Giant Invertebrates: Scientists Deadliest Accidents or Competitive Superiority Through Evolution?
Image credit: Movie poster advertisement for Tarantula (1955), Public Domain, Image Cropped
In the mid twentieth century, movies like ‘Tarantula’ and ‘Them!’ set the scene for what life would be like if giant bugs existed. Scenes filled with nightmares arisen from experiments gone wrong, children-stealing ants, great taglines like “Science’s Deadliest Accident”, and some unbelievably outdated special effects.
Luckily, giant bugs don’t exist… Anymore.
Let’s rewind 358.9 million years ago, to the Carboniferous Period, when the giant landmasses of Laurasia and Gondwana collided and fused together in a mountain-building event, forming the Appalachian-Hercynian orogeny. These landmasses were dominated by huge forests of vascular plants, such as giant club mosses, huge conifers, and large ferns. Here, other giants also existed, without the need for mad scientists or radioactive waste dumps.
Rather than needing a mad scientist or a toxic waste dump, scientists believe that huge invertebrates arose because gigantism allowed them to achieve competitive superiority. Note that not all insects during this time were gigantic, but those that were larger than the average, were very… very…large. Let’s go through the titans of the Carboniferous, starting with the invertebrates.
The largest insect to ever have existed is Meganeuropsis. A 43 cm long dragonfly-like creature with a wingspan reaching 72cm. These primitive ‘griffinflies’ or Meganisopterans lived from the Late Carboniferous to the Late Permian period (317 to 247 mya). The largest species of dragonfly today, the giant darner (Anax walsinghami), has a wingspan of just 12.7 cm. The Meganeuropsis is 5.5 times larger, about the size of a seagull.
The largest arthropod to ever have existed is the Arthropleura, a giant millipede that grew around 2.5m in length, and weighed about 50kg. Currently, only 3 fossils of Arthropleura exist, however no fossilized heads have been found. These invertebrates disarticulated (separated at each body segment) as they died so scientists still know little about them. The largest living millipede in the world today is the African giant black millipede (Archispirostreptus gigas), which can grow up to 30cm. That is over 8 times smaller than Arthropleuras!
The largest land scorpion to ever have existed was Pulmonoscorpius. Reaching 70cm from head to tip of its stinger. Roughly the size of a house cat, it is three times larger than today’s largest land scorpion, the Giant Forest scorpion (Heterometrus swammerdami titanicus) which reaches 23cm in length.
So how did they get so big?
Huge forests dominated the land, expelling oxygen into the atmosphere at tremendous rates. Without decomposers in the environment, as they had not yet evolved, there was no additional carbon dioxide being recycled back into the atmosphere. This allowed the atmospheric oxygen level to rise to about 35%, which is about 1.7 times higher than today’s levels.
With this excess oxygen, organisms would experience damage to cells, tissues, and organs caused by excess oxygen levels or higher than normal partial pressure of oxygen. This can lead to oxygen toxicity, which can cause death. To compensate for excess oxygen, and to escape oxygen toxicity, organisms evolved to have increased body size. Invertebrates use tracheal tubes (breathing tubes) to expel oxygen, and as they increase in body size, more of their body is taken up by these tracheal tubes. Scientists have found organisms living with high oxygen concentrations had smaller tracheal volume, meaning these tubes were smaller allowing the individual to grow much larger than one living in a time-period with lower oxygen concentrations.
Excess oxygen is the commonly held hypothesis, however, some scientists believe other factors may have been at play. Another potential cause for such gigantic insects is the lack of aerial vertebrate predators. There is some supporting evidence for this theory. Researchers at the University of California, Santa Cruz found that insect size did in fact track the amount of oxygen in the atmosphere, increasing and decreasing for about 200 million years. This continued all the way through the Carboniferous Period, through the whole of the Permian Period (251.9 million years ago), right up until the end of the Jurassic Period, 145 million years ago. However when the Jurassic period ended, the oxygen level increased but insect size did not. This period coincides with the evolution of birds. Whether birds took over the niche of larger insects or were a novel predator preying upon these large species is unknown. However, evidence suggests birds did influence insects’ ability to grow large, even when there was an influx of oxygen in the atmosphere.
Another hypothesis is the evolutionary arms-race in body size between plant-feeding prey and their predators. With such large plant species growing during this time, it would allow for the prey species to grow larger in size, therefore allowing the predator species to grow larger.
Where did they go?
At the end of the Permian period, a mass extinction occurred that wiped out 90% of all life on earth and was considered the “mother of mass extinctions”. This event led to the largest extinction of insects we’ve yet recorded, which are thought to be “survivor” organisms. It resulted in nine entire orders becoming extinct and left another ten in sharp decline. Although oxygen fluctuations continued after this period as life continued, no insect has ever become giant again.
Jennifer Merems is a writer and researcher focusing on behavioral and nutritional ecology. She is currently a PhD candidate in the Department of Forest and Wildlife Ecology with the University of Wisconsin-Madison. You can learn more about Jennifer by following her on Twitter at @atyourcervid.