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The cretaceous atmosphere had a higher oxygen concentration (roughly 50% more oxygen than today). It was denser, but the idea that it provided structural suppor

The cretaceous atmosphere had a higher oxygen concentration (roughly 50% more oxygen than today). It was denser, but the idea that it provided structural support for large animals is ridiculous. Part of the reason animals could be larger then (from "giant" insects to genuinely-giant dinosaurs) is that the extra oxygen meant smaller lungs (or better absorption for non-lung-havers) required to support higher energy use. It isn't because the atmosphere literally stopped them from falling down. It's still a gas, and gravity hasn't changed.

londons_explore 5y ago
But at say 10 bar of pressure, flight would be much easier... Is there any evidence for or against the surface pressure of earth being higher in the past?
- lazide 5y ago
  Hah, while that could help a bird (if ACTUALLY 10 bar, which is insanely high - 145 psi, compared to 14ish now), it wouldn’t help as much as you would think. At STP, air is 1.3kg per cubic meter, and at 10 bar, it’s still only 11kg per cubic meter. Water is 1000kg/cubic meter. So yeah you could get some bigger birds, but t-Rex or brontosaurus weren’t meaningfully supported by the atmosphere in such a scenario even if they had some kind of lifting bladder, which no one has seen any fossil evidence of anyway. The amount of additional mass required to have a 100 bar atmosphere instead of 1 bar is rather incredible, and there is so far no plausible method known for such mass to disappear. If it was somehow subducted, then hey maybe. But we also don’t know of any way for that to occur right now.
  - 988747 5y ago
    >> The amount of additional mass required to have a 100 bar atmosphere instead of 1 bar is rather incredible, and there is so far no plausible method known for such mass to disappear. Earth's atmosphere is constantly being blown away by solar wind[1]. I guess after 65 millions years this would add up. But I'm still not convinced if it's possible to loose enough air this way to lower pressure 100 times. [1] https://www.sciencealert.com/earth-loses-hundreds-of-tons-atmosphere-to-space-every-day https://www.sciencealert.com/earth-loses-hundreds-of-tons-at...
    - lazide 5y ago
      If 'several hundred' is, say 500 tons (not sure which definition of ton exactly there, but we'll go with the biggest one of 2240 (or a metric long ton - 1016kg). That would put it at 508,000kg/day (or 5.08 x 10^5 kg) If there are 365 days in a year, and 65 million years, we're talking 23.7 billion days (give or take). So roughly, if the 'high end' estimate was always the case (who knows), and has been happening every day since 65 million years ago, we'll have lost 1.2 x 10^16 kg of atmosphere. Which is a lot of gas. But how much gas mass is in the atmosphere? I ran across this interesting article/paper [https://journals.ametsoc.org/view/journals/clim/18/6/jcli-3299.1.xml https://journals.ametsoc.org/view/journals/clim/18/6/jcli-32...], which tries to quantify the dry mass and variations caused by water vapor. They estimate the total current mass at approximately 5.1352 ± 0.0003 × 10^18 kg With yearly shifts in mass due to water vapor on the order of 1.2 to 1.5 × 10^15 kg. As large as the amount of gas that has boiled off (making those assumptions), and if my math is correct, the total amount of gas is 2 orders of magnitude larger. Doing some random scribbling, I THINK it would add up to .2% of the total CURRENT amount of dry air in the atmosphere. So not enough to make any noticeable difference.
