The mass and size of the dinosaurs and prehistoric insects continues to amaze me. I read from an earlier post that oxygen was more abundant during the prehistoric era. However supporting the weight for me is still a mystery. Is it possible that the air was denser, allowing slight decrease in weight from greater buoyancy?

  • $\begingroup$ How much denser should the air be? Unless it's not becoming a liquid, you can forget about this. $\endgroup$
    – Chris
    Feb 2, 2015 at 21:53
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    $\begingroup$ @Chris - actually this is a question pertaining to the flight of large reptiles as well, isn't it? So it's not far fetched. Not that I buy that the atmosphere was significantly different then from now. $\endgroup$ Feb 3, 2015 at 3:04
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    $\begingroup$ @anongoodnurse If you go up to 10.000m the air is getting considerably thinner. Yet still birds can fly over the Himalaya without much problem. To have a real impact, the atmosphere had to be much thicker. $\endgroup$
    – Chris
    Feb 3, 2015 at 7:27
  • $\begingroup$ @Chris - agreed. I read a couple of papers on this, and the thought was (not mine) that there was more CO2 then, maybe twice as much, which might be enough. In any case, I don't know about aerodynamics. I agree the atmosphere was not likely so very different once sources of CO2 were being sequestered and the oceans were sucking it up out of the atmosphere. $\endgroup$ Feb 3, 2015 at 7:30
  • $\begingroup$ At least for arthropods, the reason for those large species during the mesozoic was simply that their maximum body size scales with the partial pressure of oxygen in the atmosphere. This is due to their relatively simple respiratory system -- they have no lungs but tubular structures that reach into each segment for passive gas exchange. $\endgroup$
    – ChrKoenig
    Feb 3, 2015 at 8:33

1 Answer 1


While it seems difficult to me to imagine any mechanisms that would allow significant changes to the total amount of atmosphere (and therefore density) in geologically recent times (< 100M year), it's quite difficult to find direct evidence about it.

In 2012 Nature published a paper that calculated the air pressure (and therefore the air density) by noting the distribution of sizes of puddles formed by raindrops in 2.7 billion year old ash. The conclusion was that the air was no more than twice as great as it is today. This amount would be quite a bit for flying creatures, but would be almost completely insignificant for supporting the weight of large creatures.

As an example, a large elephant has a volume of slightly more than $10m^3$, and a mass of over $5000kg$. The buoyancy of the air only takes off the equivalent of about $12kg$ of that mass. Even an atmosphere 10 times the modern density would be little help in supporting its weight.


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