Arthropleura is widely regarded as the largest terrestrial invertebrate of all time. It belongs to a group of bugs known as the myriapods, which includes several other, many-legged bugs such as millipedes and centipedes.
What was Arthropleura?
Arthropleura is neither a millipede nor a centipede, but rather an ancestor of both. It has millipede-like traits, such as body segments (known as tergites) that bear two pairs of walking legs, and centipede-like traits, such as a pair of small mandibles.
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Its strange combination of traits, some of which are absent in today’s millipedes and centipedes, identify Arthropleura as a stem-millipede and a member of a long-extinct sister group. This group includes several other, >1m-long giants, but none were quite as big as Arthropleura.
What’s the difference between a myriapod and an insect?
The term ‘insect’ is often used to describe a wide range of invertebrates, including spiders, scorpions, millipedes, centipedes, and beetles, to name just a few. However, only the latter of these aforementioned creepy crawlies - beetles - are actually insects; or hexapods to give the group its scientific name.
As a group, insects are characterised by a three-part body plan - an anterior head, thorax, and posterior abdomen - and three pairs of legs. On the other hand, myriapods have long bodies made up of numerous, similarly sized segments and many pairs of jointed legs.
As for spiders and scorpions, they belong to another related but distinct group of invertebrates known as chelicerates. This group differs from insects and myriapods in that they have a pair of modified, front-facing appendages (chelicerae) that are often used to help them catch prey - the fangs of spiders, for example.
Where did Arthropleura live?
Arthropleura lived in what is now Europe and North America from 346 to 290 million years ago. It lived during a time when these two landmasses were part of a single, expansive continent that straddled the equator and was covered by densely forested swamps.
For a long time, it was thought that Arthropleura lived in such swamps, but based on more recent discoveries it’s now thought that it preferred sparse woodlands, floodplains, and even coastal environments.
Arthropleura was discovered in 1854 and is largely known from fossils found in central Europe and the UK. A number of significant discoveries have also been made in North America where several trackways, believed to have been made by Arthropleura, have been found.
Why was Arthropleura so big?
Prior to 2018 and the discovery of a remarkable specimen in Howick Bay, Northumberland, it was suggested that Arthropleura was capable of growing up to 2m in length. This new specimen raised that bar to 2.6m, which is roughly the length of an average-sized car.
It’s often said that Arthropleura, and other giant bugs that lived during the Carboniferous, grew so large in response to an increase in the amount of oxygen in the atmosphere. Today, Earth’s atmosphere is made up of 21% oxygen, but during the Carboniferous it comprised more than 30% and peaked at a whopping 35%.
However, based on the supersized specimen found in Northumberland and the fact that it was found in rocks that predate the peak in atmospheric oxygen, it’s now thought that high levels of atmospheric oxygen did not set Arthropleura on the path towards gigantism.
A more likely driver of gigantism in Arthropleura is simply a lack of serious competition. It was, by far, the largest animal in its environment and it lived during a time when tetrapods - four-legged vertebrates that eventually gave rise to amphibians, reptiles, and mammals - were only just starting to diversify and adapt to life on land.
What did Arthropleura eat?
Arthropleura was a detritivore, just like today’s millipedes, and ate pretty much anything it came across, provided it was dead and decaying. This meant that most things were on the menu for Arthropleura, from commonly available leaf litter, to less-frequent banquets of animal remains.
Not everyone agrees that Arthropleura was so unfussy when it came to choosing what it had for its dinner. In the early 1900s, researcher Gérard Waterlot suggested that Arthropleura may have been a predator that preyed on the smaller animals that shared its environment.
It might sound a little far fetched, but centipedes - some of Arthropleura’s closest living relatives - are predatory and are known for possessing pincer-like front legs that are capable of injecting lethal doses of venom into their prey.
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However, no evidence of centipede-like weaponry has ever been found on the fossilised remains of Arthropleura, nor any specific adaptations that suggest it may have been a predator. The trackways found in North America and believed to have been made by Arthropleura also imply that it was very slow and probably incapable of pursuing prey, even if it wanted to.
Why did Arthropleura become extinct?
Arthropleura lived for approximately 56 million years and weathered a series of significant, globe-spanning climate changes that consigned many other animals to extinction. That said, it eventually disappeared in the Early Permian (about 290 million years ago), during a time when our planet grew increasingly hotter and more arid.
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It’s widely believed that the desertification of Earth’s equatorial regions during the Early Permian spelt the beginning of the end for Arthropleura. It’s thought that increased competition from the descendants of the tetrapods that had once skulked in its shadows may have also contributed towards its extinction.
After the extinction of Arthropleura, bugs decreased dramatically in size and dominion over land was handed to a new group of animals, reptiles. While they may be smaller and far from the dominant denizens they once were, bugs - or invertebrates - are still the most abundant and diverse animals on Earth, making up approximately 97% of all species.
Of living invertebrates, myriapods (the group that includes Arthropleura) contribute roughly 15,000 species. That’s more than the total number of living reptile species (12,000), and more than double the total number of living mammal species (6,500)!
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Top image: A digital reconstruction of the giant stem millipede Arthropleura by Prehistorica CM, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons