I’m working on a paper right now for my vertebrate paleontology class. The thesis of the paper is that climatic conditions in the Cretaceous, particular levels of O2 and CO2, allowed for terresterial vertebrates to exist which were up to 5x larger than anything which appeared after the KT extinction. The theory being that with higher levels of oxygen in the atomshpere, and CO2 which would allow for higher levels of dissolved oxygen in the blood, terresterial vertebrates would be able to grow larger.
There has been quite a bit of work done showing a link between invertebrate growth and oxygen levels, but not much with vertebrates.
At this point, I think I might just end up debunking the theory. The problem, as I’ve discovered, is that there is a limit to the amount of oxygen your blood can hold. (one corralary to this is that football players who wear an oxygen mask may not be getting any more oxygen than if they would from just breathing deeply) Currently, the atmosphere is about 20% oxygen, and it was about 30% oxygen when the big dinosaurs lived (saurapods).
We of course have no clue what the blood of dinosaurs was like, but if it was in any way like that of extant vertebrates, I’m not sure if they oxygen would have made that much of a difference.
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I thought (and this is just an “I heard once that” not something I actually learned in a class or anything) that the oxygen level could never get more than a few percentage points above where it is now because it would instantly cuase fires to burn much longer/harder/forcefully whatever. THat is to say, I thought there was a point far below 30% where oxygen would essentially cuase fires to last forever (or, more accurately, proceed until they’d fixed enough CO2 that atmospheric option dropped to the point that they could go out). Am I crazy?
So, not knowing what their blood was like seems like a pretty big hole in making any pronouncements, no? I know nothing about this, so this is not really a challenge, but that sounds like a big “if”.
You can make assumptions based on what living creatures who had direct ancestors who lived then. For example, crocodiles are a very old clade of animals, which existed during the Cretaceous. There were giant crocodiles. Given that crocodiles and other clades haven’t evolved that much, we can work under the assumption that their blood make up would be similar.
It is true that it might not be the case and that the level of hemegloben in their blood cells has dropped, but its the best guess we can make given what we know.
As it turns out, most animals (reptile or mamal) don’t have a big variance in hemegloben levels. Either that level was enough to transport the required oxygen necessary for behemouth sized creatures, or it wasn’t. That can be determined with physics.
When dealing with the soft parts of fossils (skin, blood, muscle) we can really only make educated guesses based on animals which are derived from what you want to study, or from the hard parts we have found. That’s all we got to go on. Prior to some impressions found in China, we assumed that dinosaurs had reptile like skin. We now think that at least some had feathers like birds.
Its actually a valid point.
The thing to remember is that the farther back you go, the fewer trees there are. You don’t have woody material, you have green plants like you find in tropics.
Most of the oxygen level experiments were done with wet paper, not with live plants.
There is still a fair amount of research on O2 levels I have to do, but I think its fairly certain that oxygen levels were well over 30% and might have been as high as 40% in the carboniferous period, which is what would have been needed to have dragonflies with three foot wing spans.