I’ve heard that one reason dinosaurs could be of such a large size is that there used to be more oxygen in the atmosphere. I wonder if it is known whether there was also higher surface air pressure.
One of the aspects of climbing Mt.Everest is that it requires oxygen tanks. Then, once at the top, climbers look up and wonder how flocks of birds can fly past at that altitude.
How do birds do that, as flying requires so much energy and thus oxygen?
It is because they are descended from dinosaurs, an animal that evolved at the end of the Permian extinction when there had been an oxygen collapse.
Birds, and dinosaurs, have a physiology that traps air inside their bodies.