Why did the giant insects disappear?

Imagine a world populated by giant dragonflies with wingspan of up to 70 centimeters. It is not science fiction. About 300 million years ago, in the late Carboniferous and early Permian period, insects reached their largest sizes. A team of scientists from the University of California in Santa Cruz (USA) has studied these giants through the fossil record and proposes that they werethe evolution of the first birds the decisive factorin the decrease in size that the insects suffered later.

Current theories attribute the dwarfing of these animals to a decrease in atmospheric oxygen, which at that time was 30 percent, compared to 21 percent today. The new study, published inPNAS, collects data on 10,500 insect fossils and relates them to the oxygen levels of each epoch. The results reveal some contradictions, as stated by Matthew Clapham, one of the authors: “The size of prehistoric insects is related to the amount of oxygen existing in a period of 200 million years. Later, at the end of the Jurassic and early period Cretaceous, about 150 million years ago,oxygen increased but insect size decreased“.

On the other hand, the scientists verified that this moment coincided with a greater specialization of the birds. This could be, according to the researchers, an important evolutionary force towards a decrease in the size of flying insects, which needed greater maneuverability to escape new predators.
However, the authors highlight the difficulty of drawing relevant conclusions, since there is a gap of 20 million in the fossil record of insects that makes it very difficult to detect the exact moment in which the size change occurred.