Dougal Dixon

“It is not just the different plants and animals that define the environment. There are all sorts of physical factors as well. Take the atmosphere, for instance. The oxygen levels became usable to us about 400 million years ago, but since then there has been a great variation in the oxygen levels. In the late Jurassic it is possible that the oxygen levels were about 35%, as opposed to 20% at the present day. Indeed this figure has been put forward to explain the survival of the very big dinosaurs, high oxygen concentrations in the breathing air being able to keep the great volumes of tissue oxygenated. On the other hand the proportion of carbon dioxide was also high. This may account for the prolific plant life at the time, carbon dioxide being essential for the good growth of plants. The difference between the composition of the Jurassic atmosphere and that of your own time may make it difficult for you to breathe when you first arrive, but your body will probably adapt to it before long.”
― Dougal Dixon, A Survival Guide: Living with Dinosaurs in the Jurassic Period