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During much of the Cretaceous period, oxygen levels were substantially higher than they are currently, with estimates indicating that around 30% of the atmosphere consisted of oxygen. Obviously, this indicates that species that evolved during this time period would be likely to require these levels of oxygen to support their metabolisms. This raises the question of how these species would fare if suddenly transported into the modern world. Obviously, there would be some difficulty, but how severe would it be? Humans are capable of acclimating to lower oxygen levels while climbing mountains, would dinosaurs have to have some kind of similar acclimatization process (possibly in an artificially oxygen-enriched transition environment where the oxygen content is decreased more gradually)?

Essentially, how much trouble would Cretaceous-period dinosaurs such as Triceratops or Tyrannosaurus Rex have in adapting to a modern atmosphere?

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    $\begingroup$ At some times during the Cretaceous oxygen levels were higher than today. At other times, not so much. And we know from practical reality that extant dinosaurs have highly advanced respiratory systems which enable them to maintain very high metabolic rates needed to allow them to fly. $\endgroup$ – AlexP Nov 19 at 12:58
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    $\begingroup$ Thanks for specifying the right time period. $\endgroup$ – Legorooj Nov 19 at 21:59
  • $\begingroup$ A creature designed for sea level O2 levels of 20% (like humans) can survive a vacation to 6000m and 10% O2 levels. It just takes time to adjust: hypoxico.com/altitude-to-oxygen-chart $\endgroup$ – Surprised Dog Nov 19 at 23:23
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    $\begingroup$ @Surprised: "and 10% O2 levels" -- nit: the percentage of O2 doesn't change with altitude. Air pressure does, and thus so does the partial pressure of O2 in the air. But the percentage remains at ~20%. $\endgroup$ – Peter Duniho Nov 21 at 3:08
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There's evidence dinosaurs in general had the same sort of respiratory system that the modern dinosaurs have kept, which is more efficient than the system used by mammals. This is suspected to be part of the reason why they could get to such a huge size; they could more efficiently process it to extract the oxygen, and there's no reason to assume they'd have it any more difficult than today.

Here's a chart of oxygen levels since the Silurian:

enter image description here

You'll note that the period of low oxygen in the Triassic was significantly lower than today...and that's when dinosaurs solidified their dominance, due in part to their better oxygen extraction system. There were subsequent times when oxygen levels dipped to near-current levels, and again, dinosaurs kept on ticking through.

Based on that, there shouldn't be any reason why dinosaurs from the Cretaceous should have any significant difficulties based on the amount of oxygen in the air. There were land mammals even larger than the dinosaurs you name who thrived at current oxygen levels, so clearly larger animals than are present now are possible.

For example, T-Rex is estimated to have massed between 4.5 to 14 tonnes (depending on assumptions about how heavily built they were), and Triceratops 6 to 12 tonnes. The Asian straight-tusked elephant Palaeoloxodon is estimated to have been up to 22 tonnes; Paraceratherium 15 to 20 tonnes; the steppe mammoth 14 tonnes; Deinotherium 10+ tonnes; and so on.

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  • $\begingroup$ Also note that different studies have come up with different answers for prehistoric oxygen levels, this one said that oxygen levels were lower than today throughout the dinosaur age. $\endgroup$ – Hypnosifl Nov 19 at 17:36
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    $\begingroup$ ~70Ma is when the T-Rex and Triceratops first appeared. That period is the last straight-ish line in the graph (at ~25%) before it drops off and they all die. 25% verses today's ~22%? meh, +1. $\endgroup$ – Mazura Nov 20 at 2:07
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    $\begingroup$ As @ Hypnosifl says, I think your graph may be outdated. It seems to originate from 2010; since then newer research suggests that the oxygen levels throughout most of the dinosaur era were lower than or similar to modern levels. See Fig 11 in Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic (from 2013). $\endgroup$ – iayork Nov 20 at 13:19
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    $\begingroup$ .. .that said an unpublished study suggests that very early dinosaurs may have evolved in a higher oxygen environment. In any case, all the work seems to agree that during the dinosaur era there were lower-oxygen periods that dinosaurs dealt with just fine. $\endgroup$ – iayork Nov 20 at 13:23
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    $\begingroup$ I find it bizarre that grasses are younger species than T. rex. $\endgroup$ – hobbs Nov 20 at 18:15
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To a lazy first approximation, the O2 concentration now is about 70% of what it was back then. To get a similar effective difference in the partial pressure of oxygen, you can go to an altitude of about 2900m. Without acclimatization, this can be quite unpleasant (or potentially life threatening) and physical activity can be challenging. It is possible to acclimatize to such environments though, and regular humans can live their entire lives at higher altitudes than this.

