The Myth of Prehistoric Oxygen
Giant sauropod Barosaurus lentus fends off a pair of giant theropods Allosaurus fragilis. Did these animals owe their magnificent size and strength to anatomical advantages or environmental crutches?
Fred Wierum (CC BY-SA 4.0)
It's a fairly common idea floating around pop-sci spaces that oxygen levels in prehistoric times were drastically higher than today1. Proponents of this idea claim that increased oxygen levels supercharged prehistoric animals' metabolisms and growth rates, leading to giants like Brontosaurus. Consequently, it is often claimed that if prehistoric animals (usually dinosaurs) were magically brought to modern times, they would immediately suffocate to death2. It's a neat and tidy story for why our modern giants, like elephants and rhinoceroses, are so unimpressive compared to the titans of the deep past.
Unfortunately, this idea is completely and horribly wrong.
Sauropods like Argentinosaurus dwarfed even other dinosaurs. But did they need a boost of O2 to get that big?
Slate Weasel (Public Domain)
For starters, let's look at everyone's favorite super-sized saurians, the dinosaurs. Across their 175 million year-long stint as Earth's dominant megafauna, numerous dinosaur species independently achieved breath-taking sizes. Giant like Tyrannosaurus rex and Giganotosaurus carolinii weighed upwards of ten metric tons3—more than most elephants. Plant-eating like Shantungosaurus giganteus could occasionally get to twice that4, while titanic like Argentinosaurus huinculensis could rival the largest whales of today at 60-100 metric tons5, equivalent to a whole herd of elephants. And unlike whales, these were land animals that had to contend with the full force of gravity.
Since terrestrial animals the size of whales are fundamentally incompatible with 'normalcy' in our modern world, it makes intuitive sense that something was fundamentally different about the prehistoric world; dinosaurs' abnormality is explained by the abnormality of the world they inhabited. But the evidence doesn't add up.
Firstly, oxygen levels during the were extremely varied. At the start of the Triassic period (~250 million years ago) they went as low as 15%6, during most of the Jurassic they hovered around today's 21%1 and during the late Cretaceous (~70 million years ago) there might have been spikes of up to 30%1. If dinosaur body size was constrained by oxygen levels, we'd expect them to stay small during the Triassic, get medium-sized during the Jurassic and balloon in size at the end of the Cretaceous. For some, like the duckbilled and horned , this is approximately the case, but for others the patterns don't match. Theropods crossed the one-tonne mark all the way back in the early Jurassic7 while sauropods crossed the ten-tonne mark even earlier in the late Triassic8. After that, theropods consistently topped out at 8-10 tonnes, while sauropods did the same at 70-80. These weight limits stay relatively consistent throughout the Jurassic and Cretaceous9, suggesting that increased oxygen is neither necessary nor sufficient for dinosaur growth.
Straight-tusked elephant Palaeoloxodon namadicus, which towers over many dinosaurs.
Asier Larramendi (CC BY 4.0)
Secondly, dinosaurs were among the only animals to achieve great size during the Mesozoic. If oxygen levels increased we'd expect all animals to benefit, but no giant mammals, amphibians, tortoises or insects shared the world with the dinosaurs, while crocodilians and lizards only grew to great size in the seas. And even some dinosaurs, like the insect-eating alvarezsaurs, the raptor-like troodontids and birds, never achieved great size.
Thirdly, giant animals didn't just all up and vanish at the end of the Mesozoic. There were five-tonne crocodilians10 in South America just 10 million years ago, while one-tonne tortoises11 and elephants heavier than some Brontosaurus species12 were roaming around Asia just 10,000 years ago. For the latter we have direct samples of the atmosphere at the time trapped in ancient Antarctic ice and those samples indicate atmospheric oxygen levels were basically the same as today13. Therefore, animals the size of T. rex or Brontosaurus are definitely possible in a modern atmosphere and there's no fundamental need for the Mesozoic atmosphere to be different.
So if it wasn't oxygen, how did so many dinosaurs get so big?
The secret to dinosaur size was a suite of sophisticated anatomy. Their bones and viscera were riddled with holes filled by air sacs like those found in modern birds14. These air sacs allowed them to pass oxygen-rich air over their lungs on both the inhale and exhale, doubling their efficiency relative to air sac-less mammals. Their cartilage was infused with blood vessels which allowed it to grow thick15, cushioning their weight-bearing joints in a way impossible for the bloodless cartilage of mammals. And, most importantly, a fast warm-blooded metabolism allowed them to grow from chicken-sized hatchlings to whale-sized adults in just a few decades16. Sauropods in particular also happened upon a digestive method that required minimal energy and got more efficient at great size9, causing an evolutionary feedback loop that drove them to freakish size, even by dinosaur standards.
