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I'm world building a planet dominated by reptiles (they are around the size of dogs) for a sci-fi story of mine. I figure that in order to have larger reptiles, I'll need the planet to have a higher oxygen content. However, humans also live in this world, and I was wondering if this is still physiologically plausible? I've read a bit on oxygen toxicity, but I'm not sure about the exact oxygen amount I should have, or get away with.

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closed as unclear what you're asking by Mołot, Ash, L.Dutch, Green, Culyx May 1 '18 at 20:28

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    $\begingroup$ Don't we already live on a world with humans and dog size reptiles? A 2200lb (salt-water crocodile could eat a pack of dogs as a light lunch. $\endgroup$ – Henry Taylor May 1 '18 at 17:35
  • $\begingroup$ Higher oxygen partial pressure wouldn't help reptiles much. What mammals have and reptiles don't is homethermy, that is, the ability to keep the temperature inside their bodies constant. Reptiles are poikilothermic animals, a.k.a. cold blooded, which means that they don't have the ability to regulate their internal temperature. In cold weather, they are sluggish or cannot move at all. If it's too hot they overheat and die unless they can find shadow. If they do a lot of effort they overheat and must stop or die, so their endurance is much lower than the endurance of mammals. $\endgroup$ – AlexP May 1 '18 at 17:52
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    $\begingroup$ I don't see what's your actual problem, given that you basically described our own planet. $\endgroup$ – Mołot May 1 '18 at 18:02
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    $\begingroup$ Insects, not reptiles, are the ones that (maybe) grow huge in high oxygen environments. $\endgroup$ – RonJohn May 1 '18 at 18:34
  • $\begingroup$ I think you'll find that other environmental factors dominate mere atmospheric oxygen content. I agree with RonJohn though. High oxygen content will benefit insects far more than reptiles. $\endgroup$ – Green May 1 '18 at 20:21
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Yes totally possible. Like 100%. For instance alligators and crocodiles are both wayyy bigger than dogs and breathe our atmosphere and are fine.

Besides more oxygen isn't really a requirement for reptiles since they do have lungs and everything its much more a requirement for insects arachnids excetra who have to rely on diffusion through spiricals. The main limiting factors for reptiles is heat since they can't control it themselves.

Alternativly if you do have additional oxygen your humans would have just evolved to be more resistant to oxygen poisoning or have a very blueberry heavy diet. Its worth noting that humans can live in partial pressures of oxygen half that at sea-level I imagine that it probably has a similar tolerance the other way too.

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    $\begingroup$ Weirdly healthy humans can apparently breath pure oxygen, at sea-level or lower pressures, for extended periods without ill-effect it's only when the partial pressure goes up over 1.6 Bar that it starts to cause issues. $\endgroup$ – Ash May 1 '18 at 18:18
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    $\begingroup$ In other words, the humans would eat a bunch of antioxidant-heavy foods to get rid of those oxygen radicals. $\endgroup$ – ani ben May 1 '18 at 19:04
  • $\begingroup$ Yup, though I don't think it'd be all that necessary tbh $\endgroup$ – Ummdustry May 1 '18 at 20:04
  • $\begingroup$ I thought the scientific verdict was till out on dietary antioxidants? $\endgroup$ – Pink Sweetener May 1 '18 at 20:10
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Higher levels of atmospheric oxygen may actually impend the success of reptiles, if it's at the expense of greenhouse gases, remember reptiles by definition cannot regulate their body temperature internally. A world with very little Carbon Dioxide, or Methane, in the atmosphere will be much cooler, given the same insolation, than one with a good thick atmosphere high in Carbon Dioxide. So you really don't need more oxygen to have big reptiles, depending on the habitat range, in terms of latitude, that you want reptiles to be prevalent in you might actually want to reduce the relative level of oxygen, particularly as Ozone in your planetary atmosphere.

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  • $\begingroup$ But during the eras when dinosaurs and large insects roamed the earth, it was both more tropical in climate AND the oxygen content was higher, right? Just wondering. $\endgroup$ – ani ben May 1 '18 at 19:13
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    $\begingroup$ @aniben Nope, those were two different Periods (actually the age of the dinosaurs spanned several periods) during different Eras Dinosaurs were around for the whole Mesozoic while giant insects and arachnids are only seen in the fossils for the Carboniferous about 100 million years earlier. $\endgroup$ – Ash May 1 '18 at 19:46
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    $\begingroup$ This is the most important point. Dinosaurs were (very likely) driven to extinction by an asteroid, but neither that nor climate is the reason why reptiles no longer evolve to become huge. As I understand it, reptiles stopped getting big because mammals started filling all of the big animal niches. $\endgroup$ – Pink Sweetener May 1 '18 at 20:13
  • $\begingroup$ @PinkSweetener Dinosaurs probably weren't reptiles, or even ectotherms so reptiles in the Mesozoic and the modern Quaternary were/are reasonably similar in size, although bigger prey meant some groups could be, and were, much larger, like crocodiles. $\endgroup$ – Ash May 5 '18 at 4:24
  • $\begingroup$ @Ash I was under the impression that there was still a lively debate in the scientific community regarding whether or not dinosaurs are endo/ectotherms. My general point was just that very large scaley creatures have come and gone throughout the epochs, but the reason they have not returned during our time probably has more to do with the arrival of mammals rather than atmospheric changes. Crocodiles, alligators, and monitors are all quite big, but they all occupy roughly the same niche - supporting the notion that the loss of large scaled animals (reptile or dinosaur) is niche-related. $\endgroup$ – Pink Sweetener May 5 '18 at 14:57

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