At 2.5m in length, Arthropleura is widely considered the largest invertebrate to ever walk the Earth, says Will Newton.
What was Arthropleura?
Arthropleura belongs to a family of bugs known as the myriapods, which includes several other, many-legged bugs such as millipedes and centipedes.
Was Arthropleura an insect?
While Arthropleura and its living relatives may be invertebrates, they’re not insects per se. The term ‘insect’ is used to describe a related but distinct family of bugs known as hexapods. This family includes lots of different types of bugs, from beetles to butterflies.
When and where did Arthropleura live?
Arthropleura existed during the Carboniferous Period - from 346 to 290 million years ago - and lived in what is now Europe and North America, back when the two landmassess were part of a single continent (known as Euramerica) that lay on the equator. Arthropleura was long thought to have lived in densely forested swamps - an iconic and incredibly extensive environment of the Carboniferous. However, based on more recent research, it’s now thought it lived in far more open environments, preferring sparse woodlands, floodplains, and even coastal environments.
How big was Arthropleura?
At 2.6m in length and nearly 50kg in weight, Arthropleura is widely regarded as the largest terrestrial invertebrate to ever live. The giant eurypterid Jaekelopterus - a ‘sea scorpion’ that lived during the Devonian Period around 405 million years ago - is thought to have grown to similar lengths, but it lived underwater and in an environment that better supported enormous body sizes.
Was Arthropleura a millipede or a centipede?
The body, or trunk, of Arthropleura was made up of 28-32 segments, known as tergites. Each of these tergites bore two pairs of walking legs, which is a key trait of modern millipedes.
This historically led to the interpretation that Arthropleura was a massive millipede, but a recent find of a fossilised Arthropleura head - something that had eluded researchers from its discovery in 1854 until just last year - revealed distinctly centipede-like traits. This suggested that Arthropleura belonged to an ancient millipede-centipede group and that today’s millipedes and centipedes are each other’s closest relative.
Why was Arthropleura so big?
It’s a commonly held view that Arthropleura, and other giant bugs that lived during the Carboniferous, grew so large in response to higher levels of oxygen in the atmosphere. In the Carboniferous, oxygen levels peaked at around 35%, which is a lot higher than the 21% they’re at today.
This surplus of oxygen may have driven other bugs towards gigantism, but based on some newly discovered Arthropleura remains found in Northumberland, England, it’s now thought that Arthropleura achieved its giant size before the peak in atmospheric oxygen.
A more likely driver of gigantism in Arthropleura is simply a lack of serious competition. The genus, Arthropleura, persisted for approximately 56 million years and, for the majority of that time, was by far the largest animal in its environment.
It was only in the last few million years of Arthropleura’s reign that tetrapods - animals with four legs and the ancestors of amphibians, reptiles, and mammals - got big and started to compete with the invertebrates that had dominated terrestrial ecosystems since the Devonian Period (419-359 million years ago).
While Arthropleura may be the largest invertebrate ever discovered, it wasn’t the only giant bug that lived during the Carboniferous. This period of Earth’s history also witnessed the rise of the griffenflies, or meganisopterans.
These giant, dragonfly-like insects were the largest flying insects to ever live. The largest, Meganeura, had a wingspan that measured 71cm across, which is about the same as a sparrowhawk’s. There was also a giant scorpion, known as Pulmonoscorpius, that exceeded lengths of 70cm and was one of the top predators in Scotland during the Early Carboniferous.
What did Arthropleura eat?
Like today’s millipedes, Arthropleura was a detritivore that fed on either dead and decaying plant matter or animal remains - on the rare occasions that it stumbled across them.
In the early 1900s, researcher Gérard Waterlot proposed that Arthropleura may have been a carnivore, like today’s centipedes, and that it preyed on other, smaller animals that shared its environment. This was called into question by other researchers who pointed to the giant bug’s closely packed legs and short stride (as evidenced by fossil trackways) as signs that it was very slow and, as a result, incapable of pursuing prey. Arthropleura also lacked the pincer-like front legs that today’s centipedes weaponise and use to inject venom into their prey.
In terms of natural predators, Arthropleura had very few if any that it had to watch out for. As mentioned above, it was the largest animal in its environment by far and its size alone would have likely deterred any would-be predators. It was also covered in thick dorsal armor made of chitin, which gave it an extra layer of protection. That said, it has been suggested that large amphibians or lobe-finned fish may have occasionally preyed on young Arthropleura that wandered a little too close to the water’s edge.
Why did Arthropleura become extinct?
Arthropleura was a survivor, persisting for more than 50 million years and weathering a series of climatic events in the Late Carboniferous before it ultimately faced extinction in the Early Permian, ~290 million years ago. To put its longevity into perspective, modern humans (Homo sapiens) have only been around for a maximum of 300,000 years, or 2.4 million years if you take the first species in our genus (Homo habilis).
It’s unclear exactly why Arthropleura disappeared, but it’s thought that the desertification of the planet’s equatorial regions during the Early Permian likely spelt the beginning of the end for the giant bug. Arthropleura probably also faced increased competition during this time. This is the point in Earth’s history that reptiles rose to ecological dominance and took over from the bugs that had ruled over the land from the Devonian to the Carboniferous.
After the extinction of Arthropleura, bugs decreased dramatically in size. Thankfully, the days of double duvet-sized millipedes are long gone, but it’s perhaps a stretch to say that ‘The Age of Bugs’ is truly over. They may no longer be the giants they once were during the Carboniferous, but bugs - or invertebrates - still make up approximately 97% of all animal species on Earth.
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