So giant dinosaurs could probably live just fine on our modern-day planet, barring any issues of diet or disease. But what about other prehistoric life?
The monstrous millipede Arthropleura was the largest arthropod in history. But was this an inevitable consequence of high O2, or an unusual outcome even with that?
Prehistorica CM (CC BY 4.0)
Well, oxygen levels were (often substantially) lower than today from the beginning of life on Earth all the way to the start of the Carboniferous period some 350 million years ago17. During the Carboniferous, the proliferation of terrestrial plants laid down vast beds of coal and shot the atmospheric oxygen level up to an absurd 35%17. The Carboniferous is also famous for massive bugs, like the crow-sized Meganeura monyi, the cat-sized scorpion Pulmonoscorpius kirktonensis and the man-sized millipede Arthropleura. High oxygen remains a popular explantion for how these arthropods got so large. Since most terrestrial arthropods do not actively breathe and just let oxygen passively diffuse into their bodies, the theory goes, they can only get large if oxygen levels are high enough to make up for their inefficient breathing18.
But even this theory doesn't hold up very well. For one, not all arthropods breathe passively; crickets, beetles and ants actively pump air through their bodies like us vertebrates19. For two, the titanopteran Gigatitan similis got to a similar size20 as Meganeura in the very low-oxygen Triassic. And for three, there weren't actually that many giant arthropods in the Carboniferous. Besides those previously mentioned there was the giant sap-sucking insect Mazothairos enormis, which was about the same size as its predator Meganeura, and the amphibious 'sea scorpion' Megarachne servinei, which was comparable to the modern coconut crab. Carboniferous roaches21, spiders22, centipedes23 and so on were generally about as large or smaller than the ones we have today. If oxygen levels were solely responsible for their size, would we not expect them all to grow?
So how and why did Meganeura, Pulmonoscorpius and Arthropleura get so big? We're not sure. Certainly the oxygen wouldn't have hurt, but it's likely that some unusual combination of open ecological space and anatomical adaptations in these species allowed them to surpass their relatives by so much24. We will only know for sure by finding more fossils and collecting more data; in the meantime, we can only speculate.
References
1 🔒Mills, B. J. W., Belcher, C. M., Lenton, T. M., & Newton, R. J. (2016). A modeling case for high atmospheric oxygen concentrations during the Mesozoic and Cenozoic. Geology, 44(12), 1023–1026. https://doi.org/10.1130/G38231.1
2 📰Cramer, J. G. (1988, July). Dinosaur breath. (Alternate View Column AV-27). In Analog Science Fiction & Fact Magazine. Retrieved September 7, 2025 from https://npl.washington.edu/av/altvw27.html
3 Persons, W. S. IV, Currie, P. J., & Erickson, G. M. (2019). An older and exceptionally large adult specimen of Tyrannosaurus rex. The Anatomical Record, 303(4), 656-672. https://doi.org/10.1002/ar.24118
4 📖Paul, G. S. (2016). The Princeton Field Guide to Dinosaurs. Princeton University Press.
5 🔒Mazzetta, G. V., Christiansen, P., & Fariña, R. A. (2006). Giants and bizarres: Body size of some southern South American Cretaceous dinosaurs. Historical Biology, 16(2-4), 71-83. https://doi.org/10.1080/08912960410001715132
6 Benton, M. J. (2018). Hyperthermal-driven mass extinctions: Killing models during the Permian–Triassic mass extinction. Philosophical Transactions of the Royal Society A, 376, 20170076. https://doi.org/10.1098/rsta.2017.0076
7 Dal Sasso, C., Maganuco, S., & Cau, A. (2018). The oldest ceratosaurian (Dinosauria: Theropoda), from the Lower Jurassic of Italy, sheds light on the evolution of the three-fingered hand of birds. PeerJ, 6, e5976. https://doi.org/10.7717/peerj.5976
8 Pol, D., Garrido, A., & Cerda, I. A. (2011). A new sauropodomorph dinosaur from the Early Jurassic of Patagonia and the origin of the sauropod-type sacrum. PLOS One, 6(1), e14572. https://doi.org/10.1371/journal.pone.0014572
9 Sander, P. M., Christian, A., Clauss, M., Fechner, R., Gee, C. T., Griebeler, E.-M., Gunga, H.-C., Hummel, J., Mallison, H., Perry, S. F., Preuschoft, H., Rauhut, O. W. M., Remes, K., Tütken, T., Wings, O., & Witzel, U. (2011). Biology of the sauropod dinosaurs: The evolution of gigantism. Biological Reviews, 86(1), 117-155. https://doi.org/10.1111/j.1469-185X.2010.00137.x
10 Aureliano, T., Ghilardi, A. M., Guilherme, E., Souza-Filho, J. P., Cavalcanti, M., & Riff, D. (2015). Morphometry, bite-force, and palaeobiology of the late Miocene caiman Purussaurus brasiliensis. PLOS One, 10(2), e0117944. https://doi.org/10.1371/journal.pone.0117944
11 📖Orenstein, R. (2001). Survivors in Armor: Turtles, Tortoises, and Terrapins. Key Porter Books Ltd.
