to eat foliage from plants and trees, meaning the larger they got the higher positioned foliage they could eat which allowed them to get bigger which required more food which allowed them….) being bigger was better.

Outside of animals, check out Pando. Its a colony of aspen that all grow from the same root system so it is actually one tree, although from above ground it looks like many trees. It weighs about 30x a blue whale

Dunno how that compares to extinct plants but it’s still pretty huge

https://en.m.wikipedia.org/wiki/Pando_(tree)

Climate stability is what allowed them to grow so large. There was a very stable climate for many millions of years, which allowed species to become hyper developed for those environments. When climate changes, the smaller species with a higher generation rate consistently fare better than larger species, often due to food scarcity and rate of change. The same thing happened in the past 15 thousand years – rapid climate change from an ice age caused most of the megafauna to go extinct.

The issue is that large species usually need an abundance of food, which requires a stable ecosystem. If 80% of the plants in an ecosystem die, all the larger herbivores and predators are going to die off, but the small herbivores will likely survive, as will their predators.

This sort of thing can be seen in real time by watching the arctic circle. When the flora freezes over and food becomes scarce, all the large fauna go away. The polar bears hibernate, the wolves and caribou move south, etc. But the foxes and the hares stick around – the hares can survive on the scarce flora, which supports a small population of foxes. Once it warms up and the flora comes back, so do the big fauna.