the atmosphere 66 million years ago wasn’t that different to the atmosphere today, the reason why pterosaurs like quetzalcoatlus was able to grow to such massive sizes was because they had extremely light skeletons. higher oxygen levels did allow arthropods to grow to giant sizes during the carboniferous, but it would have little effect on how large vertebrates could grow.
the atmosphere 66 million years ago wasn’t that different to the atmosphere today, the reason why pterosaurs like quetzalcoatlus was able to grow to such massive sizes was because they had extremely light skeletons. higher oxygen levels did allow arthropods to grow to giant sizes during the carboniferous, but it would have little effect on how large vertebrates could grow.