12 Larramendi, A. (2016). Shoulder height, body mass and shape of proboscideans. Acta Palaeontologica Polonica, 61(3), 537-574. https://doi.org/10.4202/app.00136.2014
13 🔒Stolper, D. A., Bender, M. L., Dreyfus, G. B., Yan, Y., & Higgins, J. A. (2016). A Pleistocene ice core record of atmospheric O2 concentrations. Science, 353(6306), 1427-1430. https://doi.org/10.1126/science.aaf5445
14 Sereno, P. C., Martinez, R. N., Wilson, J. A., Varricchio, D. J., Alcober, O. A., & Larsson, H. C. E. (2008). Evidence for avian intrathoracic air sacs in a new predatory dinosaur from Argentina. PLOS One, 3(9), e3303. https://doi.org/10.1371/journal.pone.0003303
15 Bonnan, M. F., Wilhite, D. R., Masters, S. L., Yates, A. M., Gardner, C. K., & Aguiar, A. (2013). What lies beneath: Sub-articular long bone shape scaling in eutherian mammals and saurischian dinosaurs suggests different locomotor adaptations for gigantism. Nature, 8(10), e75216. https://doi.org/10.1371/journal.pone.0075216
16 Botha, J., Choiniere, J. N., Benson, R. B. J. (2022). Rapid growth preceded gigantism in sauropodomorph evolution. Current Biology, 32(20), 4501-4507.e2. https://doi.org/10.1016/j.cub.2022.08.031
17 Mills, B. J. W., Krause, A. J., Jarvis, I., & Cramer, B. D. (2023). Evolution of atmospheric O2 through the Phanerozoic, revisited. Annual Review of Earth and Planetary Sciences, 51, 253-276. https://doi.org/10.1146/annurev-earth-032320-095425
18 📖Holmes, T. (2008). March Onto Land: the Silurian Period to the Middle Triassic Epoch. Infobase Publishing.
19 🗞️Powell, K. (2003, January 24). Bug breathing exposed. Nature. https://doi.org/10.1038/news030120-9
20 Schubnel, T., Legendre, F., Roques, P., Garrouste, R., Cornette, R., Perreau, M., Perreau, N., Desutter-Grandcolas, L., & Nel, A. (2021). Sound vs. light: Wing-based communication in Carboniferous insects. Communications Biology, 4, 794. https://doi.org/10.1038/s42003-021-02281-0
21 Schneider, J. W., & Rößler, R. (2023). The early history of giant cockroaches: Gyroblattids and necymylacrids (Blattodea) of the late Carboniferous. Diversity, 15(3), 429. https://doi.org/10.3390/d15030429
22 🔒Selden, P. A., Corronca, J. A., & Hünicken, M. A. (2005). The true identity of the supposed giant fossil spider Megarachne. Biology Letters, 1, 44-48. https://doi.org/10.1098/rsbl.2004.0272
23 📖Minelli, E. (Eds.). (1990). Proceedings of the 7th International Congress of Myriapodology. Brill Publishers.
24 🔒Davies, N. S., Garwood, R. J., McMahon, W. J., Schneider, J. W., & Shillito, A. P. (2022). The largest arthropod in Earth history: insights from newly discovered Arthropleura remains (Serpukhovian Stainmore Formation, Northumberland, England). Journal of the Geological Society, 179(3), jgs2021-115. https://doi.org/10.1144/jgs2021-115
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