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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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.
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.