- caymanjim 5y ago
  A thicker atmosphere would provide more static lift, but flapping wings in thicker atmosphere would require larger muscles and use more energy. There are scholarly writings out there on this topic, but I'm not familiar enough with any of them to have an opinion. I know there's a school that argues pterosaurs couldn't fly today solely due to lower atmospheric density, and a school that argues they could. I think the lower oxygen levels would still be the primary barrier due to energy requirements.
Flatcircle 5y ago
Pressurized gas can act like a liquid. And at sea level, thick atmosphere would certainly be pressurized. smaller lungs aren't relevant to the issue that comes up regarding extremely large dinosaurs. Because it's not an issue of lung capacity, or oxygen supply per liter of blood. We're not talking about animals that are comparable to modern day elephants. Have you seen the size comparisons of dreadnoughtus to blue whales? They are enormous. The physics start to come into question. Just the sheer weight of skin and muscle hanging on bone. Even if the bones are hallow and even if the muscle is somehow more efficient due to increased oxygen in the blood. Like a whale plopped on dry land, they'd collapse under their own weight standing up. Much less, hunting, mating, and fighting. furthermore the thick atmosphere theory helps solve another tricky mystery which is the, Mesozoic Paleoclimate Paradox During the Mesozoic era a remarkably homogeneous flora of tropical and temperate plant species covered the Earth. Plants such as ferns, laurels, palm trees, and Magnolia that could not withstand freezing, thrived at 70 degrees north and south latitude. Many of the same plants that existed near the equator were also thriving at the Polar Regions of the Earth. For decades paleoclimatologists have tried to explain the Mesozoic paleoclimate paradox. If they matched the vegetation of the lower and middle latitudes then their climate models were too cold at the higher latitudes. If they matched the warm temperatures of the Polar Regions then their models projected an unrealistic hot sauna for the rest of the planet that again conflicts with the geological evidence. The reason their climate software models fail to match the Mesozoic climate is because perhaps the paleoclimatologists make the incorrect assumption that the Mesozoic atmosphere was the same thickness as the present. As stated earlier, the variation of temperatures around a planet whether it is day or night or according to latitude, is a function of the thickness of a planet’s atmosphere. Planets or moons with no atmosphere will have the most extreme difference in temperatures, planets such as the present day Earth and Mars that have relatively thin atmosphere will still have these differenced in temperatures but much less extreme, while extreme thick atmosphere planets such as Venus and the Mesozoic Earth show almost no temperature difference at all according to latitude.
- throwawaylinux 5y ago
  > Like a whale plopped on dry land, they'd collapse under their own weight standing up. Much less, hunting, mating, and fighting. Just to be clear, you're talking about the buoyancy effect that a denser atmosphere provides to land animals?

