Travel back in time, 300 million years, and you will most likely be surrounded by massive swamp forests that stretch beyond the horizon, with the ground covered in mosses and weird looking trees resembling massive ferns. This is the Carboniferous period.
A bird is flying towards you, but as it gets closer, you realise that it is actually an enormous dragonfly. This massive insect is called Meganeuropsis permiana and belongs to a family of estimated insects called griffinflies, a cousin to today’s dragonflies. From wing tip to wing tip it was a gigantic 71 centimetres (about the size of a pigeon). Today the largest dragonfly measures a mere 17 centimetres from wing to wing – four times smaller than the Meganeuropsis.
In the Carboniferous period arthropods were extremely common and much larger than their descendants today. There were no mammals and no birds. Insects and other arthropods ruled the skies, seas and the ground. Millipedes that were up to 2 meters long and 50 centimetres wide could come crawling along towards you.
Drawing of the Carboniferous Period (Source: https://commons.wikimedia.org/wiki/File:Our_Native_Ferns_-_Carboniferous_Pteridophyta.jpg)
The largest fossilized millipede is called Aurthoplaura and was probably the biggest arthropod that ever lived. There were mammoth cockroaches and massive scorpions. The largest fossil of a scorpion belongs to the Jaekelopterus rhenaniae, which was bigger than a tall man with claws up to 46 centimetres – not something you would like to meet while taking a quick dip!
Fossils show us that these massive creatures flourished, but where are they now? To understand why they could exist then and not now we need to look at oxygen concentration in the air.
The oxygen content of the atmosphere was 14% higher than today (35% compared to today’s 21%). The concentration of oxygen made a difference in the size of these insects due to their respiration system.
A respiration system is how an animal takes in oxygen. For humans, we breathe air into our lungs, and the oxygen enters our blood, which brings it to every single cell in our body.
Insects, on the other hand, do not have lungs or blood. Instead, they have openings on the outside of their bodies that allow air to enter. These opening are called spiracles, and they get smaller and smaller and allow oxygen to diffuse into every cell. It is this system that puts a size limit on arthropods. In the Carboniferous period, the oxygen content of the air was high; arthropods could take advantage of this and grow massive.
In labs, it has been shown that by slowly increasing the concentration of oxygen to a population of insects, each generation gets bigger and bigger. This experiment shows how the size of insects is correlated to the amount of oxygen in the atmosphere and suggests a new theory as to why arthropods got so big.
Arthropods got so big not because they could but because they needed too. Researchers at Michigan State suggested that arthropods got bigger as small larva could not handle the high oxygen levels as they could not regulate their oxygen intake. Bigger larva could handle the oxygen levels and went on to produce bigger adults, which meant that arthropods evolved to be larger because it was the only way to survive.
But why was the oxygen level so high in the first place? During this period the land was covered in plants, from mosses to some of the earliest trees. These plants were taking in carbon dioxide from the atmosphere and pumping out oxygen like they do today, but the bacteria that decomposed the plants when they died was not around yet. So the carbon dioxide did not re-enter the atmosphere, and the oxygen level increased, allowing for bird-sized dragonflies and massive millipedes.