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?
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.