It's likely the larvae of many ancient insects also passively absorbed oxygen from water and were not able to regulate their oxygen intake very well—a big danger when oxygen levels were so high.
One way to decrease the risk of oxygen toxicity would have been to grow bigger, since large larvae would absorb lower percentages of the gas, relative to their body sizes, than small larvae.
"If you grow larger, your surface area decreases relative to your volume," Verberk explained.
Read more about the study at National geographic News. Link
(Image credit: Ned M. Seidler, National Geographic)