Did ancient life forms during periods of high atmospheric oxygen have larger individual cells?

Submitted by eyaf20 t3_zkofje in askscience

Dinosaurs, enormous birds, giant sloths — there used to be an abundance of huge organisms compared to the size range covered across species today. I've often heard the ability for organisms to grow to such sizes is correlated with higher concentration of oxygen in the atmosphere, is this accurate? And beyond that, for organisms that grew to be so massive, did they simply contain a larger number of cells not much different from the animal and plant cells we see today (granted there's variation within those categories), or were individual cells also larger themselves? Perhaps the surface area to volume ratio could have differed based on oxygen availability and thus enhanced cell transport?

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horsetuna t1_j054qat wrote

High oxygen content is not directly correlated to animal size, although it does seem to cause gigantism in certain kinds of insects.

A research paper exposed and grew various species of insects in oxygen levels comparable to the Carboniferous. Some of the species got larger, but some of them did not.

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horsetuna t1_j05564m wrote

The sauropod dinosaurs were able to grow to the largest land animals ever because they found a way to make their skeletons much lighter, using the same thing that birds do. Air sacs!

The air sacs have been shown to invade the bones, making them more lightweight. The aie circulatory system allowed them to shed heat more quickly which is a large problem for large animals. As well as take in more oxygen

(Disclaimer; since we don't have any lungs or actual air sacs from sauropods, we look at the shape of the bones to infer they were there by comparing them with modern reptile and bird bones for similar marks)

If I remember right, the giant ground sloths lived in an environment without too much more oxygen than we do today. Oxygen alone is not the only control factor for big size. The ground slots probably got big to help avoid predators, not unlike modern elephants.

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atomfullerene t1_j07nflq wrote

Replying to horsetuna (#925,268)

Quite right. And ground sloths weren't really unusually large...they filled the same sort of large-mammal ecosystem niches that rhinos and elephants fill today. Really they were just standard-sized big mammals, the unusual part isn't that land mammals were large in the past, it's that large land mammals are so scarce in the modern world.

It's really only the giant insects that grew large due to high oxygen (and lack of competition from flying vertebrates)

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