londons_explore 5y ago

But at say 10 bar of pressure, flight would be much easier... Is there any evidence for or against the surface pressure of earth being higher in the past?

lazide 5y ago
Hah, while that could help a bird (if ACTUALLY 10 bar, which is insanely high - 145 psi, compared to 14ish now), it wouldn’t help as much as you would think. At STP, air is 1.3kg per cubic meter, and at 10 bar, it’s still only 11kg per cubic meter. Water is 1000kg/cubic meter. So yeah you could get some bigger birds, but t-Rex or brontosaurus weren’t meaningfully supported by the atmosphere in such a scenario even if they had some kind of lifting bladder, which no one has seen any fossil evidence of anyway. The amount of additional mass required to have a 100 bar atmosphere instead of 1 bar is rather incredible, and there is so far no plausible method known for such mass to disappear. If it was somehow subducted, then hey maybe. But we also don’t know of any way for that to occur right now.
- 988747 5y ago
  >> The amount of additional mass required to have a 100 bar atmosphere instead of 1 bar is rather incredible, and there is so far no plausible method known for such mass to disappear. Earth's atmosphere is constantly being blown away by solar wind[1]. I guess after 65 millions years this would add up. But I'm still not convinced if it's possible to loose enough air this way to lower pressure 100 times. [1] https://www.sciencealert.com/earth-loses-hundreds-of-tons-atmosphere-to-space-every-day https://www.sciencealert.com/earth-loses-hundreds-of-tons-at...
  - lazide 5y ago
    If 'several hundred' is, say 500 tons (not sure which definition of ton exactly there, but we'll go with the biggest one of 2240 (or a metric long ton - 1016kg). That would put it at 508,000kg/day (or 5.08 x 10^5 kg) If there are 365 days in a year, and 65 million years, we're talking 23.7 billion days (give or take). So roughly, if the 'high end' estimate was always the case (who knows), and has been happening every day since 65 million years ago, we'll have lost 1.2 x 10^16 kg of atmosphere. Which is a lot of gas. But how much gas mass is in the atmosphere? I ran across this interesting article/paper [https://journals.ametsoc.org/view/journals/clim/18/6/jcli-3299.1.xml https://journals.ametsoc.org/view/journals/clim/18/6/jcli-32...], which tries to quantify the dry mass and variations caused by water vapor. They estimate the total current mass at approximately 5.1352 ± 0.0003 × 10^18 kg With yearly shifts in mass due to water vapor on the order of 1.2 to 1.5 × 10^15 kg. As large as the amount of gas that has boiled off (making those assumptions), and if my math is correct, the total amount of gas is 2 orders of magnitude larger. Doing some random scribbling, I THINK it would add up to .2% of the total CURRENT amount of dry air in the atmosphere. So not enough to make any noticeable difference.
caymanjim 5y ago
A thicker atmosphere would provide more static lift, but flapping wings in thicker atmosphere would require larger muscles and use more energy. There are scholarly writings out there on this topic, but I'm not familiar enough with any of them to have an opinion. I know there's a school that argues pterosaurs couldn't fly today solely due to lower atmospheric density, and a school that argues they could. I think the lower oxygen levels would still be the primary barrier due to energy requirements.

Flatcircle 5y ago

Pressurized gas can act like a liquid. And at sea level, thick atmosphere would certainly be pressurized. smaller lungs aren't relevant to the issue that comes up regarding extremely large dinosaurs. Because it's not an issue of lung capacity, or oxygen supply per liter of blood. We're not talking about animals that are comparable to modern day elephants. Have you seen the size comparisons of dreadnoughtus to blue whales? They are enormous. The physics start to come into question. Just the sheer weight of skin and muscle hanging on bone. Even if the bones are hallow and even if the muscle is somehow more efficient due to increased oxygen in the blood. Like a whale plopped on dry land, they'd collapse under their own weight standing up. Much less, hunting, mating, and fighting. furthermore the thick atmosphere theory helps solve another tricky mystery which is the, Mesozoic Paleoclimate Paradox During the Mesozoic era a remarkably homogeneous flora of tropical and temperate plant species covered the Earth. Plants such as ferns, laurels, palm trees, and Magnolia that could not withstand freezing, thrived at 70 degrees north and south latitude. Many of the same plants that existed near the equator were also thriving at the Polar Regions of the Earth. For decades paleoclimatologists have tried to explain the Mesozoic paleoclimate paradox. If they matched the vegetation of the lower and middle latitudes then their climate models were too cold at the higher latitudes. If they matched the warm temperatures of the Polar Regions then their models projected an unrealistic hot sauna for the rest of the planet that again conflicts with the geological evidence. The reason their climate software models fail to match the Mesozoic climate is because perhaps the paleoclimatologists make the incorrect assumption that the Mesozoic atmosphere was the same thickness as the present. As stated earlier, the variation of temperatures around a planet whether it is day or night or according to latitude, is a function of the thickness of a planet’s atmosphere. Planets or moons with no atmosphere will have the most extreme difference in temperatures, planets such as the present day Earth and Mars that have relatively thin atmosphere will still have these differenced in temperatures but much less extreme, while extreme thick atmosphere planets such as Venus and the Mesozoic Earth show almost no temperature difference at all according to latitude.

throwawaylinux 5y ago
> Like a whale plopped on dry land, they'd collapse under their own weight standing up. Much less, hunting, mating, and fighting. Just to be clear, you're talking about the buoyancy effect that a denser atmosphere provides to land animals?