Modern archosaurs (in the form of avians and crocodilians) have got fancier lungs than mammals do, and may fare better. The more energetic the lifestyle of the dinosaur, the more they'll suffer from the oxygen deficit. A herbivore might cope OK, but a predator might not be able to run fast enough and pterosaurs might not be able to get airborne at all.

There may, of course, be other unknown issues with dinosaur metabolism that make them less able to weather the change; we're not likely to discover those without actually conjuring up some suitable dinosaurs to experiment on.

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    $\begingroup$ Counterpoint: the modern dinosaurs (aka birds) have very energetic lifestyles, yet tolerate significantly lower oxygen partial pressure. Birds migrate over the Himalayas, with the common crane observed at over 10,000 meters altitude. Swans were tracked over Ireland at 8200 meters, and even regular mallard ducks were observed at 6400 meters. Note that none of these birds are generally considered high-altitude dwelling animals. For those that are, Ruppell's vultures routinely cruise at 6,000m, and one was sucked into a jet engine at 37,000 feet (11.3 km). A mere 2900 meters? Pshaw. $\endgroup$ – Keith Morrison Nov 20 at 3:06
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    $\begingroup$ TL;DR: They're probably fine. $\endgroup$ – Gloweye Nov 20 at 9:09
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    $\begingroup$ @KeithMorrison not all birds are created equal. The fact that some birds can do it does not imply that all birds can do it and definitely doesn't say anything about ancient bird ancestors adapted for a different enivronment. $\endgroup$ – Starfish Prime Nov 20 at 10:33
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    $\begingroup$ When multiple birds from lineages that have been separated since the Cretaceous display similar capabilities, it's reasonable to conclude that capability is likely ancestral to birds in total. And all birds have 10 times greater surface area to gas exchange volume than mammals, meaning all birds have greater capacity to extract oxygen from air. $\endgroup$ – Keith Morrison Nov 20 at 15:52
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    $\begingroup$ Several answers mention that the lungs of dinosaurs and modern archosaurs are "fancier". In fact they breathe in a loop instead of in and out like humans do, which allows more efficient oxygen extraction, and is indeed fancy and cool =) $\endgroup$ – Cireo Nov 20 at 16:19
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The Cretaceous period was long. There were periods when oxygen was 30% and there were periods (after massive volcanic eruptions) when there were 18%. I can't say it had no effect on the biosphere, but dinosaurs (and T-rex especially) kept their apex positions in both cases.

But these changes were slow, taking place over the course of generations. So these dinosaurs had time and conditions to adapt.

But if we just put these dinos out of their age (where oxygen concentration was high) to our time then ther might be some problems, but not much.

T-Rex was a long-walker, but a short-runner, about hundreds of meters - like cheetah. It was running on inner reserves (like cheetah do now) and the amount of reserves does not depend on outer conditions. It would just take more time for replenishing these reserves. So it would be able to do this run not say (I don't know exact numbers) once an hour, but once one and a half hour. On large scale it will reduce "net meat income" for T-Rex population, but for single animal it would not make a big difference.

Same thing is for other types of dinosaurs - they will get tired faster, but that would not matter much on individual scale (unless you are doing some beast-pit battles), since their predators (or preys) would also tire faster.

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I would also be gravely concerned about the changes in microscopic life since Mr. Dinosaur has been away. On that matter, I’d expect a hunter/scavenger carnivore like Rex to show a little more resilience given his lifestyle.

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