Arthropleura was a millipede relative longer than a person. The largest individuals grew to about 2.6 m long and 55 cm wide, making it the largest arthropod, the group that includes insects, spiders and crabs, ever known to have lived on land. It crawled across the equatorial lowlands of Europe and North America for about 50 million years, from the Carboniferous into the early Permian.
The largest arthropod on land
Most large Arthropleura fossils are not bodies but shed skins, left behind as the animal moulted and grew. In January 2018 a section of one of these shed skins, about 326 million years old, was found at Howick Bay in Northumberland, England. A 2021 study of this specimen estimated that the whole animal was between 1.9 and 2.63 m long and weighed about 50 kg. Fossil trackways at Joggins in Nova Scotia are up to 50 cm wide.
Its body was made of 28 to 32 segments in adults, each covered by an armoured plate in three lobes, typically studded with bumps or spines, and each carrying two pairs of legs. Not every Arthropleura was a giant: tracks at Montceau-les-Mines in France suggest that the kind living there reached only about 40 cm, though larger animals cannot be ruled out.
A head at last
For more than 150 years no complete head was known, because the large fossils do not preserve one. In 2024 a team scanned the fossils of two young animals, about 2.3 and 4 cm long, from Montceau-les-Mines, using micro-CT and a synchrotron. The heads had antennae, stalked compound eyes, internal jaws and two pairs of further mouthparts.
The result was a surprise: a head with features like a centipede’s on a body like a millipede’s, but with none of the venom claws that centipedes use to kill. The team’s main analysis placed Arthropleura as an early offshoot of the line leading to millipedes, although adding a fragmentary older relative pushed it further down the myriapod family tree. They concluded that it was most likely a detritivore, though no direct evidence of its diet has been found.
A 2025 study described heads on two further specimens from northern France, from animals of at most about 50 cm, so the heads of the largest animals are still inferred from smaller ones.
Not a swamp monster
Arthropleura has often been drawn crawling through the dense coal swamps of the Carboniferous. The 2021 study argued that it preferred more open, sparsely wooded ground near water, such as coasts and floodplains. Its trackways cross both ground that was underwater and ground that was dry, so it may sometimes have waded. A 2025 study found pores on some of its plates that could have been openings for breathing tubes, which would confirm it as a land animal.
Why so big?
Giant Carboniferous animals are often explained by the high oxygen levels of the time. Arthropleura does not fit that story: it was already large before oxygen rose, and its largest known fossil comes from a time when the air held about 23 per cent oxygen, only slightly more than today. Plenty of food and little competition may matter more.
It disappeared in the early Permian, about 292 million years ago. The 2021 study suggested that the drying out of the equatorial regions and competition from four-limbed animals finished it off, though the cause is still uncertain.
