Carboniferous Mega-Insects. Why?
Artist’s recreation of a Carboniferous swamp forest.
Image from:http://www.extra-life.de/
Imagine taking a stroll in a beautiful swamp forest about 300 million years ago. You’re surrounded by a staggering amount of plant diversity, much of it fern-like in appearance. There are club mosses, lycopods (such as the glorious Lepidodendron, pictured here), ferns, massive horsetails, and other beautiful now extinct life forms. Vascular plants are flourishing after their triumphant invasion of the land, and primitive amphibians and reptiles are following close behind. The climate is humid and tropical. Pangaea is forming. Life is good.
Then a dragonfly with a 70 cm wingspan appears, and two and a half feet of flying invertebrate careens over your head. Meganeura is one of the largest known flying insect genuses ever discovered, and it thrived, along with a wide variety of other super-massive insect species, during the Carboniferous period. Meganeura is thought to be predatory, as dragonflies are today, but due to its size, it was likely capable of preying on small amphibians as well as its fellow insects. The Carboniferous saw a huge explosion of insect diversity, with over 100 different families (including cockroaches) evolving during this phase of Earth’s history. Many of the insects were huge. Mayflies were the size of canaries, millipedes could grow to be eight feet long, and, of course, Meganeura ruled the skies. But why aren’t these massive insects around today? More importantly, why are all currently living insects so small?
The answer lies with the oxygen content of the Carboniferous atmosphere. Insects have their own way of breathing. They don’t have circulatory systems to move oxygen around to their organs from the environment like we do. Instead, they have tiny pores along the length of their bodies that allow oxygen to diffuse directly from the atmosphere into their tissues. However, there is a limit to how far oxygen can diffuse freely, which means that insects must be small-bodied to allow the oxygen to reach all of their aerobic tissues.
Nowadays, oxygen makes up about 20% of the atmosphere. But in the Carboniferous, it made up 35%. This is a substantially increased amount of oxygen compared to current levels. The Carboniferous is sometimes called the “Oxygeniferous” for this reason. Because the atmosphere was so oxygen-dense, insects could be much larger because the oxygen could diffuse further. In other words, their diffusion-dependent size limit was much larger than it is now, allowing dragonflies like Maganeura to reach record-breaking sizes.
Meganeura as compared to a human. Image from: http://www.prehistoric-wildlife.com/species/m/meganeura.html
The start of the Carboniferous was also when our first tetrapod ancestors were just beginning to heave themselves out of the primordial ocean and onto the land. It’s possible that they were aided in this endeavour by the high oxygen content of the Carboniferous atmosphere, because it allowed them to obtain more oxygen with one gulp and avoid the dehydration of their early fish-like bodies. It’s common knowledge that a frog or a salamander needs to be moist to survive, and this was likely a major concern for our pioneering ancestors as well.
The Carboniferous was followed by the Permian period, and oxygen percentages began to decline, threatening the survival of the Carboniferous mega-insects. The Permian ended with the largest mass extinction the Earth has ever seen, in which 95% of species perished (including Meganeura), possibly due to asteroid bombardment or climate change. Life, however, survived - thankfully including those early tetrapods which eventually gave rise to anatomically modern humans approximately 200,000 years ago. Though only a distant (and possibly revolting) memory today, the Carboniferous mega-insects remain an impressive and beautiful example of the power of evolution to continually defy our wildest ecological imaginations.